Electronically unsaturated mixed-metal cluster complexes containing platinum with bulky phosphine ligands have been shown to add hydrogen readily at room temperature. The reactivity of some unsaturated new platinum-rhenium complexes toward hydrogen will be described. The mechanisms of hydrogen activation have been investigated by theoretical methods. Some new pentanuclear platinum-osmium cluster complexes have also been shown to add hydrogen, reversibly, at room temperature by opening and reclosing of some of the Pt – Os bonds. It has also been shown that the Pt(P-t-Bu₃) group activates metal-hydrogen bonds toward alkyne insertion reactions in certain mononuclear metal complexes. Some di- and trimetallic ruthenium-tin and platinum-ruthenium-tin cluster complexes have been shown to be precursors to multi-metallic nanoparticles when deposited on mesoporous supports. These nanoparticles have been shown to be superior catalysts for the selective hydrogenation of cyclododecatriene to cyclododecene and hydrogenation of dimethylterephthalate to cyclohexanedimethanol.