Lability of the metal-water bond in pentamethylcyclopentadienyl bipyridine aqua complexes of cobalt, rhodium and iridium: A molecular orbital study

Abstract

Metal complexes [M(Cp ∗)(bpy)(H 2O)] 2+, with M  Co III, Rh III and Ir III, have been investigated by means of molecular orbital (MO) calculations with partial inclusion of relativistic effects. Quasi-relativistic INDO/1 and effective potential ab initio methods have been used to calculate the electronic structure of these complexes. Based on various local criteria the lability of the MOH bond has ben estimated and correlated with experimental kinetic data on the anation of complexes [M(Cp ∗)(bpy)(H 2O)] 2+ by ligand X. The energy profile of a possible reaction pathway has been calculated, taking into account both [ M(Cp ∗)(bpy)] 2+ and [M(Cp ∗)(bpy)Cl(H 2O)] + as transition states. The former is clearly energetically unfavourable, whereas the latter is more probable. The rate-controlling step is the dissociation of the aqua ligand from it and the dissociation energy now correlates with the rate constant of the ligand interchange. After inclusion of the solvent effect a non-monotonous trend was obtained ΔE ∗ ( X  Cl − ) = 2.60, 2.44 and 2.69 eV versus ln k i (298 K, averaged) = −0.51, 7.37 and 5.39 for M  Co, Rh and Ir, respectively.