Cobalt-Cobalt Multiple Bonds in Homoleptic Carbonyls? Co2(CO)x (x = 5-8) Structures, Energetics, and Vibrational Spectra
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- 作者：Joseph P. Kenny ; R. Bruce King ; and Henry F. Schaefer ; III
- 刊名：Inorganic Chemistry
- 出版年：2001
- 出版时间：February 26, 2001
- 年：2001
- 卷：40
- 期：5
- 页码：900 - 911
- 全文大小：189K
- 年卷期：v.40,no.5(February 26, 2001)
- ISSN：1520-510X
文摘
Homoleptic binary cobalt carbonyls with multiple cobalt-cobalt bonds have been examined theoretically usingestablished levels of density functional methodology. These species include 19 structures ranging from theexperimentally well characterized dibridged (CO)3Co(CO)2Co(CO)3 to the proposed monobridged (CO)2Co(CO)Co(CO)2 structure with a formal quadruple bond. Consistent with experiment, three energetically low-lyingequilibrium structures of Co2(CO)8 were found, of C2v (dibridged), D3d (unbridged), and D2d (unbridged) symmetry.For Co2(CO)8, the BP86 method predicts the dibridged structure to lie 3.7 kcal/mol below the D2d structure and6.3 kcal/mol below the D3d structure. The D2d and D3d structures thus have the opposite energetic ordering of thatdeduced from experiment by Sweany and Brown. A satisfactory harmony between theoretical and experimentalvibrational frequencies and IR intensities is found, although the D2d and D3d structures are essentiallyindistinguishable in this regard. For Co2(CO)7 the unbridged asymmetric structure suggested by Sweany andBrown is confirmed with the BP86 method, and with perhaps one exception the vibrational features agree wellfor theory and experiment. For Co2(CO)6 only one vibrational feature has been assigned from experiment, butthis band (2011 cm-1) fits very well with BP86 predictions for the monobridged D2d symmetry structure with aformal Co
Co triple bond. For the Co2(CO)5 molecule, for which no experimental results exist, the most interestingstructure is the monobridged closed-shell singlet with a very short (2.17 Å) cobalt-cobalt bond, to which weassign a formal bond order of four. Potential energy distributions have been analyzed to identify the principalvibrations with cobalt-cobalt stretching contributions. The condensed phase Raman analysis by Onaka and Shriverof the Co-Co stretches for the three known isomers of Co2(CO)8 is remarkably consistent with the presentpredictions for the gas-phase species. Prospects for the synthesis of these and related dicobalt compounds arediscussed.
Co triple bond. For the Co2(CO)5 molecule, for which no experimental results exist, the most interestingstructure is the monobridged closed-shell singlet with a very short (2.17 Å) cobalt-cobalt bond, to which weassign a formal bond order of four. Potential energy distributions have been analyzed to identify the principalvibrations with cobalt-cobalt stretching contributions. The condensed phase Raman analysis by Onaka and Shriverof the Co-Co stretches for the three known isomers of Co2(CO)8 is remarkably consistent with the presentpredictions for the gas-phase species. Prospects for the synthesis of these and related dicobalt compounds arediscussed.