ASSOCIATIVE REACTIONS OF METAL-CARBONYL CLUSTERS - SYSTEMATIC KINETIC-STUDIES OF SOME RUTHENIUM AND OTHER CLUSTERS

Rate constants for associative reactions of metal carbonyl dusters with a range of P-donor nucleophiles of various basicities and sizes can always be fitted successfully to the equation log k(2) = alpha+beta(pK(a)'+4) + gamma(theta-theta(th))lambda. The evolution of this equation, and the use of the electronic parameter pK'(a) and steric parameter theta assigned to particular P-donor nucleophiles, is outlined. lambda is a switching function which is zero when theta less than or equal to theta(th), (the steric threshold) and becomes unity when theta > theta(th). The data can be fitted to the equation by a least-squares programme developed initially by Professor W.P. Giering. The significance of the parameters alpha (defined as the standard reactivity of the carbonyl), beta (the susceptibility of the carbonyl to the sigma basicity of the nucleophiles), theta(th) (the Tolman cone angle of the nucleophile where steric effects begin to be apparent, i.e. the steric threshold) and gamma (the sensitivity of the rates to the cone angles of the nucleophiles when theta > theta(th)) are discussed in detail and their interrelationships considered. In particular the sharpness of the steric threshold is taken to imply that a substrate carbonyl undergoes an isomerization reaction when approached by the nucleophile. This produces a so-called transition state isomer (TSI) that contains a roughly conical opening into which all nucleophiles with theta less than or equal to theta(th) can fit without steric difficulty. When theta > theta(th) the nucleophiles can attain the same metal-P-donor bond distance in the transition state if the TSI opens up further, so that gamma is a measure of the flexibility of the TSI. Alternatively the flexibility of the TSI may be so low that it cannot accommodate large nucleophiles unless they penetrate less deeply into the space in the TSI. The application of this systematic kinetic approach to the delineation of the dynamic character of metal carbonyl clusters is illustrated by examples, many of them unpublished, that involve Ru-3(CO)(11)L clusters (where the effect of varying the P-donor substituent is probed), some Rh-4 and Ir-4 clusters, some high nuclearity Ru and Fe clusters, and a variety of osmium clusters. Not only are the clusters characterizable in this way but also the actual mechanistic paths followed, and the products formed, can undergo major changes which depend mainly on the size of the nucleophiles.