The thermodynamic and kinetic study of reactions involving rhodium(II) carboxylates with ligands isonicotinic acid and niacin

The kinetics and thermodynamics of the formation of 1:1 and 1:2 adducts of rhodium(II) carboxylates have been studied by the use of spectrophotometry and temperature-jump techniques. The rhodium(II) carboxylates used were rhodium(II) acetate, propionate and methoxyacetate and the ligands used were niacin, isonicotinic acid and picolinic acid. The stability constants indicate that the rhodium-ligand axial bonding involves some [pi]-interaction. Rhodium(II) methoxyacetate has been found to form the most stable adduct with isonicotinic acid which has the highest [pi]-acceptor ability of all the ligands studied. The kinetic study indicates that these reactions are describable by Eigen's mechanism. Of the three carboxylates, rhodium(II) methoxyacetate has the highest rate of reaction with all the ligands. The variation of the rate with changing R group in the parent carboxylate has been explained by the desolvating and inductive effect. The propionate group exhibits the desol tion effect by shielding the axial position of the rhodium(II) carboxylate from solvent contacts. Thus, the energy will be less in breaking the water-metal bond and will allow a hydrophilic ligand to react with a faster forward reaction rate. The inductive effect is exhibited by the methoxyacetate group which induces the lowest electron density on the two rhodium atoms, thus allowing the water molecules solvating the axial positions to be rapidly replaced by an incoming electron-donor ligand.