Transition metal polyhydride complexes. 11. mechanistic studies of the cisto trans isomerization of the iridium(III) dihydride Ir(H)(2)(CO)L (L = C6H3(CH2P(H)(2))(2))

Density functional theory (B3LYP) has been used to investigate the mechanis m of the cia-trans isomerization in Ir(H)(2)(CO)L (L = C6H3(CH2P(H-2))(2)), a model, for Ir(H)(2)(CO)L' (L' C6H3(CH2P(i-Pr-2))(2). This complex is ini tially formed as the cis isomer by bubbling H-2 through a benzene solution of Ir(CO)L' at room temperature. Over period of 18 h hat 90 degrees C, the cis isomer quantitatively converts to the trans isomer (Organometallics 199 7, 16, 3786). Five possible mechanisms are examined in detail: (i) CO disso ciation and reassociation; (ii) CO migratory insertion into the Ir-H bond a nd subsequent elimination; (iii) Ph-H: reductive elimination to an arene in termediate and subsequent oxidative addition; (iv) phosphine dissociation, complex rearrangement, and phosphine reassociation; and (v) nondissociative trigonal twist. The preferred mechanism involves two consecutive trigonal twists in which the complex passes through a distorted octahedral intermedi ate.