Hybrid breeding has been shown to effectively increase rice productivity. However, identifying desirable hybrids out of numerous potential combinations is a daunting challenge. Genomic selection holds great promise for accelerating hybrid breeding by enabling early selection before phenotypes are measured. With the recent advances in multi‐omic technologies, hybrid prediction based on transcriptomic and metabolomic data has received increasing attention. However, the current omic‐based hybrid prediction has ignored parental phenotypic information, which is of fundamental importance in plant breeding. In this study, we integrated parental phenotypic information into various multi‐omic prediction models applied in hybrid breeding of rice and compared the predictabilities of 15 combinations from four sets of predictors from the parents, that is genome, transcriptome, metabolome and phenome. The predictability for each combination was evaluated using the best linear unbiased prediction and a modified fast HAT method. We found significant interactions between predictors and traits in predictability, but joint prediction with various combinations of the predictors significantly improved predictability relative to prediction of any single source omic data for each trait investigated. Incorporation of parental phenotypic data into various omic predictors increased the predictability, averagely by 13.6%, 54.5%, 19.9% and 8.3%, for grain yield, number of tillers per plant, number of grains per panicle and 1000 grain weight, respectively. Among nine models of incorporating parental traits, the AD‐All model was the most effective one. This novel strategy of incorporating parental phenotypic data into multi‐omic prediction is expected to improve hybrid breeding progress, especially with the development of high‐throughput phenotyping technologies. Hybrid rice and maize are the most successful cases that have greatly mitigated the global food crisis. However, finding the desirable hybrids from all potential crosses is a tremendous challenge in hybrid breeding. It is practically impossible to evaluate the performance of all potential hybrids in multi‐year and multi‐location trials due to limited resources. Genomic selection (GS) has been proposed as a promising strategy to confront these challenges. GS uses genome‐wide DNA markers and phenotypes from a training sample to predict the genetic values of candidates in a test sample, where the latter have been genotyped but not phenotyped . With the rapid development in molecular technologies, transcriptomic and metabolomic information has been available for prediction , a special case of K‐fold CV with K equal to the number of observations (n), can be used to quantify the predictability without random errors due to sample partitioning , the least absolute shrinkage selection operator (LASSO), Bayesian methods, random forest, neural networks, support vector machine (SVM), and reproducing kernel Hilbert spaces regression (RKHS). Several studies have compared these prediction methods with empirical and simulated data (Xu imes. Xu  developeThe objectives of this work are to (1) modify the original HAT method for multi‐omic prediction and verify its feasibility, (2) evaluate the performance of hybrid prediction based on all combinations of the four sets of predictors including genomic data (G), transcriptomic data (T), metabolomic data (M) and phenomic data (P) of the parents using BLUP with the modified HAT method, (3) test whether incorporating parental phenotypic data can improve hybrid prediction and (4) further investigate the optimum model for incorporating parental phenotypic data.n‐fold using two publicly available data sets of wheat and maize. The wheat data set, available in the R package BGLR, consisted of 599 inbred lines genotyped with 1279 DArT (Diversity Array Technology) markers  together with the additive and additive‐dominance models Figure , where GWe performed an analysis of variance for the predictabilities of all 15 × 4×2 = 120 predictor‐trait‐model combinations. All the main effects and two‐way interaction effects are significant Table . We thenAccording to the above studies, integrating predictor P with any other predictors has significantly improved the predictability. How to incorporate P into a current omic prediction model becomes an important issue. The parental phenotypic data are regarded as random variables so far in this study, while such data can also be regarded as fixed effects. A total of nine models  of incorporating predictor P were compared. Here, the Random model treats the effects of predictor P as random effects, while the other models treat the effects of predictor P as fixed effects. In the A‐One, D‐One, AD‐One and P‐One models, we used parental phenotypic data just from the target trait, while in the A‐All, D‐All, AD‐All and P‐All models, we used parental phenotypic data from all traits. A detailed description of the models is provided in the Experimental procedures section. The predictabilities of the four traits from the seven predictor combinations containing predictor P  were calculated using the nine models Figure . For eacBased on the AD‐All model, we integrated predictor P with other predictors and then evaluated their predictabilities. The predictabilities of the four traits from the predictor combinations with P and those without P are illustrated in Figure The training sample consisting of 278 hybrids represents a small subset of all 21 945 crosses derived from 210 recombinant inbred lines (RILs). Based on the parameters estimated from the training sample, we predicted all potential crosses for the four traits. Due to the low predictability of predictor P, we did not use predictor P alone but combined predictor P with other predictors to predict hybrids with the AD‐All model. The predicted phenotypic values of the 21 945 crosses from 14 predictors are listed in Data et al., et al., et al., et al., For the first time, we integrated parental phenotypic information (phenome) into current multi‐omic prediction models for hybrid breeding of rice. The traits incorporated so far are all agronomic traits. Advanced phenotype facilities allow investigators to measure phenotypes of thousands of traits simultaneously. The biological functions of many such traits may not be known, but they can be used collectively to predict agronomic traits, just like SNPs of genome. If the predictor includes a large array of phenotypes, the method is called phenomic prediction. Although hybrid prediction from the phenotypes of inbred parents alone is not very effective Smith, , we founDespite the benefits of combining multiple predictors for hybrid prediction, the costs of obtaining omic data should not be ignored in practical breeding. Therefore, it is necessary to balance the improvement of predictability and cost. Results from this study show the benefits of incorporating parental phenotypic data for hybrid prediction. Combining genomic data and parental phenotypic data seems to be a cost‐effective choice for hybrid prediction. Although the joint predictor GTMP has the highest predictability, some problems exist in practical use of transcriptome and metabolome. Generally, transcript and metabolite profiles are dynamic and susceptible to environmental variation. A particular challenge in prediction based on transcriptomic and metabolomic data is the selection of appropriate tissue and sampling time  between the 10‐fold CV and 10‐fold HAT and between n‐fold CV and n‐fold HAT for all the four traits are approximately equal to 1, indicating very strong consistency between the two methods. The computational speed of the n‐fold HAT method is approximately n times faster than that of n‐fold CV. To reduce the computation burden, a modified multi‐omic HAT method can be used instead of the lengthy CV to measure the predictability of BLUP. Here, we focused on the HAT method for BLUP, and we will try to modify this method for other prediction models like LASSO and Bayesian methods. Previously, Gianola and Schön  derived from a cross between Zhenshan 97 and Minghui 63. The mating design and genetic characteristics of the population were described in detail previously  and m is the number of independent variables for each set of predictors in the model . For the genomic data, the numerical code of marker k  for individual j  is defined as Mjk=Fjk=1 for the homozygote of the major allele A1, Mjk=Fjk=0 for the heterozygote A1A2, and Mjk=Fjk=‐1 for the homozygote of the minor allele A2. The additive genotype of the hybrid is defined asZjk=12(Mjk+Fjk), and the dominance genotype is defined as Wjk=12|Mjk‐Fjk|. Assume that the genotypes of Zhenshan 97 and Minghui 63 are A1A1 and A2A2, respectively. The corresponding hybrid predictors are defined as Let Zjk=12(Mjk+Fjk), and the dominance coding is defined as Wjk=12|Mjk‐Fjk|, where Mjk and Fjk represent the measurements of the corresponding predictor  for male and female parents, respectively. Details of coding system were described in our previous study , W= \{Wjk\} is an n × m design matrix for the dominance effect γd, and ε is a vector of residual errors with an assumed N distribution. Assume that γa∼N and γd∼N, where ϕa2 and ϕd2 are polygenic variances of additive and dominance effects, respectively. The expectation of y is E(y)=Xβ and the variance–covariance matrix isKa and Kd are kinship matrices for additive and dominance effects. The variance components ϕa2,ϕd2,σ2 are estimated with the restricted maximum likelihood (REML) method. The whole sample is partitioned into a training sample and a test sample. Parameters estimated from the training sample are used to predict the phenotypic values of the test sample. Here, y1 is an n1×1 vector of phenotypic values for the training sample and y2 is an n2×1 vector of phenotypic values for the test sample. The partitioned model is written asKa11 and Kd11 are the kinship matrices for the training sample, Ka22 and Kd22 are the kinship matrices for the test sample, and Ka21 and Kd21 are the kinship matrices between individuals in the test sample and individuals in the training sample. The predicted phenotypic values for the test sample is interpreted as the conditional expectation of y2 given y1, which is expressed asThe hybrid prediction model based on a single source of predictors is defined asThe above prediction model includes additive and dominance effects, so it is called the additive‐dominance model. If only the additive effect is involved, the model becomes the additive model.ZG, ZT, ZM and ZP are the design matrices for the additive effects of G, T, M and P predictors, respectively; WG, WT, WM and WP are the design matrices for the dominance effects of the corresponding predictors. Let the polygenic variances of additive effects γGa, γTa, γMa and γPa be ϕGa2, ϕTa2, ϕMa2 and ϕPa2, respectively, and the polygenic dominance variances corresponding to the dominance effects γGd, γTd, γMd and γPd be ϕGd2, ϕTd2, ϕMd2 and ϕPd2, respectively. The expectation of y is E(y)=Xβ and the variance–covariance matrix isWhen using multiple predictors, the fully combined model is modified asThe unknown parameters are estimated using the REML method whose likelihood function isOnce the parameters are estimated based on the training sample, the prediction of the test sample follows the procedure described in equation  by integy denotes a vector of the phenotypic values; Xβ represents the fixed effects; ξ1 and ξ2 are random effects for two sets of predictors with ξ1∼N and ξ2∼N distributions, respectively; and ε∼N is a vector of residual errors. The expectation of y is E(y)=Xβ and the variance–covariance matrix of y is var(y)=V=K1σξ12+K2σξ22+Iσ2, where K1 and K2 are kinship matrices for the two sets of predictors. The parameters β,σξ12,σξ22,σ2 are estimated with the REML method. Let ξ=ξ1+ξ2=y‐Xβ^ be the ‘observed’ effects for predictors (these are the phenotypic effects adjusted by the fixed effects), and let ξ^=ξ^1+ξ^2=y^‐Xβ^ be the predicted effects. By solving the mixed model equations, ξ^ based on the whole sample is derived asTo simplify the argument, two predictors are considered in the multi‐omic prediction model. The prediction model with two predictors is reformulated asξ^=Hξ, HR=(K1σ^ξ12+K2σ^ξ22)V‐1 should be the HAT matrix of the random effects. For the K‐fold HAT method, the sample needs to be partitioned into K parts. Let e^=ξk‐ξ^k be the estimated residual error for the sample in the kth part and HkkR be the diagonal block of HR for the sample in the kth part . The predicted residual error for the kth part is ek=I‐HkkR‐1e^k, and this has been proved in . Similar to CV, the n‐fold HAT method is a particular case of K‐fold HAT with K = n. For the n‐fold HAT, PRESS is modified as.Hjj is the jth diagonal element of the HAT matrix. To avoid sample partitioning errors, we used the n‐fold HAT method to evaluate the predictability in this study.The predictability is defined as(1)P=12(PM+PF)βa,(2)P=12|PM‐PF|βd,(3)P=12(PM+PF)βa+12|PM‐PF|βdand(4)PMβM+PFβF, where P is a set of phenotypic predictors, PM and PF are the phenotypic data for the male and female parents of the corresponding hybrids; βa, βd, βM and βF are the model effects. These four models are called A, D, AD and P models, respectively. For the first three models, 12(PM+PF) represents the mid‐parent value and 12|PM‐PF| represents the difference between the two parents. Model (4), the P model, does not involve any recoding but simply takes the original phenotypic values of the parents as predictors. Additionally, the PM and PF can be the parental phenotypic data from all traits (PM‐All and PF‐All) or just from the target trait (PM‐One and PF‐One). Here, a total of nine scenarios  were considered when combining the phenotypic data of parental lines. The fully combined models incorporating four sets of predictors are summarized in Table https://github.com/yangxu89/GS2020).The phenotypic data of parental lines are treated as random variables in the prediction model described above, but such data can also be considered as fixed variables. To explore the full potential of parental phenotypic information, we incorporated the phenotypic data using various models and compared their predictive performances. These models includeThe authors declare no conflict of interest.C.X., S.X. and Y.X. designed the research; Y.X., Z.Y. and X.Z. performed the research; Y.Z., X.W., Y.M. and P.L. analysed the data; and Y.X. wrote the paper. All authors read and approved the final manuscript.Figure S1 Predictabilities of four traits from seven predictor combinations comprising the predictor P with nine models.Figure S2 Predictabilities of four predictors  for four traits.Table S1 Analysis of variance of predictabilities from a 15×2×4 factorial design with 15 predictors, two prediction models, and four traits.Table S2 Analysis of variance of predictabilities from a 9×7 factorial design with nine models and seven predictors for four traits.Table S3 Predictabilities of the HAT and CV methods under 10‐fold and n‐fold for four traits from 15 predictor combinations.Click here for additional data file.Data S1 Predicted four traits for all 21,945 potential crosses derived from 210 recombination inbred lines from 14 predictors.Click here for additional data file.


