Binuclear cyclopentadienylmetal carbonyl derivatives of the oxophilic metal niobium

Comparison of the lowest energy Cp₂Nb₂(CO) _n structures (n = 7, 6, 5, 4, 3, 2, 1) with the corresponding Cp2V2(CO)n structures using density functional theory predicts structures having fourelectron donor η²-μ-CO groups and short M-O distances to be more prevalent for niobium than for vanadium, in accord with the greater oxophilicity of niobium relative to vanadium. The lowest energy Cp ₂Nb₂(CO)₇ structure consists of CpNb(CO) ₄ and CpNb(CO)₃ units joined by a relatively long ~3.4 Á Nb-Nb bond and is marginally stable by ~9 kcal/mol with respect to dissociation into mononuclear fragments. The lowest energy Cp₂Nb ₂(CO)₆ structure contains a rj2-pt-CO group with a Nb-Nb single bond of length ~3.2 A. The lowest energy Cp₂Nb ₂(CO)₅ structure is similar to that of the known Cp ₂V₂(CO)₅ with a predicted Nb=Nb triple bond length of ~2.7 A. Two η²-μ-CO groups are found in the lowest energy Cp₂Nb₂(CO)₄ structure with a predicted Nb=Nb double bond length of ~3.0 Å. The lowest energy Cp ₂Nb₂(CO)₃ structure is predicted to have two η²-μ-CO groups and a Nb=Nb triple bond of length ~2.8 Å. The lowest energy Cp₂Nb₂(CO)₂ structure is also predicted to have two η²-μ-CO groups but a significantly shorter Nb-Nb quadruple bond of length ~2.6 Å. All of these structures are singlets with 18-electron configurations for both niobium atoms. The lowest energy structure for Cp₂Nb₂(CO) is a triplet with a η²-μ-CO group and a Nb-Nb quadruple bond of length ~2.6 A. The lowest energy structures for Cp₂Nb₂(CO)₅ and Cp₂Nb₂(CO)₃ are both stable with respect to disproportionation into Cp₂Nb₂(CO)_n+1+Cp ₂Nb₂(CO)_n-1, whereas Cp₂Nb ₂(CO)₆ and Cp₂Nb₂(CO)₄ are unstable with respect to such disproportionation. The reaction of Cp ₂Nb₂(CO)₅ with a CpNb(CO)₃ fragment to give Cp₃Nb₃(CO)₈ is predicted to be exothermic by ~13 kcal/mol, thereby suggesting a possible mechanism for the experimentally observed formation of the trinuclear derivative Cp ₃Nb₃(CO)₆(η²-μ-CO) in the photolysis of CpNb(CO)₄.