Thiolato-bridged dinuclear d8 iridium(I) complexes and their hydrogenation to form dihydridodiiridium(II)(Ir-Ir) complexes. Crystal and molecular structures of [Ir(.mu.-SBu-t)(CO)(P(OMe)3)]2 and of [Ir(H)(.mu.-SBu-t)(CO)(P(OMe)3)]2