Titanium octacarbonyl Ti(CO)8 is unstable as a result of steric repulsion between the CO ligands. The 20‐electron Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18‐electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)8 and Hf(CO)8 are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7. The heptacarbonyl complexes have shorter metal−CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable.Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight‐coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six‐coordinate Ti(CO) Bonding discoveries: The coordinatively saturated mononuclear carbonyl complexes of Group 4 elements, TM(CO)8  and Ti(CO)7, were prepared in the gas phase and/or in solid neon matrix. The analysis of the bonding situation shows that the formal 20‐electron complexes observe the 18‐electron rule and that Ti(CO)8 is unstable for the loss of one CO because of steric repulsion. It has long been known that molecules possess a particular stability when their atoms have a certain number of electrons in a shell structure in which the outermost shell is wholly or partially connected to other atoms. This was formulated by Langmuir following the electron‐pair model of Lewis5 and hexavalent sulfur molecules like SF6.There are exceptions to the electron‐counting rules, which have been mysterious to chemists for some time, and which could only be explained when the quantum chemical nature of covalent bonds was understood.4, Fe(CO)5 and Cr(CO)6, are well‐known stable homoleptic carbonyl complexes that follow the 18‐electron rule. The metal−CO bonds can be straightforwardly explained with the Dewar–Chatt–Duncanson (DCD) bonding model.6 with 17 valence electrons is a highly reactive but isolable homoleptic metal carbonyl complex.7  formally satisfy the 18‐electron rule and were theoretically predicted to be stable species by Luo et al.6 complexes, rather than the 18‐electron TM(CO)7 complexes, are formed in solid matrices.n with higher coordination number than n=6 are not stable, at least not for neutral species.The present work deals with a similar situation for the 18‐electron rule for transition‐metal complexes. Homoleptic transition‐metal (TM) carbonyl complexes are archetypical examples for demonstrating the metal–ligand bonding and the 18‐electron rule.7+, which was observed and investigated by using guided ion beam mass spectrometry in the gas phase.7+ complex is relatively small ((0.54±0.07) eV), it is stronger than expected for a ligand in the second coordination shell. Recent infrared photodissociation spectroscopic studies in the gas phase revealed that the 15‐electron TM(CO)6+ complexes, rather than the 17‐electron TM(CO)7+ complexes, should be characterized to be the fully coordinated complexes for TM=Ti, Zr, Hf.7+  and TM(CO)8+ , but the lighter Sc+ and V+ ions form only the 16‐electron complexes Sc(CO)7+ and V(CO)6+, respectively.8+ and V(CO)7+ were predicted to be stable, and higher‐temperature experiments gave the expected 18‐electron complex V(CO)7+ for vanadium.6+ cation was isolated as a stable salt compound.The first report of a seven‐coordinate homoleptic carbonyl was on the heptacarbonyl cation Ti(CO)8− , which are isoelectronic with the neutral Group 4 complexes TM(CO)8 TM=Ti, Zr, Hf.8−  as coordinatively saturated complexes. The latter systems are formally 20‐electron systems when all valence electrons of the metal−CO bonds are counted. Our theoretical analysis showed that the complexes possess cubic (hO) symmetry, in which one of the metal−CO valence MOs with a2u symmetry has no AO coefficient at the metal.8−  fulfill the 18‐electron rule suggested by Langmuir if only those valence electrons are counted that by symmetry bind to the metal.The situation for negatively charged metal carbonyl complexes is different. Very recently, we reported the observation of the anions TM(CO)8  in low‐temperature solid neon matrix.hO) symmetry and that the metal−CO bonding exhibits the same pattern as the transition‐metal carbonyl complexes. Very recently, we even reported the observation of the isoelectronic dinitrogen complexes M(N2)8.8  revealed that it can be understood with the DCD model in terms of the M←CO σ donation and M→CO π back‐donation, for which the metal AOs of the latter term come from the valence d AOs.8. The complexes are formally 18‐electron systems, but the occupied valence a2u MO of the cubic (hO) structures has a node at the metal atom, and thus, the complexes are actually 16‐electron systems. The degenerate eg HOMO is only doubly occupied, and therefore, the complexes M(CO)8  have a triplet ground state.A very surprising result was the recent observation by us that the heavy alkaline‐earth elements calcium, strontium, and barium can also bind eight CO ligands forming the eight‐coordinate neutral complexes M(CO)8 rather than heptacarbonyl complexes TM(CO)7 . Here we report a combined infrared spectroscopic and theoretical study on coordinatively saturated Group 4 metal carbonyl complexes prepared in the gas phase and/or solid neon matrix. With respect to coordinative saturation of transition metals, we refer to the 18‐electron rule. The results show that although titanium only forms the Ti(CO)6+ and Ti(CO)7 complexes, both Zr and Hf form eight‐coordinate carbonyl neutral and positively charged adducts. The first experimental observation of the coordinatively saturated Group 4 carbonyl complexes closes a gap in the field of homoleptic transition‐metal complexes.With the above knowledge, we re‐investigated the question of the stability of the coordinatively saturated Group 4 carbonyl complexes, which may be realized in octacarbonyl complexes TM(CO)−1 in the range of 1500–2200 cm−1. The wavenumber of the OPO laser was calibrated by using the CO absorptions. When the infrared laser was on resonance with a vibrational fundamental of the ion complex, absorption took place, which resulted in the dissociation of the complex. The fragment and parent ions were reaccelerated and mass analyzed by the second TOFMS. The photodissociation spectrum was obtained by monitoring the yield of the fragment ion as a function of the dissociation IR laser wavelength and normalizing to the parent ion signal. Typical spectra were recorded by scanning the dissociation laser in steps of 2 cm−1 and averaging over 300 laser shots at each wavelength.The Group 4 metal carbonyl cation complexes were generated in the gas phase by using a pulsed laser vaporization/supersonic‐expansion source and were studied by infrared photodissociation spectroscopy.−1 by using a Bruker Vertex 80 V spectrometer at 0.5 cm−1 resolution. A liquid‐nitrogen‐cooled broad‐band HgCdTe (MCT) detector was used. Bare window backgrounds, recorded prior to sample deposition, were used as references in processing the sample spectra. After the infrared spectrum of the initial deposition had been recorded, the samples were warmed up to the desired temperature and quickly re‐cooled and more spectra were taken. Alternatively, for some experiments, broad‐band photoexcitation was performed by using a high‐pressure mercury arc lamp with glass filters. The CO/Ne mixture was prepared in a stainless‐steel vacuum line by using standard manometric techniques. CO  and isotopically labelled 13C16O  and 12C18O  were used without further purification.The Group 4 metal carbonyl neutral complexes were prepared by the reactions of metal atoms and carbon monoxide in solid neon and were investigated by using Fourier transform infrared absorption spectroscopy as described in detail previously.The quantum chemical calculations using density functional theory (DFT) were carried out with the M06 functional developed by Truhlar and ZhaoΕint) between two fragments is decomposed into three energy components →(CO)8 π back‐donation followed by [TM(d)]←(CO)8 σ donation d AOs. Figure ρ(1)–Δρ(5) of Zr(CO)8, which are associated with the most important pairwise orbital interactions ΔEorb(1)–ΔEorb(5) given in Table ρ(1)–Δρ(5) of Ti(CO)8 and Hf(CO)8 look very similar and are shown in Figures S14 and S15 (Supporting Information). The color code of the charge migration is red→blue. Only one component of the degenerate orbital interactions is shown. The shape of Δρ(5) shows that the stabilizing polarization of the (CO)8 cage involves a charge migration from oxygen to carbon.The numerical EDA‐NOCV results of the TM(CO)on Table . The twoCv3) field of seven CO ligands in TM(CO)7 and the interactions with the occupied σ and vacant π* valence MOs of (CO)7. Considering the lower symmetry of the heptacarbonyl complex, all of the occupied σ valence MOs of (CO)7 can donate electronic charge to the metal, satisfying the 18‐electron rule in TM(CO)7. The shape of the occupied valence orbitals of Ti(CO)7 displayed in Figure n)s, (n−1)d, and (n)p AOs, which split into a1 and e orbitals in the Cv3 field could mix into all occupied a1 and e valence orbitals of the complex. Inspection of the metal AO coefficients suggest that the 1a1 MO of the complex has mainly contributions from the (n)s AO, the 2a1 MO has mainly contributions from the (n)p AO, and the 3a1 MO has mainly contributions from the (n−1)d AO. The 1e MO of the complex has mainly contributions from the (n)p AO, the 2e MO has mainly contributions from the (n−1)d AO, and the 3e MO has contributions from the (n−1)d and (n)p AOs. The latter AOs serve as polarization of the occupied (n−1)d AOs.Figure 7, which provide a quantitative account of the orbital interactions that are qualitatively sketched in Figure 7 π back‐donation followed by [TM(d)]←(CO)7 σ donation, which has two (2e and 3a1) components. The overall metal–ligand bonding situation in the heptacarbonyl complexes is thus very similar to those in the octacarbonyl complexes. However, why is then TM(CO)8 favored over TM(CO)7? A comparison of the EDA‐NOCV results for the two sets of complexes in Tables n attraction ΔEint of the octacarbonyl complexes is (as expected) stronger than that in the heptacarbonyl complexes. However, the attractive components ΔEelstat and ΔEorb in TM(CO)7 are much stronger than that in TM(CO)8. The covalent bonding and the electrostatic attraction in the octacarbonyl complexes are weaker than that in the heptacarbonyl complexes, which agrees with the shorter TM−CO bonds in the latter, but the octacarbonyl complexes are yet the energetically more favored complexes. This is because the destabilizing component of the repulsive Pauli term ΔEPauli is much higher in TM(CO)7 than in TM(CO)8 (Tables Cv3 structures of TM(CO)7 as a result of the shorter TM−(CO)7 bonds, which overcompensates the stronger attraction compared with TM(CO)8. The role of Pauli repulsion is often neglected in the discussion about bond strength, which usually only considers covalent and electrostatic (ionic) attraction. It has been shown that Pauli repulsion prevents the maximum overlap of the bonding orbitals at the equilibrium bond distance2.