Article pubs.acs.org/JPCA
Infrared Spectra and Structures of Anionic Complexes of Cobalt with Carbon Dioxide Ligands Benjamin J. Knurr and J. Mathias Weber* JILA and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: We present infrared photodissociation spectra of [Co(CO2)n]− ions (n = 3−11) in the wavenumber region 1000−2400 cm−1, interpreted with the aid of density functional theory calculations. The spectra are dominated by the signatures of a core ion showing bidentate interaction of two CO2 ligands with the Co atom, each forming C−Co and O−Co bonds. This structural motif is very robust and is only weakly affected by solvation with additional CO2 solvent molecules. The Co atom is in oxidation state +1, and both CO2 ligands carry a negative charge.
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INTRODUCTION Transition metals and metal organic complexes have found interest and applications in a wide variety of chemical contexts. In particular, transition metal complexes are important catalysts, but their detailed function on a molecular level is often poorly understood. Detailed, molecular-level studies of catalysts in the condensed phase are difficult, particularly under turnover conditions, because the complexity of the condensed phase environment (particularly regarding speciation) greatly complicates the response of the sample to spectroscopic probes. Spectroscopy of mass-selected ions affords a particularly attractive avenue to studying the properties of transition metals in complexes with molecular ligands. Cobalt is of interest as a catalyst in several prototypical reactions and in many metal−organic compounds, but only a few spectroscopy experiments on mass-selected cobalt-containing ions exist that involve a net negative charge on the ion. In the context of Fischer−Tropsch synthesis, Gerhards and coworkers recently addressed the interaction of alcohols with atomic and cluster anions of Co using infrared spectroscopy of mass-selected cluster ions.1,2 The interaction of CO2 with Co is of recent particular interest, because CO2 is often a small component of feedstock from biomass, which is increasingly used in Fischer−Tropsch applications.3 A related area where an extensive search for new catalysts is in progress is the reduction of CO2 with the aim of producing feedstock for the generation of chemical fuels from renewable sources. Similar to other areas, the molecular level details of reductive activation of CO2 are not well understood. In the simplest case of one-electron reduction of CO2, the use of negative ions as charge donors seems to be an interesting approach. We have recently demonstrated4,5 that CO2 in anionic complexes with atomic Au and Ag can serve as model systems for activated complexes of single atom catalysts for one-electron CO2 reduction. In these systems, [MCOO]− ions (M = Au, Ag) constitute the main charge carriers. For silver complexes, we also find CO2− and (CO2)2− as charge carrying © XXXX American Chemical Society
species, on the basis of the infrared responses of mass-selected cluster ions. For both Ag and Au complexes, solvation plays a crucial role in the nature of the possible charge carriers and for the charge distribution in [MCOO]− complexes. A natural question is how the electronic structure of the metal partner influences the interaction with CO2. Although silver and gold are similar in their electronic structure, there are very interesting differences in their interaction with CO2, showcasing the influence of subtle differences in the electronic properties of the metal on the CO2 activation behavior. Even so, the activated [MCOO]− complexes are similar in structure in the case of both Ag and Au. Here, we ask how a more drastic deviation of the electronic structure of the metal partner from the [nd10(n + 1)s] electron configuration will change the interaction with CO2 molecules. In the context of our previous studies4,5 on [M(CO2)n]− ions (M = Au, Ag), we seek information on the structural motifs governing the interaction of Co and CO2 in the presence of an excess electron. In particular, we are interested in the charge distribution in [Co(CO2)n]− complexes and the influence of solvation on the structural and electronic properties of the charge carrying species. We address these questions using a combination of infrared spectroscopy of mass-selected cluster ions and quantum chemical calculations.
