Mechanisms of Ligand Exchange in Complexes of Cobalt, Nickel and Zinc

Abstract

<p>One of the simplest types of reactions encountered in inorganic chemistry involves the exchange of a molecule coordinated to a metal ion (the ligand) with another molecule free in solution. This type of reaction is fundamental to much of synthetic inorganic chemistry and also constitutes the building blocks from which the complex multistep mechanisms typical of homogeneous catalysis are built. There has naturally been a great deal of research carried out on ligand exchange mechanisms. However, the majority of this research has been concerned with relatively slow reactions carried out in aqueous solution. Data on rapid ligand exchange reactions in non-aqueous solutions are relatively sparse. Such reactions are in many respects, the more interesting and certainly have greater relevance to the understanding of homogeneous catalysis. In particular, comparative studies of the effect of changes of metal ion, geometry of the complex and ligand substituent are lacking. The present thesis presents the results of such a study.</p> <p>The group of compounds chosen for study are thiourea and substituted thiourea complexes of cobalt, nickel and zinc. Nuclear magnetic resonance has been used as the principal experimental technique. This technique provides information on both the solution structures of the complexes and the kinetics and mechanisms of the ligand exchange processes.</p> <p>Two introductory chapters review previous work on ligand substitution and the use of nuclear magnetic resonance to study chemical rate processes. The thiourea ligands chosen can themselves undergo a rate process involving restricted rotation about the C-N bond and a third chapter describes some preliminary experiments designed to clarify the interpretation of the ligand spectra. The cobalt complexes are paramagnetic and tetrahedral in solution. Some replacement of thiourea by solvent occurs and equilibrium constants and thermodynamic data for this process have been obtained. A detailed study of the kinetics of ligand exchange has demonstrated two competing mechanisms. Rates and activation parameters for both processes for a series of complexes have been measured. Changes in both the enthalpies of activation and the entropies of activation are important in determining differences in reaction rate among the substitution reactions.</p> <p>The nickel complexes exist in solution as equilibrium mixtures of paramagnetic tetrahedral, diamagnetic square-planar and paramagnetic octahedral compounds. These equilibria have been disentangled and the factors determining geometric structure in Ni(II) compounds are discussed. Rate and mechanism data for ligand exchange reactions are presented. An associative mechanism is demonstrated for the tetrahedral and square-planar compounds and a dissociative mechanism for the octahedral complexes. NMR studies of a further series of nickel thiourea complexes containing π-allyl ligands are described. These compounds are known to be catalytically active and the ligand exchange reactions prove to be too fast for measurement with the techniques used.</p> <p>The zinc complexes are diamagnetic and tetrahedral in solution. The small chemical shifts preclude quantitative kinetic studies but qualitative comparisons with the analogous cobalt and nickel compounds are made. All of the complexes studied are very labile by the usual criteria and the overall study has allowed a more complete discussion of the factors determining ligand exchange rates in such compounds than has hitherto been possible.</p> <p>An appendix describes some studies of complexes of diacetamide.</p>