ρ(1)–(6), which are associated with the pairwise orbital interactions ΔEorb(1)–(6) in TM(CO)7, are shown in Figure 7) and in Figures S16 and S17 (Supporting Information) (for Ti(CO)7 and Hf(CO)7).Table  has two e and 3a1 has two e and 3a18 is unstable for the loss of one CO. EDA‐NOCV calculations using TM(CO)7+CO as interacting fragments provide an answer. Table Eelstat and ΔEorb in Ti(CO)8 are stronger than that in their heavier homologues. However, the Pauli repulsion ΔEPauli in the titanium octacarbonyl is significantly larger, which makes the intrinsic interaction energy ΔEint in Ti(CO)8 weaker than that in the heavier octacarbonyl complexes, albeit it is still attractive. What makes Ti(CO)8 energetically unstable for the loss of one CO is the assembling preparation energy ΔEprep, which is clearly larger than that for Zr(CO)8 and Hf(CO)8 )8 Table . Thus, T7 and TM(CO)8  appear as coordinatively saturated Group 4 carbonyl complexes. The EDA‐NOCV results of the cations shall only be shortly discussed. Table 8+ using TM+ and (CO)8 as interacting fragments. The [TM(d)]+→(CO)8 π back‐donation has two components from orbitals having a1g and b2g symmetry as a result of the lower Dh4 symmetry. The contribution of the π back‐donation in the cations is (as expected) smaller than that in the neutral species (Table Eorb. The [TM(d)]+←(CO)8 σ donation also has two components (eg and b1g), which contribute 30 %–35 % to ΔEorb. The results for Zr(CO)8+ must be taken with some caution because the self‐consistent field (SCF) calculation of Zr+ did not fully converge to the (n)s0(n)p0(n−1)d3 valence electronic configuration but showed a hybrid orbital of 70 % dz2and 30 % s character. The EDA‐NOCV results for TM(CO)6+ shown in Table hO) symmetry of the hexacarbonyl cation. The [TM(d)]+→(CO)6 π back‐donation and [TM(d)]+←(CO)6 σ donation have about the same strength in the three systems, providing about 36 % each to the total orbital interactions ΔEorb. Thus, the most important valence orbital of the metal atoms is the (n−1)d AO also in the cations. The deformation densities of TM(CO)8+ and TM(CO)6+ are shown in Figures S18–S23 (Supporting Information).The bonding analysis of the neutral complexes is presented and discussed in detail to explain the peculiar observation that Ti(CO)8 have cubic (hO) geometries, whereas the cations TM(CO)8+ have Dh4 structures. In contrast, titanium only forms the stable six‐coordinate cation complex Ti(CO)6+ and seven‐coordinate neutral complex Ti(CO)7. Titanium octacarbonyl Ti(CO)8 with hO symmetry is an energy minimum, but it is unstable with regard to loss of one CO ligand due to steric repulsion between the CO ligands. The Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals with 20 valence electrons. The molecules still fulfill the 18‐electron rule, because one doubly occupied valence orbital has a node at the metal atom, and thus, it does not mix with any of the metal valence AOs. A detailed bonding analysis by using the sophisticated EDA‐NOCV approach suggests that the major contribution of the covalent metal−CO bonding in the neutral complexes and in the cations comes from the [TM(d)]→(CO)8 π back‐donation. The most important valence orbitals of the metals are the d orbitals with the s and p functions playing an equally minor role. The bonding analysis shows that Zr(CO)8 and Hf(CO)8 are stable for loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7. The heptacarbonyl complexes have shorter metal−CO bonds than that of the octacarbonyl complexes as a result of stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes lower in energy.We report the first experimental observation of coordinatively saturated neutral and positively charged homoleptic Group 4 metal carbonyl complexes, which have been prepared in the gas phase and/or in solid neon matrix. Combined infrared photodissociation spectroscopy and matrix isolation infrared absorption spectroscopy studies reveal that both zirconium and hafnium form eight‐coordinate carbonyl neutral and cationic complexes. The neutral octacarbonyl complexes TM(CO)22+ is irrelevant for synthesis, but it is important to learn that the interference of the wave functions that give rise to covalent bonding can overcome the immense Coulomb repulsion of 200 kcal mol−1.The described complexes of this work appear exotic and may not attract the interest of chemists who are mainly interesting in “compounds in the bottle”. It is indeed unlikely that the Group 4 octacarbonyl complexes will find a wide application in synthesis and chemical technology. However, they increase our knowledge of the most important bonding modes of the Group 4 metal atoms and affect the 18‐electron rule, which belongs to the arsenal of elementary rules of chemistry that are fundamental to chemical research. The dication HeThe authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.


Two new heterobimetallic cages, a trigonal‐bipyramidal and a cubic one, were assembled from the same mononuclear metalloligand by adopting the molecular library approach, using iron(II) and palladium(II) building blocks. The ligand system was designed to readily assemble through subcomponent self‐assembly. It allowed the introduction of steric strain at the iron(II) centres, which stabilizes its paramagnetic high‐spin state. This steric strain was utilized to drive dynamic complex‐to‐complex transformations with both the metalloligand and heterobimetallic cages. Addition of sterically less crowded subcomponents as a chemical stimulus transformed all complexes to their previously reported low‐spin analogues. The metalloligand and bipyramid incorporated the new building block more readily than the cubic cage, probably because the geometric structure of the sterically crowded metalloligand favours the cube formation. Furthermore it was possible to provoke structural transformations upon addition of more favourable chelating ligands, converting the cubic structures into bipyramidal ones. Lively exchanges: Two new heterobimetallic cages with iron(II) cations in a sterically strained coordination sphere undergo complex‐to‐complex transformation. Subcomponent‐exchange reactions with sterically less crowded building blocks changed the spin state from high‐ to low‐spin iron(II), whereas ligand exchange with chelating ligands affected the structural transformation from cubic to bipyramidal cages. These complexes contain sterically strained iron(II) centres in the paramagnetic high‐spin state. We adopted previously reported subcomponent exchanges and expanded these principles to heterometallic systems. Chemical stimuli transferred the heterobimetallic paramagnetic complexes to their diamagnetic analogues upon reduction of steric strain. In addition, we could perform ligand‐exchange reactions, resulting in the structural transformation of cubic to bipyramidal cages.1 was selected to achieve the formation of C3‐symmetric iron(II) tris(pyridylimine) moietiesC2‐ and C4‐symmetric palladium(II) pyridine moieties. Reaction of 3 equivalents of aldehyde 1 with 1 equivalent of TREN 2 and 1 equivalent of iron(II) tetrafluoroborate hexahydrate in acetonitrile yielded the mononuclear complex ML‐1(BF4)2 as a bright red solid . Mixing 1 equivalent of ML‐1(BF4)2 with 1.5 equivalents of 1,3‐bis(diphenylphosphino)propane palladium(II) triflate ([(dppp)Pd(OTf)2]) or 0.75 equivalents of tetrakis(acetonitrile) palladium(II) tetrafluoroborate ([Pd(CH3CN)4](BF4)2) in acetonitrile resulted in the formation of heterometallic pentanuclear bipyramidal assembly BP‐1(OTf)6(BF4)4 or the tetradecanuclear cubic assembly CU‐1(BF4)28, respectively, each as a bright red microcrystalline solid in high yields (Scheme Ditopic building block BP‐1(OTf)6(BF4)4 were proven by 1H NMR and UV/Vis spectroscopy and ESI‐MS (see the Supporting Information). When vibrating sample magnetometry and the Evans’ methodΧmT=6.0 cm3 K mol−1 was revealed, which is in very good agreement with the expected value for two uncoupled iron(II) cations in the high‐spin state (6.001 cm3 K mol−1).BP‐1(OTf)6(BF4)4 in acetonitrile gave X‐ray diffraction quality single crystals that could be used to determine its crystal structure 6(BF4)4 crystallises in the monoclinic space group C2/c, with both homochiral enantiomers [ and ] present in the unit cell as a racemic mixture and is best described as a trigonal bipyramid or helicateThe identity and purity of bipyramid The iron cations are coordinated in an octahedral ligand sphere with an average Fe−N bond length of 2.179 Å, consistent with a paramagnetic high‐spin iron(II) complex.CU‐1(BF4)28 could also be proven by 1H NMR and UV/Vis spectroscopy and ESI‐MS (see the Supporting Information). An Evans’ experiment revealed a magnetic susceptibility of XmT=24.1 cm3 K mol−1, proving its fully paramagnetic character.CU‐1(BF4)28 was completed after only 16 hours at 50 °C, whereas the self‐assembly of a previously reported diamagnetic analogueCU‐1(BF4)28 to the larger opening angle of ML‐1(BF4)2, compared to the metalloligand without the additional methyl groups on the pyridine binding to the iron(II) ion. Due to the steric strain around the iron centre, the opening angle becomes larger, and therefore, should be better preorganized to form the heterobimetallic cube.The identity of the heterobimetallic cube CU‐1(BF4)28 over 3 months yielded single crystals, which diffracted up to 1.2 Å using synchrotron radiationVery slow evaporation of solvent from an acetonitrile solution of CU‐1(BF4)28 crystallises in the cubic space group Fm−3‾c with both homochiral enantiomers [all (Δ) and all (Λ)] present in the unit cell as a racemic mixture. Only one‐third of one ligand and one iron ion is found in the asymmetric unit due to the threefold symmetry axis. All three Fe−N bonds are crystallographically equivalent and show a bond length of 2.105(11) Å, which is consistent with a high‐spin iron(II) complex. The tetravalent palladium ions with their four coordinated pyridine moieties are located on the fourfold symmetry axes of the cubic cage. Eight tetrafluoroborate anions occupy the void of the cage, adapting the cubic arrangement themselves.ML‐1, BP‐1, and CU‐1 results in an elongation of the Fe−N bond lengths and the stabilization of the paramagnetic high‐spin state.3  to the complex solutions and were monitored by 1H NMR spectroscopy. In such a scenario a quantitative incorporation of 3 would lead to a 50:50 or 1:1 ratio of both free aldehydes 1 and 3. Please note that we had to analyse the signals of the individual aldehyde subcomponents as probes here as integration of 1H NMR signals of paramagnetic species is generally not possible, when mixed species with different magnetic properties are present.The steric strain in 3 to a solution of ML‐1(BF4)2 released free 3 equivalents of subcomponent 1 when 3 was incorporated almost quantitatively into the complex, resulting in the formation of previously reported diamagnetic metalloligand ML‐2(BF4)2 in high yield 2 (see the Supporting Information).Following this approach, addition of 6 equivalents of BP‐1(OTf)6(BF4)4 quantitatively into the corresponding diamagnetic cage BP‐2(OTf)6(BF4)4. After 16 hours at 40 °C the equilibrium ratio of 3 and 1 was determined to be 1:1 and 1H NMR signals arose that can be assigned to BP‐2(OTf)6(BF4)4 (see the Supporting Information). Also the formation of BP‐2(OTf)6(BF4)4 could be shown by UV/Vis spectroscopy (see the Supporting Information). This transformation impressively demonstrates the strong driving force of this subcomponent exchange and the highly dynamic behaviour of this heterobimetallic system. In the course of this exchange reaction, one covalent imine bond, two coordinative Fe−N bonds, and one coordinative Pd−N bond have to be broken, the new building block has to