Study of 4d and 5d transition metal cluster complexes in aiming at systematic understanding of metal-to-metal interaction.

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A number of new metal cluster complexes of heavy transition elements such as molybdenum, tungsten, ruthenium, osmium, rhenium, and platinum, have been prepared, and their structures have been determined by X-ray structural analysis. A series of doubly oxide bridged complexes include single to triple metal-metal bonded complexes including bond-order of 1.5 and 2.5 provide good correalation between bond-order and metal-metal distances. They show multi-step one-electron redox behavior, spectroelectrochemical results of which provide information on electronic states of different oxidation states. Remarkably high comproportionation constants (ca. 10^<20>) for the (III,IV) mixed valence states are noted for osmium complexes of various bridging moieties such as di (mu-O), mono (mu-O) and (mu-O) (mu-RCOO). Ruthenium and rhenium complexes with similar structures show much smaller values. Several new mixed-bridged complexes of (mu-O) (mu-RCOO) and (mu-O)(mu-RCOO)_2 bridging group have been prepared for ruthenium and osmium. They also show multi-step one-electron transfer behavior. Of particular interest is the proton-coupled redox reactions in acetonitrile as studied by adding various proton donors with different acidity. By controlling the pKa of added acid, we found the condition to show proton-coupled one-step two-electron transfer behavior. Proton-coupled redox behavior has not been observed for the di (mu-O), and mono (mu-O) complexes, indicating much higher reorganizing barrier for the protonation processes of these complexes.