The Photochemistry and Electronic Structures of Metal Carbonyl Cluster Complexes

The electronic structures and spectra of the M3(CO)12 (M = Fe, Ru, Os) complexes are discussed. Using techniques such as single crystal polarized spectroscopy and MCD spectroscopy it is shown that the two lowest energy electronic transitions in these complexes are the σ → σ* (xz (bonding) → xz (antibonding)) and σ*' → σ* (z2 (antibonding) → xz (antibonding)) transitions. Photolysis of Ru3(CO)12 leads to fragmentation of the cluster complex; however, a photochemical study of Os3(CO)12 showed that Os - CO dissociation is a more efficient photoprocess than Os - Os bond cleavage. The different photochemical behaviors of the two cluster complexes is ascribed to their different lowest excited states. The lowest excited state of Ru3(CO)12 is σ → σ*, a state which certainly leads to metal-metal bond cleavage. The σ*' → σ* state is lowest in Os3(CO)12. Electronic excitation to this state does not lead to efficient cluster fragmentation. The effects of bridging ligands on photochemical metal-metal bond cleavage were also studied. It was found that the bridging CO groups prevent photofragmentation of the (h5-C5H5)2Fe2(CO)4 molecule and also of the Fe3(CO)112- complex. In the latter molecule, Fe-CO dissociation is the primary photoprocess. In the case of (h5-C5H5)2Fe2(CO)4, the primary photoproduct is a CO-bridged dimer which has no metal-metal bond.