Pressure as a kinetic parameter in mechanistic studies of chemical reactions induced by flash photolysis and pulse radiolysis

A detailed outline is given of how pressure can be used as a mechanistic indicator in kinetic studies of chemical reactions in solution that are induced by flash photolysis and pulse radiolysis. These techniques have mainly been applied to tlie study of organometallic and bioinorganic reactions including processes such as ligand substitution, binding of small molecules, formation and breakage of metal-carbon bonds, 0-elimination and electron transfer. Typical examples are presented and an account of our most recent work i n this area is given. INTRODUCTION The application of high pressure kinetic techniques in the study of inorganic, organometallic and bioinorganic reaction mechanisms in solution, has received significant attention from numerous groups over the past decade (ref. 1 to 4). Such studies have added a further dimension to mechanistic investigations by introducing pressure as an additional, and in many cases a decisive, parameter for the elucidation of the underlying reaction mechanism. The fundamental principles involved, the determination of activation and reaction volumes, the construction of reaction volume profiles, and the interpretation of the observed volume changes have been treated in detail elsewhere (ref. 1 to 4) . The basic idea is that the volume of activation represents the change in partial molar volume on going from the reactant to the transition state of the process, such that the reaction voluine profile can be analyzed in t e r m of volume changes along the reaction coordinate. In this contribution we will focus on examples mainly from organoinetallic and bioinorganic chemistry where the combination of high pressure kinetic and flash-photulysis of pulse-radiolysis techniques has been eniployed i n an effort to improve our understanding of the intimate mechanism of the process. Work performed by our own group has benefitted greatly from intensive collaboration with other groups mentioned in the cited references. In the remainder of this contribution examples for various types of reactions will be presented, followed by ;I few conclusive remarks. LIGAND SUBSTITUTION REACTIONS Ligand substitution reactions of transition metal complexes have been the topic of many mechanistic investigations because of the fundamental importance of such reactions in many chemical and biochemical processes. There are basically three simple pathways along which ligand substitution reactions can occur: the dissociative (D) process with an intermediate of lower coordination number: the associative (A) process with an intermediate of higher