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, have central Co4 butterflies with five bonding Co-Co interactions rather than Co4 tetrahedra with six bonding Co-Co interactions. Slightly higher energy Co4(CO)n (n = 10, 9) butterfly structures are found with a unique μ4-CO group bridging all four cobalt atoms.