ACTIVATION PARAMETER CORRELATION FOR NUCLEOPHILE ADDITION TO (MU(2)-H)(2)OS-3(CO)(10)

Kinetic studies of the addition reactions of a large number of P-donor nucleophiles to the unsaturated cluster (mu(2)-H)2(O)s(3)(CO)(10) wer e performed as a function of pressure and temperature. The second-orde r rate constants, K-2, for reactions in chlorobenzene at 30 degrees C follow the same empirical equation, log k(2) = alpha + beta(pK(a)' + 4 ) + gamma(theta - theta(th))lambda, that has been reported elsewhere f or reactions in heptane. pK(a)' represents the sigma-donicity of the n ucleophiles in nonpolar solvents and theta the Tolman cone angle for t he individual P-donors. No steric effect was observed for ligands with cone angles below the steric threshold (theta less than or equal to t heta(th) = ca. 147 degrees; lambda = 0). For larger ligands (theta > t heta(th); lambda = 1) k(2) is strongly affected by the size of the nuc leophile, as shown by the large, negative gamma value of -0.18 +/- 0.0 2 deg(-1). Reactions with PPh(3) in heptane and nitromethane show that the rate constant is independent of the polarity of the solvent at am bient temperatures. In chlorobenzene as solvent the rate constants are slightly smaller, possibly due to the solvent donor properties, but t he beta and gamma values and, therefore, the transition states of the addition step remain essentially unchanged. The volumes of activation, Delta V-double dagger, determined in chlorobenzene are all negative. They show no significant dependence on the pK(a)' values, which indica tes that the volume collapse is controlled by steric rather than elect ronic factors. The Delta V-double dagger values show a remarkable corr elation with the cone angles. For theta < 160 degrees, Delta V-double dagger decreases linearly (R = 0.91) with tan(2)-(theta/2), a measure of the relative volumes of the nucleophiles. This suggests that the Os ...P bond lengths in the transition states are the same for all these nucleophiles, and the slope of the correlation enables a rough estimat e to be made of the penetration (ca. 24 pm) of the P-donor ligands ins ide the periphery of the cluster in the transition state. Extrapolatio n to tan(2)(theta/2) = 0 shows that formation of the cluster moiety in the transition state unexpectedly involves a compression of the ligan d polytope. At theta = 160 degrees there appears to be a new type of s teric threshold (defined in terms of Delta V-double dagger rather than log k(2) or Delta G(double dagger)) above which the Delta V-double da gger values become less negative, the penetration of the nucleophiles becomes less, and the Os...P bond lengths become progressively longer. Other correlations of the activation parameters are considered.