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METHODS Experimental Section. The experimental apparatus has been described in detail previously.6 Briefly, [Co(CO2)n]− clusters were formed by using the third harmonic of a nanosecond pulsed Nd:YAG laser (355 nm) to ablate a rotating cobalt target, creating a cobalt vapor. The metal vapor was then entrained in a pulsed supersonic expansion of CO2 Received: March 31, 2014 Revised: May 14, 2014
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(stagnation pressure 5.5 bar) from an Even−Lavie valve, resulting in [Co(CO2)n]− clusters in addition to other species, e.g., [CoO(CO2)n]− and (CO2)n− clusters. The anions were accelerated and mass selected by a Wiley−McLaren time-offlight mass spectrometer. Attempts to obtain rare gas solvated [Co(CO2)n]·Rgm− ions by entraining CO2 and laser-vaporized Co into a supersonic expansion of Ar or Kr were unsuccessful. The mass-selected clusters were irradiated with the tunable output of an optical parametric converter (LaserVision) in the range 1000−2400 cm−1 with a bandwidth of 2 cm−1 and pulse duration of 7 ns. We note that there is currently a gap in our tuning range from 2150 to 2250 cm−1. Irradiation occurred in a multipass cell based on design by Liu and co-workers.7 Multiphoton effects were tested for by varying the number of passes in the multipass cell and monitoring the fragment ion signal for the main transitions; no multiphoton effects were observed. The formation of fragment ions resulting from the loss of a CO2 molecule was monitored using a reflectron as a secondary mass analyzer. Typical fragment ion peak intensities are on the order of 1% of the parent ion intensities but depend strongly on the ion and the spectral region, because laser fluence varies substantially across the tuning range of the optical parametric converter. The resulting action spectra were corrected for laser fluence and multiple spectra were averaged over different days to show reproducibility and increase the signal-to-noise ratio. The repetition rate of the experiment was 20 Hz. Computational. We carried out DFT calculations using the TURBOMOLE V. 5.9.1 suite of programs8 to asses possible structures for [Co(CO2)n]− clusters (n = 1−4). The B3-LYP functional9,10 with dispersion correction11 was employed for all calculations with def2-TZVPP basis sets12 assigned to all atoms. Vibrational spectra were calculated for all clusters using the AOFORCE program.13,14 Scaling factors were applied to all reported calculated vibrational frequencies to account for anharmonicity, dependent on the structural motif at play. For structures with a single CO2 ligand covalently bound to the Co atom in a formate-like motif, the correction factor applied was 0.9380, because the same factor was used in our previous studies on Au4 and Ag5 containing metal−CO2 clusters of a similar electronic and geometric structure. For all other structures, a scaling factor of 0.9752 was used, on the basis of the comparison of the experimental value for the antisymmetric stretch of free CO215 with that of a calculation for the same vibrational motion. Partial charges were calculated using a natural population analysis.
Figure 1. Experimental spectra of [Co(CO2)n]− (n = 3−6) monitoring the loss of one CO2 molecule. Numbers denote the number of CO2 molecules in the cluster. All spectra are individually normalized so the left and right traces are on different scales (see text for discussion).
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RESULTS AND DISCUSSION The signatures we observe in the infrared spectra of [Co(CO2)n]− ions (Figures 1 and 2) are CO stretching modes arising from the CO2 ligands. In principle, a CO2 molecule can act as part of the ionic charge carrier or as a solvent molecule in the vicinity of the charge carrier. On the basis of infrared spectra of other clusters containing CO2 as solvent,4,5,16−19 solvent CO2 molecules retain their chemical identity. Consequently, their antisymmetric stretching frequencies typically exhibit very little deviation from that of neutral, free CO2, which is found at 2349 cm−1.15 The spectral features on the high wavenumber side of Figures 1 and 2 are consistent with this picture. The peaks stay centered around 2345 cm−1 without substantial deviation as cluster size changes. If a CO2 molecule is part of the charge carrying species, a negative charge on a CO2 ligand will weaken the CO bonds and
Figure 2. Experimental spectra of [Co(CO2)n]− (n = 7−11), analogous to Figure 1. Due to the lower parent ion intensities compared to n = 3−6, only data above 1600 cm−1 were acquired.
lead to a red shift of its CO stretching frequencies corresponding to the type and degree of interaction with the metal.4,5 Depending on the structure of the charge carrier, such a CO2 molecule can in fact lose its chemical identity, and its vibrational modes may no longer be similar in character to those of free CO2. B
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The infrared spectra of all [Co(CO2)n]− ions under study (Figures 1 and 2) show two dominant peaks around 1750 and 1785 cm−1 in addition to a number of weaker features in the range 1000−2000 cm−1. All of the peaks below 2000 cm−1 can be attributed to CO stretching modes of the charge carrier, because the only vibrational mode of solvent CO2 molecules in this region would be the Fermi doublet of the symmetric stretch and the bend overtone, which both have small oscillator strengths. Unlike [Ag(CO2)n]− and [Au(CO2)n]− clusters where only one CO2 ligand binds covalently to the metal center,4,5 the multiple peaks in the region below 2000 cm−1 implies there are either multiple CO2 molecules that are part of the charge carrier, multiple isomers with different infrared signatures, or a combination of both. No laser-induced fragmentation was observed in [Co(CO2)]− and [Co(CO2)2]−, despite the large abundance of both clusters (Figure 3). Together with the high
Figure 4. Comparison of the positions of dominant peaks and shoulders in the IR spectra of [M(CO2)n]− clusters for M = Au4 (top), Ag5 (center), and Co (bottom).
