The Remarkable Nb2(CO)12 with Seven-Coordinate Niobium: Decarbonylation to Nb2(CO)11 and Nb2(CO)10

The dissociation of Nb2(CO)12 into two Nb(CO)6 units is predicted to require ∼13 kcal/mol so that Nb2(CO)12 rather than Nb(CO)6 is the anticipated initial oxidation product of the known Nb(CO)6(-) anion. This differs from the corresponding vanadium carbonyl chemistry where V(CO)6 rather than V2(CO)12 is found experimentally to be the oxidation product of V(CO)6(-). The lowest energy Nb2(CO)12 structure consists of two Nb(CO)6 fragments joined by a Nb-Nb bond of ∼3.4 Å length so that each niobium atom is heptacoordinate, counting the metal-metal bond. These niobium coordination polyhedra can be approximated by capped octahedra. Among unsaturated binuclear niobium carbonyls the lowest energy Nb2(CO)11 structure has a formal four-electron donor bridging η(2)-μ-CO group and a formal Nb-Nb single bond rather than only two-electron donor carbonyl groups and a formal Nb═Nb double bond. The Nb2(CO)11 structures with formal Nb═Nb double bonds and exclusively two-electron donor carbonyl groups lie more than 13 kcal/mol above this low-energy Nb2(CO)10(η(2)-μ-CO) structure. However, Nb2(CO)11 is predicted to be thermodynamically disfavored, owing to disproportionation into Nb2(CO)12 + Nb2(CO)10, a slightly exothermic process by ∼4 kcal/mol. The Nb2(CO)10 structures with formal Nb≡Nb triple bonds and all two-electron donor carbonyl groups appear to be particularly favorable, as suggested by high CO dissociation energies and viability toward disproportionation. Such structures are isolobal with Cp2Mo2(CO)4, which was the first stable metal carbonyl to be discovered with a short metal-metal distance, corresponding to a formal triple bond. Considerably higher energy Nb2(CO)10 structures (by more than 20 kcal/mol) have two four-electron donor bridging carbonyl groups and long niobium-niobium distances. Such structures can be considered to consist of a bidentate Nb(CO)6 "ligand" coordinating to a Nb(CO)4 unit through the two η(2)-μ-CO groups.