Transition metal polyhydride complexes. 10. Intramolecular hydrogen exchange in the octahedral iridium(III) dihydrogen dihydride complexes IrXH2(eta(2)-H-2)(PR3)(2) (X = Cl, Br, I)

Density functional calculations (B3LYP) on IrXH2(eta(2)-H-2)(PR3)(2) for X = Cl, Br, I and R = H, Me and inelastic neutron scattering studies for X = Cl, Br, I and R = Pr-i are used to elucidate the mechanisms for the intramo lecular dihydrogen/hydride exchange. The two lowest energy processes are ro tation of the dihydrogen ligand and oxidative addition of the dihydrogen to form an intermediate Ir(V) tetrahydride, which undergoes rapid reductive e limination to interchange the dihydrides and the dihydrogen. The use of PMe 3 as a model phosphine is essential to bring the calculated barriers for th e dihydrogen/hydride interchange into agreement with the experimental obser vations. The activation energy for site exchange (1.9 kcal/mol) is found to be in excellent agreement with the experimental result obtained for X = Cl (1.5(2) kcal/mol), and the calculations show a slight decrease in this val ue from X = C1 to I. Comparison between calculated rotational barriers (0.3 to 0.7 kcal/mol) and experimental values obtained for IrXH2(eta(2)-H-2)(PR 3)(2) (X = Cl, Br, I; R = Pr-i) (0.5 to 1.0 kcal/mol) also demonstrates tha t the quantitative estimate of the barrier to rotation requires PMe3 as the minimal model ligand.