Figure 3. Low mass region of the mass spectrum of laser vaporized Co entrained into a supersonic expansion of CO2.
abundance of [Co(CO2)2]−, this suggests that the dominant charge carrier involves two CO2 molecules strongly bound to the metal. The lower frequency component of the dominant doublet is more intense and shifts on average by −1 cm−1 per additional CO2 molecule, whereas the other shows an even smaller average red shift. Both peaks have largely unresolved substructures that are likely due to solvent conformers (vide infra). The weak dependence of the peak positions on cluster size indicates that the charged species responsible for these features is very robust and hardly affected by solvation (Figure 4), in contrast to [AuCO2]− and [AgCO2]−.4,5 Electronic structure calculations allow a more detailed analysis of the experimental infrared spectra. Complexes involving a single cobalt atom, CO2, and an excess electron allow the existence of singlet and triplet states. The lowest lying calculated isomers of [CoCO2]− are structurally and electronically very different (Figure 5). The ground state is calculated to be a triplet state where the Co atom is inserted into a CO bond, in agreement with previous calculations by Castleman and coworkers.20 We assume that there is a substantial barrier to the formation of this insertion structure (AT) due to the necessary breaking of a CO bond. The “formate” motif4,5 observed in [AuCO2]− and [AgCO2]− is calculated to be a higher lying
Figure 5. Lowest energy [CoCO2]− structures with relative energies. Subscript letters denote spin multiplicity (T for triplet, S for singlet). Co atoms are shown in gray, C in black, and O in red.
triplet state, whereas the lowest lying singlet structure shows a bidentate interaction of the CO2 ligand with the Co atom, forming both a C−Co and an O−Co bond. The energy of this isomer is far (1.3 eV) above the energies of the triplet structures. Interestingly, structures BT and CS shown in Figure 5 are predicted to leave the cobalt atom close to neutral and most of the negative excess charge on the CO2 moiety. Structure AT shows positive charge on the Co and C atoms, and the excess negative charge is localized on the O atoms. Because photodissociation of [CoCO2]− was not observed and attempts to prepare [CoCO2]−·Ar or [CoCO2]−·Kr proved to be unfeasible, we cannot determine the dominant structure of [CoCO2]−. C
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The three calculated structures of [CoCO2]− provide templates of structural motifs for [CoC2O4]−, but the relative energies are reordered (Figure 6). The insertion motif (DT) is
[CoC2O4]− indicates that isomer (HT) is not significantly populated, at least not at this cluster size. Calculated binding energies for all other isomers for n = 1 and 2 are significantly higher than the photon energy range in this experiment, whichtogether with the absence of isomers (BT) and (HT) explains why photodissociation is not observed for these cluster sizes. These high binding energies may render atomic Co unsuitable for CO2 reduction catalysis, because the active site is likely to be poisoned by the reactant. As mentioned above, laser-induced fragmentation is first observed in [Co(CO2)3]−, and the infrared spectra do not change significantly with increasing cluster size. Therefore, comparison of the predicted infrared spectra of the minimum energy structures recovered for n = 2 to the experimental spectrum of [Co(CO2)3]− should allow identification of the dominant structural motif of the “core” ion. Figure 7 shows
Figure 6. [CoC2O4]− structures with relative energies. Subscript letters denote spin multiplicity (T for triplet, S for singlet). Co atoms are shown in gray, C in black, and O in red.
