LABILITY OF THE METAL-WATER BOND IN PENTAMETHYLCYCLOPENTADIENYL BIPYRIDINE AQUA COMPLEXES OF COBALT, RHODIUM AND IRIDIUM - A MOLECULAR-ORBITAL STUDY

Metal complexes [M(Cp)(bpy)(H2O)](2+), with M = Co-III, Rh-III and Ir -III, have been investigated by means of molecular orbital (MO) calcul ations with partial inclusion of relativistic effects. Quasi-relativis tic INDO/1 and effective potential ab initio methods have been used to calculate the electronic structure of these complexes. Based. on vari ous local criteria the lability of the M-OH bond has been estimated an d correlated with experimental kinetic data on the anation of complexe s [M(Cp)(bpy)(H2O)](2+) by ligand X. The energy pro file of a possibl e reaction pathway has been calculated, taking into account both [M(Cp )(bpy)](2+) and [M(Cp*)(bpy)Cl(H2O)](+) as transition states. The for mer is clearly energetically unfavourable, whereas the latter is more probable. The rate-controlling step is the dissociation of the aqua li gand from it and the dissociation energy now correlates with the rate constant of the ligand interchange. After inclusion of the solvent eff ect a non-monotonous trend was obtained Delta E (X = Cl-) = 2.60, 2.4 4 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. Copyright (C) 1996 Elsevier Scien ce Ltd.