Kinetics and mechanisms of CO substitution reactions of metal carbonyls

This article mentions briefly some of our early work on the kinetics and mechanisms of CO substitution reactions of metal carbonyls, and it gives an overview of our more recent studies on three different types of systems. systems are (1) compounds where ring slippage seems to be involved in reaction, (2) heterocyclic metal carbonyls, and (3) 17-electron metal complexes. Experimental details are not given, but these are available in the referenced publications. These INTRODUCTION Ligand substitution reactions are essential for the use of transition metal organometallic compounds as homogeneous catalysts. Therefore, it is important that we know what factors affect the rates of reaction and why. is available1 on the reactivity of organometallic compounds. detailed kinetic studies that give information on the mechanisms of ligand substitution, and on what factors contribute to rates of reaction in these systems. Studies in our laboratory on three different types of systems are described here. Much information, both qualitative and quantitative, What is needed are more RING SLIPPAGE MECHANISM About three decades ago we initiated a study of CO exchange using 14C0, and of CO substitution with P-ligands on binary metal carbonyls. had been done at the time, our observations were often a source of some surprise. example, Ni(C0)h was found3 to exchange CO by a dissociative process, although one might have expected the 4-coordinate complex to readily expand its coordination number to permit a low energy associative pathway for reaction. in terms of Tolman's4 16-, 18-electron rule. require the formation of an unstable 20-electron transition state or active intermediate, whereas a dissociative process goes through a more stable 16-electron active intermediate. Imagine our surprise at the time to find5 that CoNO(C0)3 and Fe(N0)2(C0)2, isoelectronic and isostructural with Ni(C0)4, both undergo CO substitution by an associative process. interpretation given this observation, before the 16-, 18-electron rule and before the bent metal nitrosyl, was that a pair of electrons are localized on the nitrosyl group freeing a low energy metal orbital for nucleophilic attack (eq. 1). Since very little work2 of this type For Today we discuss and understand this behavior An associative substitution for Ni(CO)4 would