still by far the lowest energy isomer. Here, the carbon and oxygen atoms are distributed into a CO ligand and a CO3 ligand, the latter conceivably formed by addition of the second CO2 molecule to the lone oxygen in structure AT (Figure 5). In the lowest lying singlet structure (ES), both CO2 ligands are now attached to the metal atom in a bidentate fashion, each forming C−Co and O−Co bonds, and we denote this structural family [CO2CoCO2]−. The OCO planes of the two ligands are twisted against one another by ca. 80°. The geometry of isomer (ES) was first optimized without symmetry constraints, then in C2 symmetry. The energies of the resulting structures coincide within less than 1 meV, and we therefore conclude that isomer (ES) is indeed of C2 symmetry. Two triplet isomers (FT, GT) based on the bidentate interaction motif are also found, but the twisting angles of the two CO2 ligands are smaller for these structures (F, 20°; G, 35°). These isomers are 80 meV (FT) and 480 meV (GT) higher in energy than the singlet structure. The formate motif solvated by one additional CO2 molecule (HT) is now ca. 850 higher in energy than the bidentate singlet structure. The lowest calculated dissociation energy of any of the isomers of [CoC2O4]− is for the loss of the CO2 ligand from the formate triplet isomer (HT), which amounts to ca. 190 meV. This is on the order of the photon energy predicted for the fundamental transition in the antisymmetric stretching mode of the strongly bound CO2 ligand in isomer (HT) (∼1665 cm −1 ) and is much lower than that of the antisymmetric stretching mode of solvent CO2 (∼2345 cm −1 ). The absence of laser-induced dissociation of
Figure 7. Comparison of n = 2 core structures DT−HT (bottom layers, see Figure 6) to the experimental spectrum of [Co(CO2)3]− (top layer). We note that the third CO2 molecule plays the role of a solvent, and that solvent position does not strongly affect the simulated spectra (see also Figure 8).
such a comparison. Only bidentate structures match the dominant doublet, consistent with the expectation that the core ion contains two CO2 molecules and the fact that two of the bidentate isomers are relatively low in energy in the group of structures that do not involve breaking a C−O bond during cluster formation. In both isomers, the higher (lower) energy component of the doublet corresponds to the in-phase (out-ofphase) combination of the antisymmetric stretching modes of the two CO2 moieties. The narrowness of the experimentally observed peaks and the calculated offset of the predicted spectra for singlet and triplet bidentate structures suggest that only one of the two is present in significant abundance. Based D
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on the band positions alone, it is difficult to unambiguously identify whether the singlet (ES) or triplet (FT) bidentate motif is in better agreement with the experimental data. However, the predicted relative intensities of the dominant features and the lower isomer energy favor the singlet structure, so we tentatively assume that the singlet structure (ES) is the dominant structural motif in [Co(CO2)n]− cluster ions. We note that the relative intensities of the peaks in the region 1000−1200 cm−1 of the experimental spectra are suppressed, because the calculated binding energy of solvent CO2 molecules is ca. 1400 cm−1, consistent with the fact that loss of a single CO2 molecule is the dominant loss channel. This suggests that only clusters with some internal energy or two photon absorption events are able to yield peaks at very low wavenumbers. The power dependence of the dominant features indicate that they are due to single photon absorption. On the basis of the observation of weaker peaks in the spectrum, it is likely that other isomers are also populated in the ion beam. The insertion structure recovers small peaks around 1660 cm−1 in addition to the feature at 1950 cm−1. For [Ag(CO2)n]− cluster ions, strong spectroscopic signatures are observed that indicate the presence of CO2− and (CO2)2− charge carriers.5 Some weaker features around 1900 cm−1 for the larger [Co(CO2)n]− clusters are consistent with CO2 dimer anions as charge carriers, but they clearly do not play a major role here. Some of the weaker peaks in the experimental spectra could also be caused by overtones and combination bands of low-energy transitions, but we refrain from definitive assignments of weak features at this time. Interestingly, insertion-type structures are not strongly populated, despite the fact that their energies are significantly lower than the bidentate structures. This suggests that the heat of reaction during the formation of the complexes under study is not sufficiently high to insert the metal into a CO bond, the insertion reaction is not sufficiently fast, or the survival probability of the inserted complexes is not sufficiently high for the formation of such complexes. The latter is consistent with the observed abundance of [CoO(CO2)n]− ions in the mass spectrum (Figure 3). Adding additional CO2 molecules to the core structures calculated for [CoCO2]− and [CO2CoCO2]− yields no new calculated core ion structures that would be compatible with the experimentally observed infrared features. Consistent with experimental observation, different solvent conformers do not strongly affect the structures, charge distributions or infrared spectra of singlet [CO2CoCO2]− core ions. This is demonstrated in Figure 8, where the predicted spectra of different solvent conformers for [CO2CoCO2]−·(CO2)2 are compared with the experimental spectrum. Although the different solvent conformers do not change the positions of the dominant peaks significantly, they can fully account for the largely unresolved substructure of these peaks. The calculated charge distribution for the singlet bidentate [CO2CoCO2]− core ion indicates that the Co atom is singly positively charged, formally corresponding to a d8 configuration (with a small admixture of 4s character). Both CO2 ligands carry a negative charge. CoI is an uncommon species in inorganic and metal−organic complexes, and [CO2CoCO2]− presents an extension of this interesting class of species. From a coordination chemistry point of view, the coordination environment of the Co atom could be interpreted as a twisted “butterfly” arrangement21 in an ML4 complex, in which the O−Co−O axis would serve as the z-axis (ligands L1
Figure 8. Comparison of the experimental spectrum of [Co(CO2)4]− (upper trace) with calculated spectra of [CO2CoCO2]−·(CO2)2 (lower four traces).