align and to reform all formerly cleaved bonds again for each exchanged subcomponent.The same chemical stimulus can be used to transfer paramagnetic CU‐1(BF4)28 with a greater excess of 100 equivalents of 3 resulted in the transformation to the analogous diamagnetic cube CU‐2(BF4)28. Unlike with ML‐1(BF4)2 and BP‐1(OTf)6(BF4)4, however, the subcomponent exchange required considerably more time. After three days at 65 °C an equilibrium ratio between 1 and 3 of 30:70 was determined, which is close to the expected 24:76 ratio. However, the use of only 48 equivalents of 3 did not lead to a complete transformation (see the Supporting Information). Therefore, these experiments show that incorporation of 3 into the cubic assembly is slightly more favoured than incorporation of 1, but less pronounced than in the case of the bipyramidal assemblies.Similar treatment of cubic cage ML‐1(BF4)2 and BP‐1(OTf)6(BF4)4. The larger opening angle of ML‐1(BF4)2 caused by the additional methyl groups seems to be a better preorganization for the assembly of cubic structures than the smaller opening angle in ML‐2(BF4)2. This assumption is corroborated by the much faster formation of CU‐1(BF4)28 compared to that of CU‐2(BF4)28, although it is difficult to dissect whether this is due to the kinetics or the thermodynamics of the assembly process.Hence, in case of the cubic cage the sterically stressed complex probably is not as disfavoured as for CU‐1 and CU‐2 leads to complex‐to‐complex transformations, yielding the bipyramidal complexes BP‐1 and BP‐2 and excess metalloligands ML‐1 and ML‐2, respectively  cations, the system also allows for structural conversions upon chemical stimulus, maintaining the initial spin state of the complexes (see the Supporting Information). The addition of dppp as a chelating ligand to cubic cages BP‐1 and CU‐1 from the same C3‐symmetric metalloligand ML‐1 adopting the well‐implemented molecular library approach. Compared to previously reported diamagnetic complexes ML‐2, BP‐2, and CU‐2In conclusion, we presented the stepwise assembly of two heterobimetallic cages ML‐1 and BP‐1 showed a very high tendency to undergo subcomponent‐exchange reactions, resulting in the first reported complex‐to‐complex transformations of heterobimetallic complexes to heterobimetallic complexes in which metal‐bridging components are exchanged. In this case, 2‐formyl‐6‐methyl‐5‐(4′‐pyridyl)‐pyridine 1 could be replaced by sterically less crowded 2‐formyl‐5‐(4′‐pyridyl)pyridine 3, transforming the high‐spin complexes into their low‐spin analogues.CU‐1, however, was found to exchange 1 much less readily, probably because the opening angle of the sterically crowded metalloligand makes it kinetically and thermodynamically favourable to assemble the cubic structure.Paramagnetic complexes CU‐1 and CU‐2 led to a ligand‐exchange reaction, in which two 4‐pyridyl donors around palladium(II) cations were replaced by the chelating ligand. These exchanges resulted in complex‐to‐complex transformations to the bipyramidal cages BP‐1 and BP‐2, respectively, maintaining the spin state of iron(II). Since the iron(II) to palladium(II) ratio decreases on going from the cubic to the bipyramidal cages, the corresponding free metalloligands ML‐1 and ML‐2 are formed as side products in these transformations. The presented complex‐to‐complex transformations beautifully show the highly dynamic behaviour of the heterobimetallic systems, allowing the change of magnetic or structural properties upon certain chemical stimuli.Addition of bidentate dppp to cubic assemblies The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.


Therefore, a fast and efficient protocol for imaging of KCa3.1 channels was envisaged. The novel fluorescently labeled small molecule imaging probes 1 and 2 were synthesized by connecting a dimethylpyrrole‐based BODIPY dye with a derivative of the KCa3.1 channel inhibitor senicapoc via linkers of different length. Patch‐clamp experiments revealed the inhibition of KCa3.1 channels by the probes confirming interaction with the channel. Both probes 1 and 2 were able to stain KCa3.1 channels in non‐small‐cell lung cancer (NSCLC) cells following a simple, fast and efficient protocol. Pre‐incubation with unlabeled senicapoc removed the punctate staining pattern showing the specificity of the new probes 1 and 2. Staining of the channel with the fluorescently labeled senicapoc derivatives 1 or 2 or with antibody‐based indirect immunofluorescence yielded identical or very similar densities of stained KCa3.1 channels. However, co‐staining using both methods did not lead to the expected overlapping punctate staining pattern. This observation was explained by docking studies showing that the antibody used for indirect immunofluorescence and the probes 1 and 2 label different channel populations. Whereas the antibody binds at the closed channel conformation, the probes 1 and 2 bind within the open channel.The Ca Open vs. closed: The KCa3.1 channel is involved in critical steps of the metastatic cascade. Analyzing KCa3.1 channel expression has predictive power with respect to patient survival. Novel fluorescently labeled small‐molecule imaging probes were able to stain KCa3.1 channels in NSCLC cells selectively and efficiently. Co‐staining using antibody‐based indirect immunofluorescence did not lead to the expected overlapping punctate staining pattern. This observation was explained by docking studies showing that the antibody and the novel imaging probes label different channel populations, closed and open channels, respectively. The BODIPY‐labeled senicapoc derivatives 1 and 2 showed the punctate staining pattern of the KCa3.1 channel in NSCLC cells after only 10 min incubation time.  between the targeting component and the BODIPY dye. >5 pixels and can therefore not be interpreted as KCa3.1 channels. Changing the permeabilization to a saponine solution led again to the typical punctate staining pattern of the KCa3.1 channel with FWHM≤5 pixels after incubation of A549‐3R cells with a 10 μM staining solution of 1 for 10 min. , only the staining of the antibody‐based indirect immunofluorescence was observed with the tetramethylrhodamine isothiocyanate (TRITC) filter set. Signals were not found after switching to the fluorescein isothiocyanate (FITC) filter set (BODIPY‐labeled senicapoc derivatives 1 and 2). However, extending the incubation time for the BODIPY‐labeled ligand 1 to 20 min  was analyzed by counting the signals in ten squares with a side length of 50 pixels, corresponding to an area of 9 μm2, which were randomly placed in a given cell. When a signal was seen in both fluorescence channels, colocalization of the signals required that the x and y coordinates of the “red” and “green” maxima did not differ by more than one pixel.The density of the K1 and 2 towards the KCa3.1 channel  was found. The staining pattern changed remarkably to larger, round signals with FWHM>5 pixels, which were not identified as KCa3.1 channels. SDS (4) is a lipophilic and ionic agent, which is able to form micelles. We assume that the lipophilic BODIPY‐dye part of the small molecule 1 can be embedded into these micelles, which are then observed as bright, round signals.  led to signals using the TRITC filter set (showing antibody‐based staining), but no signals were observed in the FITC filter set displaying 1 or 2. However, signals were observed using both filter sets after extending the incubation time with 1 or 2 to 20 min (protocol 4B). For this reason, the staining order was changed: After permeabilization, A549‐3R cells were incubated with the senicapoc derivatives 1 or 2 first and then with the antibodies for the antibody‐based indirect immunofluorescence (protocol 5). Signals were observed for both filter sets. The signals received from protocols 4B and 5 were analyzed with a line scan showing the intensity profile of the signals in x‐ and y‐direction using the MetaVue software. Signals were counted as colocalized when the difference of the maxima of two signals from the TRITC and the FITC filter set was ≤1 pixel1 or 2 or antibody‐based indirect immunofluorescence only. This was not the case: Summing up all signals/μm2 (only red+only green+yellow dots) gave similar total numbers as obtained by staining with BODIPY‐labeled senicapoc derivatives 1 or 2 (protocol 1) or antibody‐based indirect immunofluorescence only as we had shown previously.1 or 2 can bind to one KCa3.1 channel protein.Co‐staining was performed in order to show colocalization of the signals obtained by indirect immunofluorescence and the BODIPY‐labeled senicapoc derivatives 1 and 2 bind to the inner pore in the open state of the ion channel as in the calmodulin‐bound open state  of the KCa3.1 channel at saturating Ca2+ concentration is 0.44.1 and 2 cannot bind to the channel.It can be expected that the antibody binds to the closed conformation, since the inactive state is the more often occurring state. The maximal open probability .These different conformations also allow interpreting the results of the co‐staining experiments. The cells were washed and fixed before staining, which may lead to an activation of the ion channels during the initial fixation process  were obtained by diluting the stock solutions with phosphate buffered saline  in a microcentrifuge tube (Eppendorf®) and dissolving the precipitate by using a Vortex (Scientific®) for 5 min. For the indirect immunofluorescence assay we used normal goat serum  und NaCl  in AmpuwaTM), anti‐KCa3.1 antibody produced in rabbit  and Cy3‐conjugated goat anti‐rabbit IgG secondary antibodies.Cell culture. We used the A549‐3R cell line, a hypotriploid epithelial cell line from a non‐small‐cell lung cancer (NSCLC) taken from a 58‐year‐old Caucasian in 1972. The suffix “3R” refers to the repeated process of intravenous administration of parental cells  into immunocompromised mice to obtain lung metastases. Cells from these metastases were isolated and cultured in vitro followed by another round of reinjection to select for tumor cells with high metastatic potential.Ca3.1 channels and thus are suitable for corresponding staining experiments.2) in cell culture dishes (Ø=10 cm). For experiments, cover slips were coated with 0.1 % poly‐L‐lysine  and washed with Dulbecco's Phosphate Buffered Saline (PBS). One cover slip was placed in each well of a 12‐well‐plate. DMEM (1 mL) was added to each well and approximately 100.000 cells were added. The cells were incubated overnight at 37 °C and 5 % CO2 prior to the experiments.Staining protocols. For each experiment, the medium was removed and the cells were washed three times with PBS and fixed with 3.5 % paraformaldehyde (PFA) in PBS for 30 min at room temperature. Then the cells were washed three times with PBS at room temperature and kept for 10 min in PBS supplemented with 100 mmol/L glycine and washed again three times. Thereafter, the staining protocols were performed:1 or 2 (10 μM in PBS) was pipetted onto Parafilm. The cover slip with the adherent A549‐3R cells was placed carefully upside down onto the drop and the NSCLC cells were incubated in a humidified dark chamber for 10 min. After washing five times with PBS, the cover slip was placed upside down onto a microscope slide.Protocol 1: A drop (30 μL) of the staining solution of Protocol 2: The cells were incubated with