Butterfly versus tetrahedral cluster structures for the unsaturated tetracobalt carbonyls Co₄(CO)_n (n = 10, 9): Major differences between cobalt and iridium

The lowest energy structures of the unsaturated Co₄(CO) _n (n = 10, 9) clusters, potentially obtained by decarbonylation of the known Co₄(CO)₁₂, are predicted by density functional theory (DFT) to have central Co₄ butterflies with five bonding Co-Co interactions rather than Co₄ tetrahedra with six bonding Co-Co interactions. Both face-bridging μ₃-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 μ₄-CO group bridging all four cobalt atoms similar to the lowest energy Co₄(CO)₁₁ structure from a previous DFT study. The lowest energy Co₄(CO)_n (n = 10, 9) structures differ significantly from their Ir₄(CO)_n analogues, which mainly have central Ir₄ tetrahedra rather than Ir₄ butterflies and only one or two edge-bridging μ-CO groups.