Homogeneous catalysis on metal clusters. The isomerization and selective hydrogenation of dienes, alkenes, and alkynes in the presence of [(η-C5H5)NiOs3(µ-H)3(CO)9] and [(η-C5H5)NiOs3(µ-H)3(CO)8L][L = PPh2H or P(C6H4Me-o)3]. Spectroscopic identification of the reaction intermediates and X-ray structure of [Os3(µ-H)(CO)9(MeCCHCMe)]