Triton X‐100 (0.25 % Octoxinol 9 in 1 % SDS/PBS) in PBS for 10 min at room temperature. After washing five times with PBS, staining protocol 1 was performed as described above.Protocol 3: The cells were incubated with saponine solution (0.5 % in PBS) for 20 min at room temperature. After washing five times with PBS, staining protocol 1 was performed as mentioned above.Ca3.1 antibody) on Parafilm and incubated for 2 h. The cells were then washed five times with PBS and incubated with Cy3‐conjugated secondary antibodies in normal goat serum  for 1 h at room temperature. At the same time, negative controls were incubated with only Cy3‐conjugated secondary antibodies . After washing five times with PBS, the cells were fixed with 3.5 % PFA in PBS for 10 min at room temperature and washed three times with PBS. Thereafter, staining protocol 1 was performed with the same coverslips as detailed above .Protocol 4: The cells were incubated with saponine solution (0.5 % in PBS) for 20 min at room temperature. After washing five times with PBS, the cover slip was carefully placed upside down onto a 30 μL drop of normal goat serum on Paraflim and incubated in a moist, dark chamber for 30 min to block unspecific binding of the antibodies. Thereafter, the cover slips were placed upside down on a 30 μL drop of primary antibodies in normal goat serum  for 20 min at room temperature. After washing five times with PBS, staining protocol 1 was performed as detailed above. Thereafter, the antibody‐based indirect immunofluorescence staining was performed as described in the preceding protocol 4. After the final fixation with 3.5 % PFA in PBS for 10 min at room temperature and washing three times with PBS, the cover slip was placed upside down onto an object slide for microscopy.Microscopy. Microscopy was performed with an inverted microscope  equipped with a x100 oil immersion objective and a digital camera (SPOT RT SE from Diagnostic Instruments Inc.). Filters were used to irradiate fluorescent dyes with light according to their absorption maxima and to filter adequate wavelengths of emitted light. The following filter sets were used: FITC  and TRITC . Signals detected in the FITC and TRITC channels are denoted as “green” and “red” fluorescence, respectively. The senicapoc‐based staining with imaging probes 1 and 2 was recorded in the FITC (green) channel and the antibody‐based staining in the TRITC (red) channel. Data acquisition and analysis were performed with Metavue software . Signals with a Full Width at Half Maximum ≤5 pixels (∼300 nm) were regarded as a single KCa3.1 channel.1 or 2 and the indirect immunofluorescence, ten squares with a side length of 50 pixels, corresponding to an area of 9 μm2, were randomly scanned in a given cell. When a signal was seen in both fluorescence channels, colocalization of the signals required that the x and y coordinates of the “red” and “green” maxima did not differ by more than one pixel.For the evaluation of colocalization of the signals of BODIPY‐labeled senicapoc derivatives Patch‐clamp experiments. A549‐3R cells were used for the experiments. Whole cell recordings were performed at room temperature using borosilicate glass pipettes  connected to an EPC‐10 amplifier . The typical electrode resistance was 3–4 MΩ, while series resistance was in the range of 5–15 MΩ. Series resistance compensation of >30 % was routinely used. Voltage‐clamp experiments on cultured cells were controlled by PatchMaster software . Current density was calculated by dividing the current amplitude determined at the end of the depolarizing voltage ramp to +60 mV by the membrane capacitance obtained from slow capacitance compensation. The following recording solutions were used: 1) Extracellular solution (mM): 140 NaCl, 5 KCl, 10 4‐(2‐hydroxyethyl)‐1‐piperazineethanesulfonic acid (HEPES), 1 MgCl2, and 1 CaCl2, pH=7.4 with NaOH. 2) Intracellular solution (mM): 140 KCl, 10 HEPES, 1.3 ethylene glycol‐bis(β‐aminoethyl ether)‐N,N,N′,N′‐tetraacetic acid (EGTA), 1.217 CaCl2, and 1 MgCl2, pH=7.4 with KOH. The calculated free Ca2+ concentration of the internal solution was 1 μM in order to obtain full activation of the KCa3.1 channels during the patch clamp experiments. 1‐EBIO (50 μM), 1 (10 μM), 2 (10 μM) and senicapoc (1 μM) were dissolved in DMSO and added to the standard extracellular solution and applied in this order for a period of 2–5 min. The final DMSO concentration was ≤0.15 %. A multibarrel application pipette with a tip diameter of about 100 μm was used for test substance application close to the recorded cell. Recordings were analyzed using FitMaster and Excel software.Statistical analysis. Data are given as mean±SEM. ANOVA was performed followed by t‐testing for paired experiments. The analysis was done using Excel software. Statistical difference was determined with the t‐test (p<0.05).The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.


RhCl3 was used in 0.2 wt% of Rh quantity on the gel surfaces, which were subsequently exposed to UV irradiation in water under a 355-W UV lamp. λ > 370 nm. Both Rh [I] and Rh [III] surface complexes were formed, depending on the gels composition. They also exhibited various efficiency in photo water splitting reaction, the photocatalysts possessing as supports titania-silica gels containing 40 and 50 mol % of TiO2, were the most effective. According to our proposal, the rhodium complexes took part in the oxidative addition and reductive elimination cycle, where two water molecules in photo water splitting reaction, yielding hydrogen – H2 and two hydroxyl radicals. The two hydroxyls radicals, catalytically converted to oxygen atom and water, in the second cycle of the investigated water splitting reaction. Titanium octahedra and silicon tetrahedra linked by Ti–O–Si bridges were thought to be the grouping responsible for activity of the investigated catalysts. Their largest amounts are on the gels containing 40–58 mol % TiO2.Titania silicates (TiO Titanium dioxide is well known as photocatalyst active in many processes e.g.: the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion –4 but alIts activity in these processes, is enhanced while determined metal or their oxide are deposed on its surface –12.2 photocatalysts activation by metals, the systems Ag–TIO2 possessing various amount of silver, may be  ions.In both cases, the greatest quantity is encountered when the gels contain 40 and 50% TiO−1 band characteristic for the isolated silanol group ≡Si–OH  Rh (OH)3 3H2O  for the following mixed oxides: Ca2Nb3O10 (−35), KNb6O17 (−37), Rb0.TaO3 (−32), and for γ-Al2O3 and SiO2 these enthalpies have positive values of 55 and 25, respectively. Negative enthalpies indicate fairly strong Rh (OH)3 carriers. However, this hydroxide deposited on aluminum oxide is stable to the degree allowing to be used as a catalyst in the transformation of aldehydes to primary amides reaction  ions was detected on them by ESCA method  3 and Rh2Cl2 C2H4) 4 complexes, described in the literature , 33 were 3 and Rh 4 comple3 3H2O must be the source of hydrogen and oxygen in this reaction. Two chlorines from two HCl molecules must originate from RhCl3 molecule, and two electrons to reduce Rh [III] to Rh [I] from Rh-Cl bonds. These effects should result from the following conversions.The water molecule from the co-ordination sphere RhCl3 and converts the complexed olefin to an aldehyde. Both products are no longer characterized by such complexing properties as phosphines and olefins and leave the co-ordination sphere of rhodium ions. These released co-ordination sites are filled by “stoichiometric” phosphines or olefins.The phosphine or ethylene molecules, as strong complexing agents, displace water molecule from the co-ordination sphere of rhodium [III]. The complexes with the possibility of forming “back donation” effect , 35, whe2O ligands, which do not bring about such effects, fail do reduce Rh [III] ions to Rh [I] ions.These conversions are specific due to the ligands forming the “back donation” effect, as for instance, H3 applied onto investigated gels: SiO2 and 10–90% Ti carriers with Ti–O–Si bonds, in an aqueous solution during irradiation with UV rays leading to the trihydroxyrhodium product [III], the Rh ion [III] undergoes reduction to Rh ion [I]. This reaction was running in the absence of organic compounds.On the other hand, in RhClThus, the reduction of Rh [III] ions needs to run only through the homolytic cleavage of two Rh-OH bonds, yielding Rh [I] ion and two hydroxyl radicals:2 complexes, where X = Cl.NH2 and OCH3, the determined bond lengths and angles between them differed slightly, depending on the X ligands, however were contained in the following ranges: Rh-O ~ 2.1 Å bonds, Rh-Rh bond length = 2.8 ÷ 3.3 Å, and X –Rh –X = 86° ÷ 90° for angles, Rh-H2O distance ~ 2.2 Å [For homogeneous homologues of rhodium [I] complex obtained, dimeric [X (cyclooctadiene) rhodium (I)] ~ 2.2 Å –38.In the investigated carriers, the titanium oxide phases have octahedral anatase structures, where the Ti–O bond length is ~ 2.0 Å, O–O equatorial =  ~ 2.8 Å . In the These bond and angular lengths should appear on the obtained rhodium [III] and rhodium [I] surface complexes.Heating of the investigated titanium-silica gels at 823 K aimed at stabilizing the oxide structures of mixed gels and silica gel, titanium oxide.Under these conditions, the titanium oxide is primarily converted to anatase and to rutile, in lower quantities. In these structures, truncated oxide octahedrons appear on their surface, where four oxygen anions are located at equatorial positions, with in-between detected titanium ion IV]. These octahedrons are interlinked through their edges.2 and isolated ≡Si–OH  will appear on their surface .6. The rhodium ion [I] in complex compounds most often adopt the structures of a flat square by binding four ligands with π i σ bonds. The electron configuration of 4d-rhodium in these complexes is d8 [The rhodium ions [III] in complex compounds most often adopt the octahedron structures by binding six ligands with π and σ bonds. The electron configuration of 4d-rhodium in these complexes is des is d8 .3 on TiO2 phases, these hydroxides must be complexed with three non-bonding electron pairs on the surface oxygen orbitals to form octahedral structure. In the case of Rh [I] OH complex, it must be co-ordinated by three non-bonding electron pairs on the surface oxygen orbitals to form a flat square structure.In investigated systems, Rh [III] (OH)These data were employed for proposing the following models of the obtained surface rhodium complexes-Fig. 3/anatase system in the octahedral rhodium complex, two TiO2 oxygen anions and two trihydroxy rhodium trihydroxy anions appear in equatorial positions, whereas one TiO2 oxygen anion and one rhodium trihydroxy hydroxyl anion in axial positions.In the model Rh(OH)2, one oxygen anion of the hydroxyl group bound with silicon ion and one anion of the hydroxyl group of the trihydroxy rhodium appear in equatorial positions. The remaining two hydroxyl groups of this hydroxide are located in axial positions.In octahedral rhodium complexes deposited on the surfaces of titania-silica 50% Ti carriers, two oxygen anions of TiO6-electron configuration in the complex of octahedron structure and when all electron spins are paired in this system (low