and L4 in the left-hand structure of Figure 9). Although the calculated O−Co−O angle is 179° and would therefore be roughly consistent with this interpretation, the highest occupied d-orbital in this case should be a strongly directional s−d hybrid orbital pointing away from the C atoms. However, the calculated highest occupied d-orbital for the [CO2CoCO2]− ion, which is also the HOMO of the molecule, is very different and has clear dz2 character with the z-axis nearly perpendicular to the O−Co−O axis (right side of Figure 9). In fact, this orbital rather resembles the highest occupied d-orbital in a bent ML2 configuration.21 Following this interpretation, each CO2 moiety could be viewed as a monodentate rather than a bidentate ligand, despite the fact that both Co−O and Co−C bonds are formed. The short CO bonds in the CO2 ligands are at the heart of this apparent discrepancy. This structural strain results in twisting the original “butterfly” complex, which stabilizes the dz2 orbital by reducing its overlap with the O ligands, similar to the orbital situation in an actual ML2 complex. The situation is reminiscent of the tendency of Co+ to coordinate with only three water molecules directly in Co+(H2O)n clusters, indicating that Co+ seems to prefer less than full coordination.22
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CONCLUSIONS The predominant species in [Co(CO2)n]− clusters is a [CO2CoCO2]− ion in a singlet electronic state of C2 symmetry, solvated by the remaining CO2 molecules for n ≥ 3. The CO2 ligands in this core ion bind to the Co atom in a bidentate fashion via a C−Co and an O−Co bond from each CO2 moiety. The Co atom is in oxidation state +1 in this isomer, with the CO2 ligands each carrying a negative charge. The ligand binding energies in this core ion are rather strong, and the core ion structure is only weakly perturbed by solvent interactions. E
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Figure 9. Orbitals in the d-block of the [CO2CoCO2]− core ion expected for a “butterfly” ML4 configuration (left) and a bent ML2 configuration (center), compared to the orbital calculated for the singlet core ion structure (right). Note that the HOMO−3 orbital is not part of the d-block but has been listed for completeness. Occupied orbitals are shown as bold horizontal lines for each configuration, calculated orbital energies are given in electronvolts. Dotted lines show how orbitals correlate in different structures. Note that the definition of the z-axis is not along the symmetry axis of the complex in this description but is defined by the dz2 character of the HOMO. (3) Diaz, J. A.; de la Osa, A. R.; Sanchez, P.; Romero, A.; Valverde, J. L. Influence of CO2 Co-Feeding on Fischer−Tropsch Fuels Production over Carbon Nanofibers Supported Cobalt Catalyst. Catal. Commun. 2014, 44, 57−61. (4) Knurr, B. J.; Weber, J. M. Solvent-Driven Reductive Activation of Carbon Dioxide by Gold Anions. J. Am. Chem. Soc. 2012, 1324, 18804−18808. (5) Knurr, B. J.; Weber, J. M. Solvent-Mediated Reduction of Carbon Dioxide in Anionic Complexes with Silver Atoms. J. Phys. Chem. A 2013, 117, 10764−10771. (6) Weber, J. M. A Pulsed Ion Source for the Preparation of Metal Containing Cluster Ions Using Supersonic Entrainment of Laser Vaporized Metal. Rev. Sci. Instrum. 2005, 76, 043301. (7) Riedel, J.; Yan, S. N.; Kawamata, H.; Liu, K. P. A Simple Yet Effective Multipass Reflector for Vibrational Excitation in Molecular Beams. Rev. Sci. Instrum. 