Oxidatively induced alkyne rotation in dicobalt complexes: structural tests of molecular orbital theory

The complexes [Co₂(CO)₂(µ-RC₂R)(µ-dppm)₂]1(R = Me, Ph or CO₂Me; dppm = Ph₂PCH₂PPh₂) undergo reversible one-electron oxidation at a platinum-bead electrode in CH₂Cl₂; chemical oxidation of 1 with the ferrocenium ion or with iodine gives stable salts of the monocations 1⁺. The X-ray structures of 1(R = Me and Ph) show the alkyne bridge to be orthogonal to a cobalt–cobalt bond which is also bridged by the two dppm ligands. The overall structure of 1⁺(R = Me), as its [PF₆]^– salt, is similar but with the alkyne bridge rotated by 12° about an axis perpendicular to the shortened metal–metal bond. The structural changes observed provide direct experimental support for previous bonding studies of the structural preferences of hydrocarbon-bridged M₂(CO)₆ complexes.