spin configuration), the Jahn–Teller effect is taking place. In such a complex, if the ligands are of a donor character, then the lengths of metal-axial ligand bond will be elongated, and such deformation is trigonal. This results in splitting of the eg orbital with two electrons located on the lower dz2 orbital.The rhodium ion [III] has the dInvestigated rhodium [III] trihydroxy surface complexes complexed with three surface donor oxygen of the carrier should be in such situation model b, Fig. 2 ions are in equatorial positions; one donor oxygen anion of the hydroxyl group bound with silicon ion, and one hydroxyl anion.In the model complex of rhodium [I] c-structure, two oxygen donor TiO4 flat square structure  complexes of MLtructure , 35, 42,Such a situation should take place in the resulting surface rhodium hydroxyl complexes [I].In all model rhodium complexes, hydroxyl groups are bonded with rhodium ions by σ-bonds, whereas the remaining bonds are coordinating ones.3/anatase system, the equatorial Rh-O bonds and Rh-O axial bonds, due to the tight crystal lattice of the carrier, will have difficulties in elongation, as ensured by Jahn–Teller effect  as well as the proposed structures of investigated rhodium [III] and rhodium [I] surface complexes: a, b, and c; the specific surface areas of the carriers and the Avogadro number, it was possible to perform calculation of surface areas occupied by examined complexes on relevant carriers.2: for two truncated octahedrons on TiO2.The calculated surface areas of rhodium [III] complexes amount to ~ 30 Å2 and SiO2 tetrahedron, the surface area of rhodium [III] complexes amounts to ~ 25 Å2.For truncated TiO3 applied per gram of carrier = 6 10–6 mol/g and on the selected specific surface areas of the carrier was used for calculating the surface concentrations of rhodium complexes. For the Rh50% TiO2, this concentration amounted to 0.14 mol%, for SiO2–0.09 mol%, whereas for TiO2 = 2 mol%.The amount of RhClThis indicates that the carrier surface constitutes the dominant area of the photocatalysts obtained and its properties must add to the properties of rhodium centers.The study of photolytic water splitting reaction against the examined photocatalysis yielded the following results, as presented in Table The obtained quantity of hydrogen in the examined reactions demonstrate that the rhodium [I] complexes deposited on the surface of amorphous silica gel exhibit a relatively poor activity.2 carrier itself demonstrated no activity, whereas the anatase – only a slight amounts.The rhodium [III] complexes deposited on the anatase surface exhibit a comparable low activity. The SiO2, the dominant phase belongs to amorphous silica gel, which barely activates rhodium complexes, however a slight increase in hydrogen yield is attributable to the increasing number of Ti–O–Si bonds.In the carriers containing from 10 to 30 mol% TiO2 carriers, the titanium-silica phase is dominant, which is made up of TiO2 octahedrons and SiO2 tetrahedra linked by Ti–O–Si bridges. These systems activate rhodium [I] complexes fairly strongly, which are deposited on these carriers, on the 50% TiO2 carrier in particular.In turn, in the 40–50% TiO2, increasing quantites of anatase crystals inactivating the rhodium [III] complexes are introduced to the titanium silica phase, however the remaining surface areas of the titanium-silica phases ensure a relatively high activity of investigated photocatalysts, although they decrease with the increasing amount of TiO2 in the carriers.In the carriers containing 60–90% TiO2 carrier, the most effective photocatalytic system. We obtained 0.20 cm3 of hydrogen.As mentioned earlier, the rhodium [I] complexes occupy a tiny part of the carrier surface areas and the possible catalytic properties of a given carrier may add to the activity of the rhodium centers. To verify this, water splitting studies were carried out in standard conditions in the presence of the 50% TiO3/g cat., 24 h) were obtained by testing the photolytic water splitting reaction on mixed titania-silica gels.Slightly higher yields ) are transferred onto antibonding molecular orbitals: 2b2*, two complexed water molecules, this is facilitated by oxygen donor ligands of the carrier, rhodium [III] being already a rhodium ion.– Two electrons from the 4d-molecular orbital of the rhodium ion [I]: b2O molecules, yielding two hydrogen atoms and two OH− groups:– The interorbital reaction of electrons with protons in the OH groups of complexed H2:– Dimerization of two hydrogen atoms to H− groups become incorporated to Rh [III] already formed.– Two OHThese intermediate stages are related to the reconfiguration of molecular orbitals of rhodium and water molecules.In the case of the rhodium complex [I] studied, which is a flat square, the molecular orbital diagram for such complexes is as follows :The bonding atomic (n + 1)s metal orbitals and σ- ligand bonds form the lowest orbitals having a comparable energy, while the molecular orbitals from the binding (n + 1)p metal orbitals (n + 1)p and π ligands also have the same energy, however they are already granted the increased energy  and to one hydroxyl group (σ-bonding):Electron d-orbitals configuration and their mutual energy values of the investigated surface L4Rh [I]: 4d8 dzx2 = dzy2 < dz22 < dxy2 ≪ dx2 – y2o .L2 6p6 .5sL = σ-ligands OH or π O surface ligands.4Rh [I] + exposition to UV light – transfer of 2 elektrons from dxy onto d x2- y2  + UV: 4d8-dzx2 = dzy2 < dz22 < dxyo ≪dx2 – y22.L4Rh [I] + UV* and deletion of oxygen carrier ligand.Electron configuration L2O to L3Rh [I] with no effect on rhodium ion oxidation.Bonding of H3Rh*[III] L3Rh*[III] + UV: 4d8-dzx2 = dzy2 < dz22 < dxy2 ≪ dx2 – y22+Transfer of two electrons from rhodium ion [I] on two water particles Rh [III] L3Rh [III] (OH)3 + H2, .Formation of octahedral complex L3Rh*[III] + UV: 4d8-dzx2 = dzy2 < dz22 < dxy2 ≪ dx2 – y22+L3Rh[III](OH)3 complex through conversion in the co-ordination sphere of the Rh[III] complex: transfer of the OH− group from the 6p-orbital to the vacant 4 dx2-y22+ orbital, resulting in a new occupation of this orbital 4 dx2-y21+(OH) and the restoration of the 6p2 –O oxygen ligand carrier, transfer of the electron from 4dxy2(OH−) orbital to the 4 dx2-y21+(OH) orbital yielding 4 dx2-y22 (OH) and 4dxy2(OH), rearrangement of the orbitals position in terms of energy from dz22 < dxy2 (OH) to dz2(OH) > dxy2, due to occupation requirements of d-orbitals d in the octahedron structure:The formation of the final octahedral L3Rh*[III]: 4d8- dzx2 -dxy2- dzy2 < dz22- dx2 – y22.Lz22 and dx2 – y22 orbitals occupy axial positions, whilst the lower energy orbitals: dzx2-dxy2-dzy2 occupy equatorial positions.Thus, the resulting rhodium [III] complex has a typical octahedral structure where dIt should be noted that this stage is formally corresponding to the reaction of water with metals of redox potential above zero. The example of such a reaction is the reaction of metallic sodium with water leading to the release of hydrogen and yielding appropriate hydroxide. In order to achieve the structure of neon, the sodium atom releases an electron, which needs to shift to water orbitals and evoke conversions in this reaction, as presented in reactions n oxidation state bound two water molecules while releasing a hydrogen molecule to form a complex with the M2+(OH)2 -group (oxidative addition stage), then in the reductive elimination hydrogen peroxide –H2O2 would be produced when exposed to UV light, with the restoration of the original complex with the –Mn ion.As regards the study of organometallic complexes with hydroxyl ligands, Piers W.  noted thIn our case, there are three hydroxyl groups in the trihydroxy rhodium [III] obtained in the oxidative addition step, and hence these two can take part in the postulated reductive elimination.3 obtained is a stable molecule and Rh [III] ion must be activated to the Rh*[III] ion for catalytic conversion. It occurs immediately, as the catalytic system is continuously exposed to UV radiation. The transfer of electrons to Rh*[III] cation from two hydroxyl ligands formed in the oxidative addition step needs to take place through the known homolytic cleavage of σ- metal–ligand  bonds, i.e. homolytic cleavage of two Rh*(OH) bonds resulting in the initial rhodium complex [I] and 2 hydroxyl radicals, in the examined case—reaction 12.The formation of hydrogen peroxide in the investigated reaction should take place through the following routes. The Rh (OH)z2 and 4dx2 – y2 orbitals, because, as previously described, octahedral complexes of transition metal with d6 electron configuration are subjected to the Jahn–Teller effect, i.e. elongation of the bonds the metal axial ligand and shortening of the bonds the metal-equatorial ligand bonds. This effect is facilitated by donor ligands as well as the energy contributed by UV radiation.This process should be applied to hydroxyl groups in the axial positions of dz2 and 4dx2 – y2 orbitals, one electron will remain i.e. 4 dz21 and 4dx2 – y21 and, for the final energy system of the orbitals to assume a plane square configuration, the electron from 4dx2 – y21 orbital will transfer to 4 dz2 orbital, which will assume 4 dz22 electron configuration and will lose the energy by positioning itself below the 4dxy orbital whose energy will increase, however the energy of vacant orbital 4dx2 – y2o will increase even more.After the departure of hydroxyl radicals on 4 dThese processes will be supported by donor ligands and UV radiation.2 was released from two molecules of complexed water on the rhodium ion [I], and at the same time this step constitutes the beginning of “oxygen” cycle, where atomic oxygen = O and water returning to the cycle, will be released from two hydroxyl radicals.This stage completes the “hydrogen” cycle with the interaction of rhodium ions that create Rh[I] Rh[III] Rh[I] cycle, where hydrogen molecule = HThe cycle runs, as follows: the first step invovles the dimerization of hydroxyl radicals to hydrogen peroxide, reaction 9 1/Ms, where the rate constant of the reverse reaction:The rate constant of this reaction during exposure to UV light is 5.2 × 10–5 1/Ms  complexes deposited on this carrier turn the most effective in the photolytic water splitting reaction. These properties are also linked to the presence of the Ti–O–Si grouping. As discussed previously, Ti [IV] ion exhibits a relatively strong Lewis acidity in that grouping.The carriers of tested photocatalysts, also with the same quantity of TiOThe “oxygen” cycle:2, leaves the reaction as a gas.This is irreversible, as its end products involve a thermodynamically stable water molecule and atomic oxygen which, after dimerization to OHydrogen peroxide exhibits Brønsted acidity , which mThe role of oxygen being released from the ionized peroxide molecule in the presence of titanium silica gel can be presented as follows: Diagram 2.