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| - Nonacarbonyldivanadium: Alternatives to Metal−Metal Quadruple Bonding
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| - The first structural characterization of the highly unsaturated nonacarbonyldivanadium V2(CO)9 is reportedusing density functional theory (DFT) with the B3LYP and BP86 functionals. A complicated collection ofminima with rather closely spaced energies was found. However, none of these many V2(CO)9 isomers wasfound to have a sufficiently short vanadium−vanadium distance for the VV quadruple bond required togive both metal atoms the favored 18-electron configuration. Triplet structures for V2(CO)9 were found to becompetitive in energy with related singlet structures. Thus, the two lowest-energy isomers of V2(CO)9 aretriplets. The four lowest-energy isomers of V2(CO)9 all have three very unsymmetrical bridging CO groups(typically “short” and “long” M−CO distances differing by 0.4−0.5 Å) rather than the symmetrical bridgingCO groups found experimentally in Fe2(CO)9 and predicted for M2(CO)9 (M = Cr and Mn) from earlierstudies. The V⋮V distances in each of these four isomers suggest a metal−metal triple bond. Next higher inenergy for V2(CO)9 are three structures with single four-electron donor bridging CO groups identified bytheir computed ν(CO) frequencies and V−O distances. The V−V distances in these three isomers suggestmetal−metal single bonds. This study of V2(CO)9 supports the following general points: (1) Metal−metalbonds of an order higher than three are not favorable in metal carbonyl chemistry. (2) The 18-electron rulefor metal carbonyls begins to break down when the metal atom, i.e., vanadium in this case, has only fivevalence electrons.
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