Carbonyl versus butadiene dissociation in binuclear butadiene cobalt carbonyls

Dicobalt octacarbonyl is known to react with diolefins to give substitution products of the types (diene)Co2(CO)6 and (diene)2Co2(CO)4. The butadiene derivatives (C4H6)Co2(CO)n (n = 6, 5, 4, 3, 2) have been investigated by density functional theory using the B3LYP and BP86 methods. The lowest energy (C4H6)Co2(CO)n (n = 6, 5, 4) structures have bridging CO groups and terminal butadiene ligands. For the (C4H6)Co2(CO)6 and (C4H6)Co2(CO)5 structures the CoCo distances of ∼2.5 Å suggest formal single bonds. However, for the lowest energy (C4H6)Co2(CO)4 structure the significantly shorter CoCo distance of ∼2.3 Å suggests the formal triple bond required to give the cobalt atom the favored 18-electron configuration. Bridging butadiene ligands are also found in (C4H6)Co2(CO)n structures including all of the lowest energy (C4H6)Co2(CO)3 and (C4H6)Co2(CO)2 structures. Both the B3LYP and BP86 methods predict butadiene dissociation from (C4H6)2Co2(CO)4 to be energetically favored over CO dissociation by ∼8 kcal/mol. For (C4H6)2Co2(CO)n (n = 3, 2) the BP86 method predicts CO dissociation to be favored energetically over butadiene dissociation by ∼8 kcal/mol. However, the B3LYP method predicts essentially equal CO and butadiene dissociation energies within ∼1 kcal/mol from (C4H6)2Co2(CO)n (n = 3, 2).