A kinetic and thermodynamic study of the complexation of tetra-[mu]-boxylatodirhodium(II) with some selected ligands

The kinetics and thermodynamics of the formation of 1:1 and 1:2 adducts of Rh[lowered 2](O[lowered 2]CR)[lowered 4], where R = CH[lowered 3], CH[lowered 3]OCH[lowered 2] or CH[lowered 3]CH[lowered 2], and ligands, 5'-AMP, L-histidine, DL-methionine, imidazole and pyridine, have been studied at 25[degrees]C in a phosphate buffer of ionic strength 0.1 and pH 7.40, using temperature-jump and 'entropy titration' techniques. The stability constants, enthalpies, and entropies of reactions indicate that the Rh-L (axial) bonding has some ir-interaction. Thus, rhodium(II) methoxyacetate has been found to form the most stable adduct with pyridine which has the highest ir-acceptor ability of all the ligands studied. The kinetic study indicates that the reactions with the nitrogen donor ligands are describable by Eigen's mechanism. Of the three carboxylates, rhodium(II) propionate has the highest rate of reaction with all of the ligands. The variation of the rate with changing R group in the parent carboxylate has been explained by its desolvating effect on the substrate. DL-methionine shows a large ligand effect which can not be explained by the S[lowered N]1IP mechanism.