Cluster synthesis. 32. Hydrogen-rich metal cluster complexes from the reaction of octadecacarbonyltetraosmiumdiplatinum with hydrogen. Synthesis and molecular structures of PtOs5(CO)16(.mu.-H)6, Pt2Os5(CO)17(.mu.-H)6, PtOs6(CO)18(.mu.-H)8, and Pt2Os7(CO)23(.mu.-H)8