The kinetics and mechanisms of some reactions of cobalt (III) bis (ethylenediamine) complexes

Abstract

A recent report on the base hydrolysis of trans [Co(en)2(OH)Cl]+ has questioned the interpretation of earlier work and presented a new mechanism from which it may be predicted that exchange should occur between free chloride ions and the complex. A re-examination 36 of this reaction using Cl has shown that little or no exchange occurs even after considerable reaction time. The isomerisation of the [Co(en)2(OH)2]+ cation has been studied in highly basic media. The results are explained in terms of an intramolecular process, with the complex 'twisting1 about four imaginary C3 axes. The isomerisation of [Co(en)2(OG2)2]3+ in dilute and concentrated acid appears to proceed through a trigonal bipyramidal intermediate. In concentrated acid the isomerisation rate is greatly increased compared with that found in dilute acid solution. This increase is discussed in terms of the greater attraction of the protons to the coordinated water ligands, weakening the cobalt-oxygen bond and increasing the rate of formation of the reactive intermediate. The activation enthalpies in dilute and concentrated acid reflect this lowering of the energy required to form the intermediate, -1 being 28.0 and 25.5 kcal.mole respectively. The racemisation of the optically active cis[Co(en)2(0H2)2]3+ in dilute acid, appears to proceed via the cis to trans isomerisation, since the rate is similar to the rate of isomerisation. The enthalpies of activation for the two reactions are almost identical being 29.4 kcal.mole-1 for the two reactions are almost identical being 29.4 lcal.mole-1 for isomerisation and 29.2 kcal.mole for racemisation. In the isomerisation of the [Co(en)2(OH2)NCS]2+, the disproportionate increases in the rate with increasing acid concentration are found at > 6M HClO4. This lower acid concentration value is thought to be due to the orientation of water molecules around the thiocyanato ligand. Preliminary studies of the equilibria in the reactions between [Co(en)2S03(0H2)] + and Cl-, Br-, and NCS- ions have shown that the equilibrium constants are approximately in the order predicted by the theory of hard and soft acids and bases.