STERIC EFFECTS ON THE ANATION REACTIONS OF PENTAAMINE COMPLEXES OF CO(III)

The kinetics of the anation reactions of [Co(MeNH2)5H2O]3+ with H3PO4/H2PO4-, H3PO3/H2PO3-, Br-, Cl- and CF3OO- and those of [Co(NH3)5H2O]3+ with Br-, Cl- and CF3COO- (for the sake of comparison) have been studied at different temperatures and at I = 1.0 M (LiClO4). This is the first study on the effects of the size of the amines on anation reactions of these complexes. All results are interpreted on the basis of an I(d) mechanism. The span of Kk values for different entering ligands, decreases slightly on going from NH3 to MeNH2, and this is interpreted as the maintenance of a dissociatively activated interchange mechanism on going from NH3 to MeNH2 complexes. Activation enthalpy values decrease for each ligand (CF3COO- excepted) on going from NH3 to MeNH2. This is interpreted on the basis of a weaker ground state Co-OH2 bond for the MeNH2 complex, as compared to the NH3 analogue, that needs less energy to be stretched to the transition sate. The variation of the activation entropy values for the same entering ligand on increasing the amine size confirms this assumption. The overall activation free energies also confirm a greater steric relief, on H2O dissociation, for the MeNH2 complexes.