An unexpected electronic preference for transfer of a β-hydrogen trans to a metal-hydride bond

Density functional calculations (R = H) on the decomposition of cis-mer-HIr(OCH₃)Cl(PR₃)₃ to cis-mer-(H)₂IrCl(PR₃)₃ via a β-H transfer process are reported. The favoured computed pathway involves transfer of a β-H to a site trans to the Ir-H bond and is consistent with that deduced experimentally (R = Me, Et). Unexpectedly however, and in contrast to earlier calculations on analogous systems, this pathway is shown to be preferred on electronic grounds and not only on steric grounds as previously thought. The β-H transfer process involves rearrangement of a π-stabilised unsaturated alkoxide intermediate with the preferred pathway being favoured by reduced interactions between trans ligands in the resultant pseudo square-pyramidal transition state.