Rates and activation parameters for solvent exchange with [M(PR3)2-(solvent)2H2]+(M = RhIII or IrIII, R = cyclohexyl or phenyl) cations

Abstract

Using ¹H n.m.r. line-broadening and, where possible, stopped-flow Fourier-transform n.m.r., rates and activation parameters have been determined for solvent exchange with complexes of the type [M(PR₃)₂(solvent)₂H₂]⁺(M = Rh^III or Ir^III; R = phenyl, solvent = MeCN or Me₂CO; R = cyclohexyl, solvent = MeCN) in [²H₂]-dichloromethane solution. In all cases the co-ordinated solvent molecules are in trans positions with respect to the hydride ligands, and at 298 K the various complexes have rate constants for solvent exchange (k₁) which differ almost by a factor of 10⁷; k₁(Rh) > k₁(lr) because for analogous species ΔH^‡(Rh) < ΔH^‡(lr), and the acetone solvates are much more labile than the acetonitrile complexes because of lower ΔH^‡ values. Values of ΔS^‡ are significantly positive (8–32 J K^–1 mol^–1 when M = Rh^III, 25–30 J K^–1mol^–1 when M = Ir^III) in line with a dissociative, D mechanism. Hydrogen-1 n.m.r. spectra of [M(PR₃)₂(solvent)₂H₂]⁺ and [Ir(PR₃)₂(OCMe₂)(L)H₂]⁺ ions (L = MeOH, MeCN, pyridine, styrene, or cyclo-octene) are reported and compared.