+, while the electrons obtained by the adjacent oxygen from O–H bond release it as the atomic oxygen. At this stage, the complexed oxygen with the acid center of the carrier affords a negative charge which forms a water molecule by binding a liberated proton. Water is complexed by an acid center, however a partially ionized hydrogen peroxide molecule will push it away from this center. This launches another cycle of releasing atomic oxygen and water, which returns to the entire water splitting cycle.The ionized hydrogen peroxide molecule becomes complexed on the surface Ti [IV] ion having the said Lewis acidity, through the oxygen ion with a partial negative charge. This entails further ionization of the peroxide, yielding a proton -HThus, this pathway starting from hydroxyl radicals to atomic oxygen and water, and requiring the specific conditions as described, adds to the stoichiometry of the photolytic water splitting reaction on the catalysts under study.2, then the “hydroxyl radical conversion” cycle leads to atomic oxygen. In order to maintain a constant standard balance of water splitting reaction, both cycles must run at the same speed.If the “hydrogen” cycle yields hydrogen molecule- HThe obtained findings and literature data allow to formulate the following conclusions:2 carriers and UV radiation with a wavelength of ≥ 370 nm.Initiation of the run of both cycles needs the activation of stable rhodium [I] and rhodium [III] complexes through the interaction of 40–50 mol% TiO2 carriers the “hydrogen” cycle as the carrier of the rhodium complexes, the “oxygen” cycle - as the catalyst for the degradation of hydrogen peroxide. These carriers contain the highest number of Ti–O–Si bonds linking TiO2 octahedras with SiO2 tetrahedra;Furthermore, both cycles require the presence of 40–50 mol% TiOo strength, appearing only on the catalysts containing 40–50 mol% TiO2 ensure effective and selective degradation of hydrogen peroxide into oxygen and water.Lewis acid centers of H2 is the most active carrier of rhodium complexes in the investigated photolytic water splitting reaction;– Titanium-silica gel obtained, containing 40–50 mol% TiOo coefficient;– These gels exhibit the highest acidity determined by H3 applied to investigated gels using UV light of λ ≥ 370 nm, has led to the formation of rhodium hydroxy complexes of various degrees of oxidation: Rh [I] complexes appeared with SiO2 and gels 10–90 mol% TiO2 as carriers, while Rh [III] complexes were formed on gels of 60–100 mol% TiO2;– Photolytic splitting of RhCl– Rhodium complexes occupy only ≤ 0.02% of the surface of titania-silica carriers;– The photolytic water splitting reaction, according to our proposal, consisted of two cycles: in the first “hydrogen” cycle, running only on rhodium complexes, in line with a typical mechanism of oxidative addition and reductive elimination, a hydrogen molecule and two OH radicals are released and the initial active rhodium [I] complex is recreated. In the second cycle, the hydroxyl radicals become converted, in the presence of Lewis acid centers appearing on the surface of 40–50% Ti carriers, into atomic oxygen and water is returning to the circulation;2O → H2 + O reaction;– Both “hydrogen” and “oxygen” cycles must run at the same rate so as to maintain the stoichiometry of H– Photocatalytically active inorganic rhodium complexes and titania-silica gels as well as the aqueous environment of the reaction ensure the absence of organic compounds in the reaction environment and thus evoke the stoichiometric run of the photolytic water splitting reaction to hydrogen and oxygen - no reaction of hydroxyl radicals with organic compounds;2, and the energy contributed by UV light of λ ≥ 370 nm.– The presence of two types of rhodium and surface catalytic centers of the carrier was necessary to run photolytic water splitting reaction, as well as the specific addition of energy: interaction on rhodium ions by carriers: 40- and in particular, 50 mol% of TiO


Herein, we describe a simple approach to vary the pH through the partial pressure of CO2 (pCO2) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)‐(2×2)R45° surface begins to dissolve at a pH 4.0–3.9 (pCO2=0.8–1 bar) and, although it is significantly roughened, the atomic‐scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X‐ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO3−) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates.Difficulties associated with the integration of liquids into a UHV environment make surface‐science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV‐compatible dosing of ultrapure liquid water and studied its interaction with TiO Dissolution studies in UHV: A simple approach to vary the pH of an ultra‐pure water drop through the partial pressure of CO2 (pCO2) in the surrounding vacuum chamber is described and used to study how metal oxide surfaces react to immersion in acidic solutions at the atomic scale. Before introducing the sample to the cell, water vapour is frozen on a cold finger located above the sample holder, and after sample transfer under UHV conditions, the ice melts and falls onto the UHV‐prepared surface. We have shown in our prior work that metal oxide surfaces exposed to liquid water in this way exhibit minimal contamination when returned to UHV, and that TiO2(110) and Fe2O3(012) are largely unaffected by pure water.3O4(001), on the other hand, responds to liquid water exposure forming a new surface phase, which we interpreted as an Fe oxy‐hydroxide layer based on scanning tunnelling microscopy (STM) and X‐ray photoelectron spectroscopy (XPS) results.2 in the water drop during immersion. Specifically, we show that the Fe3O4(001) surface changes with decreasing pH, and begins to dissolve at pH≈4. On the other hand, the TiO2(110) surface remains unaffected by an acidic solution, as expected on the basis of the Ti Pourbaix diagram.Most of the existing theories of Earth‐abundant‐mineral degradation are based on kinetic measurements. For example, great progress towards a microscopic understanding of dissolution has been made using X‐ray reflectivity to measure the dissolution rate in real time,hat TiO21 and Fe2Othat TiO20 and Fe2pCO2).2+ ions are thought to be necessary for the dissolution reaction to proceed to completion,Besides serving as a proof of principle, our results are relevant for the study of chemical weathering of Fe oxides, which plays a substantial role in the regulation of the atmospheric partial pressure of carbon dioxide (3O4(001) single crystal (SurfaceNet GmbH), prepared in UHV by cycles of 1 keV Ar+ sputtering and 900 K annealing. Every other annealing cycle was performed in an O2 environment  to maintain the stoichiometry of the crystal selvedge. The surface analysis was performed in a UHV system with a base pressure <10−10 mbar, furnished with a commercial Omicron SPECTALEED rear‐view optics and an Omicron UHV STM‐1. XPS data were acquired using non‐monochromated Mg Kα x‐rays and a SPECS PHOIBOS 100 electron analyser at grazing emission . All STM images presented in this work were corrected for distortion and creep of the piezo scanner, as described in Ref. [22]The experiments were performed on a natural Fe2O, experiments with a sample were only conducting after “washing” the walls by three cycles of filling the side chamber with 6 mbar water vapor at RT. In the actual experiment, the water freezes on the cold finger, which is held at cryogenic temperature and acts as a localized cold spot. Following the evacuation of the side chamber using a LN2‐cooled cryo‐sorption pump, the UHV‐prepared sample is transferred directly below the cold finger. After closing the valve between the side chamber and the rest of the UHV system, the cold finger is heated until the icicle thaws, and ultra‐pure water drops onto the sample surface. The chamber is then re‐evacuated, and the sample transferred back to the main chamber for analysis. This procedure ensures that the surface is exposed to water only, and no air contamination occurs.The magnetite surface was exposed to an ultra‐pure liquid‐water drop in a custom‐designed side chamber described in detail elsewhere.2 (purity 99.995 % from Linde HiQ® MINICAN) into the side chamber while the water drop was already on the sample surface. The pressure of the CO2 was monitored and controlled by a capacitance pressure gauge. The specified CO2 pressure was kept for ca. 20 min to allow for the gas to dissolve into the water. We assume that we reached equilibrium in this timescale, because longer exposure did not lead to different results. The evacuating process, as well as the transfer back to the UHV system, were identical to that described for the pH‐neutral water drop.Acidic water drops were prepared by additionally introducing CO2 dissolves in water, it exists in chemical equilibrium with carbonic acid (H2CO3). The composition of a carbonic acid solution is fully determined by the pCO2above the solution. The amount of dissolved CO2 increases with increasing pCO2surrounding the water drop, and it dominates by 2 orders of magnitude over the next highest component, HCO3−. The concentration of dissolved CO32− is always negligible in the range of pCO2considered here.Details about the equilibria involved in the dissolution of carbon dioxide in water and how this affects the pH of the acidic solution are in the supplemental information (SI). Briefly, when CO22.13O4(001) surface exhibits a (√2×√2)R45° reconstruction due to an interstitial tetrahedrally coordinated iron in the second layer (Fetet), which replaces two octahedrally coordinated iron atoms (Feoct) in the third layer.3O4(001) surface over a wide range of O2 chemical potentials relevant for UHV studies.3O4(001) surface, which exhibits the characteristic rows of protrusions running in the [110] direction due to the surface Fe cations. Bright protrusions on the Fe rows are caused by surface OH groups (i. e. hydrogen atoms adsorbed at surface oxygen atoms), which modify the density of states of the nearby Fe cations, causing them to appear brighter in empty‐states STM images.The UHV‐prepared Fe3O400 surface 3O4(001) surface following exposure to an ultra‐pure water drop for 20 minutes. The surface exhibits bright chains with an apparent height of ≈2.1 Å covering ≈40 % of the surface. We recently interpreted these chains as representative of an Fe‐(oxy)hydroxide phase, formed when two or more OwaterH groups from dissociated water coordinate tetrahedrally coordinated Fetet atoms extracted from the subsurface.Figure 3O4(001) surface imaged following exposure to acidic water drops performed with a pCO2=20 mbar (pH≈4.8) and pCO2=1 bar (pH≈3.9), respectively. The former surface (c) exhibits bright chains consistent with the growth of an iron(oxy)hydroxide phase, as observed at pH 7, but with the coverage of the bright chains decreased to ≈30 % of the surface. In the corresponding LEED pattern (inset), the (√2×√2)R45° reconstruction spots remain absent, and a (1×1) symmetry is observed. Following exposure to the more acidic solution (pH 3.9), the bright chains are completely absent and the surface appears rough in STM, suggesting that etching has occurred. Nevertheless, terraces with rows of protrusions rotated 90° with respect to each other  remain visible, suggesting the atomic‐scale structure of the surface layer remained intact. LEED shows a (1×1) pattern of similar quality to that observed at neutral pH. Isolated protrusions with an apparent height between 2.3–3.2 Å are common on the surface, but occupy positions both on the rows and in between them. Based on the XPS data shown in the next section, we suspect these to be related to carbonaceous species.Figure 2.23O4(001) surface, and following exposure to water in different