Abstract Theoretical studies on the known trinuclear cobalt carbonyl derivatives ECo3(CO)9 (E = CH, CF, P, As) predict structures with carbonyl groups bridging each edge of the Co3 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 FCCo3(CO)9 to 10 ± 3 kcal/mol for AsCo3(CO)9. The global minima for the unsaturated ECo3(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 Co3 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 FCCo3(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 ECo3(CO)n derivatives.