Mononuclear and binuclear cobalt carbonyl nitrosyls: comparison with isoelectronic nickel carbonyls

Abstract

The cobalt carbonyl nitrosyls Co(NO)(CO)_n (n = 4, 3, 2, 1) and Co₂(NO)₂(CO)_n (n = 5, 4, 3, 2) have been studied by density functional theory. The lowest energy structures for the mononuclear derivatives Co(NO)(CO)_n (n = 3, 2, 1) parallel those of the corresponding isoelectronic Ni(CO)_n+1 derivatives. In addition, a thermodynamically unstable Co(NO)(CO)₄ structure is predicted with a bent nitrosyl group. The potential energy surfaces of the binuclear cobalt carbonyl nitrosyl derivatives Co₂(NO)₂(CO)_n (n = 5, 4, 3, 2) are complicated by the similar energies of analogous structures with bridging carbonyl or bridging nitrosyl groups. However, these structures are analogous to those previously predicted for the isoelectronic binuclear nickel carbonyls Ni₂(CO)_n+2. Thus the lowest energy structures of Co₂(NO)₂(CO)₅, Co₂(NO)₂(CO)₄, and Co₂(NO)₂(CO)₃ have one, two, and three bridging groups, respectively, just like the isoelectronic Ni₂(CO)_n+2. For Co₂(NO)₂(CO)₅ and Co₂(NO)₂(CO)₄ the otherwise analogous structures with bridging carbonyl and/or nitrosyl groups are of very similar energies. However, for Co₂(NO)₂(CO)₃ there appears to be a particularly stable triply carbonyl bridged structure with an unusually short CoCo distance consistent with the formal triple bond required to give the cobalt atoms the favored 18-electron configuration.