2008, 79, 033105. (8) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. ElectronicStructure Calculations on Workstation Computers - the Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165−169. (9) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior. Phys. Rev. A 1988, 38, 3098− 3100. (10) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the ElectronDensity. Phys. Rev. B 1988, 37, 785−789. (11) Grimme, S. Accurate Description of Van der Waals Complexes by Density Functional Theory Including Empirical Corrections. J. Comput. Chem. 2004, 25, 1463−1473. (12) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (13) Deglmann, P.; Furche, F. Efficient Characterization of Stationary Points on Potential Energy Surfaces. J. Chem. Phys. 2002, 117, 9535− 9538. (14) Deglmann, P.; Furche, F.; Ahlrichs, R. An Efficient Implementation of Second Analytical Derivatives for Density Functional Methods. Chem. Phys. Lett. 2002, 362, 511−518. (15) Herzberg, G. Molecular Spectra and Molecular Structure; Krieger Publishing Co.: Malabar, FL, 1991; Vol. III, p 598.
The dominant observed isomer is significantly higher in energy (ca. 1 eV) than the global minimum structure, which is a triplet configuration based on the insertion of the Co atom into one of the CO bonds. Other calculated triplet structures that also exhibit the bidentate interaction between the CO2 ligands and the Co atom are higher in energy and less consistent with the observed infrared spectrum than the singlet isomer. Our original motivation was to compare Co with Au and Ag as model systems for single atom catalysts. The drastic differences in electronic structure between Co and the nobler metals makes this comparison difficult at best. However, this study has brought interesting insight into the interaction of Co with CO2 from a fundamental point of view, and the use of such complexes as chemical building blocks has not been explored thus far.
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AUTHOR INFORMATION
Corresponding Author
*J. M. Weber: phone, ++1-303-492-7841; e-mail, weberjm@ jila.colorado.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the National Science Foundation for funding through Grant CHE-0845618 (for graduate student support of B.J.K.) and for instrumentation and maintenance through Grant PHY-1125844. We thank Michael Thompson for helpful discussion.
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REFERENCES
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(16) Weber, J. M.; Schneider, H. Infrared Spectra of X−·CO2·Ar Cluster Anions (X = Cl,Br,I). J. Chem. Phys. 2004, 120, 10056−10061. (17) Schneider, H.; Boese, A. D.; Weber, J. M. Infrared Spectra of O2−·(CO2)n Clusters (n = 1−6): Asymmetric Docking at the π* Orbital. J. Chem. Phys. 2005, 123, 074316. (18) Shin, J. W.; Hammer, N. I.; Johnson, M. A.; Schneider, H.; Glöss, A.; Weber, J. M. An Infrared Investigation of the (CO2)n− Clusters: Core Ion Switching from Both the Ion and Solvent Perspectives. J. Phys. Chem. A 2005, 109, 3146−3152. (19) Kamrath, M. Z.; Relph, R. A.; Johnson, M. A. Vibrational Predissociation Spectrum of the Carbamate Radical Anion, C5H5NCO2−, Generated by Reaction of Pyridine with (CO2)m−. J. Am. Chem. Soc. 2010, 132, 15508−15511. (20) Johnson, G. E.; Reveles, J. U.; Reilly, N. M.; Tyo, E. C.; Khanna, S. N.; Castleman, A. W. Influence of Stoichiometry and Charge State on the Structure and Reactivity of Cobalt Oxide Clusters with CO. J. Phys. Chem. A 2008, 112, 11330−11340. (21) Jean, Y. Molecular Orbitals of Transition Metal Complexes; Oxford University Press Inc.: New York, 2005; pp 37−84. (22) Furukawa, K.; Ohashi, K.; Koga, N.; Imamura, T.; Judai, K.; Nishi, N.; Sekiya, H. Coordinatively Unsaturated Cobalt Ion in Co+(H2O)n (n=4−6) Probed with Infrared Photodissociation Spectroscopy. Chem. Phys. Lett. 2011, 508, 202−206.
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