Unsaturation in trinuclear cobalt carbonyl compounds of the type ECo 3(CO) n (E = CH, CF, P, As; n = 9, 8, 7, 6) with Co 3E tetrahedrane structures

Abstract

Theoretical studies on the known trinuclear cobalt carbonyl derivatives ECo 3(CO) 9 (E = CH, CF, P, As) predict structures with carbonyl groups bridging each edge of the Co 3 triangle in contrast with experiment where structures with all terminal carbonyl groups are found in all cases. However, the energy differences are predicted to be rather small ranging from 4 ± 2 kcal/mol for FCCo 3(CO) 9 to 10 ± 3 kcal/mol for AsCo 3(CO) 9. The global minima for the unsaturated ECo 3(CO) n ( n = 8, 7, 6) derivatives generally have two (for n = 8) or three (for n = 7 and 6) carbonyl groups bridging the edges of the Co 3 triangle. However, structures with all terminal carbonyl groups are also found in all cases as well as higher energy structures in which one of the carbonyl groups bridges all three cobalt atoms. The fluoromethinyl derivatives FCCo 3(CO) n ( n = 9, 8, 7) are anomalous since their unbridged structures or structures with a carbonyl group bridging all three cobalt atoms are closer in energy to the doubly or triply bridged global minima than is the case for the other ECo 3(CO) n derivatives.