The lowest energy structures of the unsaturated Co 4 (CO) n (n=10, 9) clusters, potentially obtained by decarbonylation of the known Co 4 (CO) 12 , are predicted by density functional theory (DFT) to have central Co 4 butterflies with five bonding Co–Co interactions rather than Co 4 tetrahedra with six bonding Co–Co interactions. Both face-bridging μ 3 -CO and edge-bridging μ-CO groups are found in such structures. Slightly higher energy Co 4 (CO) n (n=10, 9) butterfly structures are found with a unique μ 4 -CO group bridging all four cobalt atoms similar to the lowest energy Co 4 (CO) 11 structure from a previous DFT study. The lowest energy Co 4 (CO) n (n=10, 9) structures differ significantly from their Ir 4 (CO) n analogues, which mainly have central Ir 4 tetrahedra rather than Ir 4 butterflies and only one or two edge-bridging μ-CO groups.