The reactivity of the coordinatively unsaturated methylene complex Ir=CH2[N(SiMe(2)CH(2)PPh(2))(2)] With olefins is described. In all cases, carbon-carbon bond formation is observed with the isolation of Ir(III) allyl hydride derivatives Ir(eta(3)-C(3)H(4)R)H[N(SiMe(2)CH(2)PPh(2))(2)] (R = H, CN, Ph, and CO(2)Me). With ethylene, exo and endo allyl hydride isomers are formed; with acrylonitrile, anti-exo and syn-endo diastereomers result and with both styrene and methyl acrylate, only the corresponding syn-endo derivative is isolated. The reaction is facilitated by electron-withdrawing olefins since propene, ethyl vinyl ether and norbomene fail to react with the methylene complex. Variable-temperature NMR studies have provided some information on the mechanism of the reaction; it is suggested that the olefin approaches the iridium-methylene complex to generate a perpendicular binding of the olefin which then undergoes C-C bond formation via migratory insertion to generate a highly puckered iridacyclobutane. Rapid beta-elimination results in the formation of an isomeric allyl hydride species (detected by NMR spectroscopy) that isomerizes to the observed final products. Molecular orbital calculations (INDO-1 using CAChe Worksystem) support the idea that the olefin binds to the metal perpendicular to the pi-system of the iridium-carbon double bond. Suggestions are made regarding the observed dependence on the nature of the metal center to direct the reaction to form allyl hydride derivatives or undergo olefin metathesis. Ir(eta(3)-C3H4CN)H[N(SiMe(2)CH(2)PPh(2))(2)], 2b, crystallizes as a (1:l) toluene solvate, monoclinic, a = 10.884(1) Angstrom, b = 18.672(2) Angstrom, c = 20.169(3) Angstrom, beta = 100.32(2)degrees, Z = 4, space group P2(1)/c; and crystals of Ir(eta(3)-C(3)H(4)Me)H[N(SiMe(2)CH(2)PPh(2))(2)], 3e, are also monoclinic, a 13.606(2) Angstrom, b = 14.687(4)Angstrom, c = 17.672(3)Angstrom, beta = 100.86(1)degrees, Z = 4, space group P2(1)/n. The structures were solved by Patterson methods and were refined by full-matrix least-squares procedures to R = 0.032 and 0.034 (R(w) = 0.028 and 0.033) for 6304 and 4226 reflections with I greater than or equal to 3 sigma(I), respectively.