Proton-catalyzed hydrogenation of a d(8) Ir(I) complex yields a trans Ir(III) dihydride.

Hydrogenation of the (PONOP)Ir(I)CH(3) complex [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] yields the unexpected trans-dihydride species (PONOP)IrCH(3)(H)(2). Mechanistic investigations have revealed that this reaction proceeds via proton-catalyzed H(2) cleavage, a pathway that circumvents the intermediacy of the typically invoked cis-dihydride isomer. Protonation yields the cationic (PONOP)Ir(CH(3))(H)(+) complex, which is then trapped by H(2) to yield an eta(2)-H(2) complex. Deprotonation of this species yields the trans-dihydride. Intermediates in the proposed pathway have been confirmed by independent low-temperature syntheses and spectroscopic observations.