Kinetic studies of the addition reactions of a large number of P-donor nucleophiles to the unsaturated cluster (U~-H)ZOS~(CO)~O were performed as a function of pressure and temperature. The second-order rate constants, kz, for reactions in chlorobenzene at 30 "C follow the same empirical equation, log k2 = a + j3(p&' + 4) + y(6 -&)A, that has been reported elsewhere for reactions in heptane. p & ' represents the a-donicity of the nucleophiles in nonpolar solvents and 6 the Tolman cone angle for the individual P-donors. No steric effect was observed for ligands with cone angles below the steric threshold (0 I 6th = ca. 147"; A = 0). For larger ligands ( 6 > 6th; A = 1) k2 is strongly affected by the size of the nucleophile, as shown by the large, negative y value of -0.18 f 0.02 deg-l. Reactions with PPb in heptane and nitromethane show that the rate constant is independent of the polarity of the solvent at ambient temperatures. In chlorobenzene as solvent the rate constants are slightly smaller, possibly due to the solvent donor properties, but the j3 and y values and, therefore, the transition states of the addition step remain essentially unchanged. The volumes of activation, AV, determined in chlorobenzene are all negative. They show no significant dependence on the p&' values, which indicates that the volume collapse is controlled by steric rather than electronic factors. The AVI values show a remarkable correlation with the cone angles. For 6 < 160", AV decreases linearly (R = 0.91) with tan2-(6/2), 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 estimate to be made of the penetration (ca. 24 pm) of the P-donor ligands inside the periphery of the cluster in the transition state. Extrapolation to tan2(6/2) = 0 shows that formation of the cluster moiety in the transition state unexpectedly involves a compression of the ligand polytope. At 6 = 160" there appears to be a new type of steric threshold (defined in terms of AV rather than log k2 or AG*) above which the A V 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.

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