The oxidative addition of H-H to iridium(l) trimethylphosphine complexes was studied and the reactivity of the resulting water soluble dihydrido iridium (III) complexes was investigated. One Dihydrido iridium(III) complex, mer-Ir(H)₂(PMe₃)₃CI <b>(2)</b> was characterized by ¹H, ³¹p and ¹³C NMR spectroscopy, CH analysis and single crystal xray diffraction. <b>(2)</b> reacted with the strong sigma donor PMe₃ to form [IrH2(PMe₃)4]CI <b>(3b)</b> without having to remove the chloride ligand. <b>(2)</b> reacted with monosubstituted acetylenes and an internal diyne in water to form the vinyl complexes Ir(H)(C(R')=C(H)-(R))(PMe₃)₃CI (R = CMe₃, C₆H₅, SiMe₃, R' = H, <b>(4)-(6);</b> R' = CCMe₃, R= Me, <b>(7)). (2)</b> also reacted in water with ethynylpyridine to form an unusual metallacycle "trimer" <b>(8)</b>. A mechanism for tltrimerizationtl is proposed. <b>(2)</b> also reacted with ethylene in water to form the diethyl iridium complex Ir(CH₂CH₃)₂(PMe₃)₃CI <b>(8)</b>. Encouraged by these results, catalytic hydrogenation of unsaturates in water was investigated. It was found that <b>(2)</b> is active as a water soluble catalyst for hydrogenation of unsaturates. A mechanism for catalytic hydrogenation was proposed. / Ph. D.