background pressures of CO2. Data corresponding to the STM images in Figure pCO2of 800 mbar (pH≈4.0). The corresponding STM image shows roughness similar to Figure The XPS spectra  in Figures 3O400 surface,3O400 surface,2p spectrum of the SCV reconstructed Fe3O4(001) surface is enriched in Fe3+ with respect to the bulk,3+ shake‐up satellite at 718.5 eV.pCO2=0, light blue curve), the overall intensity of the Fe 2p peak area is slightly attenuated and the Fe3+‐related satellite peak at 718.5 eV disappears, while the Fe2+‐related intensity at 715 eV increases. The enhancement of the Fe2+ in the surface region is also visible as an increased Fe2+ shoulder on the low‐binding‐energy side (at 708.1 eV) of the main 2p3/2 peak.2+‐related3O4(001) surface at RT to form bidentate formate species and hydroxyl groups,The Fe 3O4(001) surface is centred at 530.1 eV, and is slightly asymmetric due to the metallic nature of the oxide.The O 1s peak from the clean Fe3O4(001) surface . Following the interaction with progressively more acidic solutions, the surface exhibits higher adventitious carbon signals, with the highest C density of 0.6 C atoms per (√2×√2)R45° unit cell (8.4×1013 C atoms/cm2) after exposure to acidic solutions at pH 3.9. More interestingly, the surface exposed to the acidic water drop at pH 4.8 exhibits a new broad peak at 288.8 eV. This is clearly shifted from the formic acid reference, and was previously assigned to surface carbonate species.3−) species.1,2 and the α3 lines of the Mg anode, and therefore the peak can be attributed to an x‐ray satellite of the α1,2 signal at 289.9 eV.The C 1s spectrum of the UHV‐prepared surface is free from carbon within our detection limit (<0.05 C atoms per √2×√2)R45° unit cell). The water‐exposed surface shows a small peak at 284.9 eV corresponding to adventitious carbon. Comparing the latter peak with a reference spectrum of saturation coverage of formate adsorbed on the Fe×√2R45° upCO2surrounding the water drop during the experiment. With no applied CO2, the peak is completely absent, and it increases slowly going from 20 mbar, to 200 and 800 mbar. Then, a steep increase is observed when going from 800 to 1000 mbar. This corresponds in STM to a significant roughening of the surface morphology, which we interpret as the result of etching. On the opposite y‐axis we plot the calculated concentration of HCO3− in the solution, assuming that the standard equilibrium equations are directly applicable. Clearly, both curves grow with increasing pCO2dissolved into the water drop; however, the surface bicarbonate concentration increases significantly when the pCO2goes from 800 mbar to 1000 mbar, deviating from the square‐root trend followed by the [HCO3−] in solution. The reaction that leads to the formation of the surface bicarbonate (which correlates with the surface dissolution) is clearly not linearly dependent on the concentrations of reactants ([HCO3−] in solution), so we can speculate that the surface‐bicarbonate concentration measured in XPS as a function of pCO2might be an indication of the reaction rate.In Figure 2 gas without the liquid water drop  surface. Empty‐states STM images of a UHV‐prepared TiO2(110) surface are dominated by five‐fold coordinated surface titanium atoms, which appear as bright rows running along the [001] direction with a 6 Å spacing2(110) surface following exposure to an ultra‐pure water drop for 20 minutes. The resulting surface looks indistinguishable to the clean one in STM, with the exception of a low density of contaminants, which are imaged as white aggregates (white blobs). This result is in agreement with Balajka et al.,2(110) surface and retains its UHV‐like (1×1) structure upon water exposure (see LEED in the inset). Figure 2(110) surface imaged following exposure to an acidic water drop in which the pCO2was set to 800 mbar (pH≈4). The resulting TiO2(110) surface remains overall intact and continues to exhibit a (1×1) periodicity in LEED (inset). However, it is possible to distinguish in STM a fractional monolayer coverage (∼0.15 ML) of adsorbate species, all located in equivalent sites and have an apparent height of 1.5–1.7 Å. These species appear similar in STM to the ones observed on the air‐exposed surface reported in Ref. [7] where it has been shown that air contamination leads to the adsorption of carboxylic acids (present in air in ppb concentrations) which arrange on the surface in a (2×1) structure. Perhaps coincidently, the coverage of these species is very similar to the saturation coverage of polarons expected for a reduced TiO2(110) surface.Figure 2.43O4(001) is also shown .In Figure 2(110) surface is free from carbon within the detection limit of the instrument. When the surface is exposed to an acidic solution at pH 4, the C 1s region of the XP spectrum shows two small peaks at 289.3 eV and at 284.9 eV. The signal at 289.3 eV has been previously assigned to carboxylate species.2 ,13 and 2.4×1013 C atoms/cm2) respectively.The C 1s spectra of the UHV‐prepared TiO2 on the TiO2 surface without a water drop. The surface was exposed to 800 mbar CO2 gas and the XP signal of the C 1s region is reported in Figure 13 C atoms/cm2). These results suggest that the acidic environment is not responsible for the formation of the adsorbate species and would speak in favor of carboxylates formation, in agreement with Balajka et al.As in the case of magnetite, we performed a control experiment to study the effect of high‐pressure CO3pCO2in the surrounding environment. As case studies, we have characterized the atomic‐scale structure of the Fe3O4(001) and TiO2(110) surface following immersion to acidic water drops. Both surfaces exhibit very little adventitious carbon contamination when returned to UHV, and the experiments maintain a similar degree of cleanliness despite the presence of high pressures of CO2. Control experiments performed using CO2 gas only (no water drop) show significantly more adventitious carbon contamination, which suggests the water drop acts as a protective film shielding the surface from hydrocarbon contaminants in the gas. Interestingly, the presence of the water drop does not change the amount of carboxylate formed on TiO2, suggesting that these species are probably a small amount of carbonaceous contamination.Our results suggest that it is possible to change the pH of an ultra‐pure water drop in a controlled way by varying the 2+ is the most stable form of Fe in acidic conditions, meaning dissolution should be expected. TiO2, on the other hand, is thermodynamically stable in the pH range considered here, so this surface should remain intact, as observed. While we do not measure the pH directly; the fact that we observe the onset of significant roughening of the Fe3O4(001) surface at pH≤4 is consistent with prior studies of Fe oxide dissolution rates,Generally speaking, our observations are consistent with the Fe and Ti Pourbaix diagrams:3O4(001) to water at pH 7 have been previously interpreted as an iron‐(oxy)hydroxide phase, which forms due to the dissociative adsorption of water.waterH groups coordinated to Fe atoms from the surface, and the growth is limited by the number of reactive O sites, which are required to accommodate the accompanying proton. As the pH is lowered from pH 7, it seems likely that an increasing proportion of the reactive O sites are protonated directly from solution, which would explain why the coverage of the bright OwaterH related protrusions decreases, until they are completely absent at pH=3.9. From STM it appears that the surface remains fully protonated (the Fe rows remain clearly visible), but it is difficult to confirm this from XPS because the OH O 1s signal overlaps with bicarbonate, and the Fe2+ signal becomes much too large to be attributed solely due to protonation.As mentioned above, the bright chains of protrusions observed after exposing Fe2+, we propose the following scenario: In Figure 3− in XPS 2. When the drop evaporates, the Fe(HCO3)2 would precipitate out and land on the surface where it accumulates.To explain the origin of the surface bicarbonate, and the additional Fe2+ contribution of the pH 3.9 surface is significantly larger than the one obtained at pH 7, where the surface was already completely hydroxylated. Moreover, the Fe2+ signal is much larger than obtained for the HCOO− monolayer reference  is complexated by the bicarbonate ions, which act as ligands and form iron bicarbonate surface species (Fe(HCO3)2),Fe2++2HCO3-→Fe(HCO3)2. The formation of these species results in the extraction of the Fe from the surface into the solution. Upon evacuation of the liquid, the iron bicarbonate species ultimately precipitate out of solution and accumulate onto the surface, where they are detected in XPS. The re‐deposition of Fe‐containing species is probably not uniform (areas where the liquid evaporates last may get a higher coverage), thus the surface structure observed by STM after etching and evaporation of the drop  to have a C density of 2 C atoms per (√2×√2)R45° unit cell.89.9 eV t2 limits the pH to ≈3.9. It would be interesting to see if the bicarbonate concentration decreased again at lower pH, as the maximum in the dissolution rate for iron oxides has been shown to depend on the counter anion.Finally, one constraint of our current setup is that the use of high pressures for any gas is limited to 1000 mbar, which in the case of COpCO2dissolved into the water drop is in the range 800–1000 mbar, suggesting that the proton concentration (pH) is not the only factor at play.In Figure 42 in the surrounding environment, while keeping the high purity of a UHV‐based surface science experiment. We have used this method to explore how two metal oxide surfaces, Fe3O4(001) and TiO2(110), react to immersion in acidic water at the atomic scale.Our study demonstrates that it is possible to tune the pH of an ultra‐pure water drop by varying the partial pressure of CO2 is predicted to be thermodynamically stable in the pH range considered in this work, and indeed we observe that the surface remains intact following exposure to an acidic solution at pH 4. On the other hand, the Fe3O4(001) surface changes with reducing pH and in presence of HCO3− ions, and begins to dissolve at pH≈4. Following earlier kinetic models, we propose that the magnetite dissolution process occurs involving three consecutive reactions, namely, surface protonation, ligand adsorption, and consequent metal detachment into solution. The chemical environment provided by the dissolution of CO2 into the water drop favours the formation of protons and bicarbonates (HCO3−) in solution. At pH 4–3.9, the surface protonation and consequent formation of surface iron‐bicarbonate species weakens the surface Fe−O bonds and favours the detachment of a Fe centre into solution. After evacuation of the liquid, the Fe(HCO3)2 precipitate from the solution and accumulate onto the surface, where are detected in XPS.Our experimental observations are consistent with the earlier predictions of the Fe and Ti Pourbaix diagrams. TiOOur approach opens the door to atomic‐scale studies of degradation at the solid‐liquid interface and can, in principle, be applied to any surface.The authors declare no conflict of interest.As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.SupplementaryClick here for additional data file.