Substituent effects on associative reactions of the clusters Ru5C(CO)14L (L = P(OPh)3 and P(C6H11)3) with phosphorus-donor nucleophiles

The substitution reactions of the high-nuclearity carbonyl clusters Ru5C(CO)14L (L = P(OPh)3 and PCy3) with a series of P-donor nucleophiles in heptane have been shown to proceed readily by associative pathways. The effects of the different electronic and steric properties of the nucleophiles at 25°C can be separated quantitatively in a way that has been previously shown to be widely successful. Comparing the fastest and slowest reactions for L = P(OPh)3 shows that increasing nucleophile basicity increases the rates by a factor of ca. 20, but this is offset by a 2800-fold retardation due to steric effects. These show up even for the smallest nucleophile etpb (P(OCH2)3CEt) so that no steric threshold is observed, but the decrease in rates with increasing nucleophile size is not exceptional. The substituents reduce the rates of reaction with etpb at 25°C by 750 (L = P(OPh)3) and 3 × 10⁴ (L = PCy3) compared with the unsubstituted cluster. These steric effects act to prevent a high degree of bond making in the transition states. The temperature dependence of many of the reactions have been studied, and an overall unfavorable increase of 16 kcal mol^-1 in ΔH‡2 is overcome by a favorable increase of 19 kcal mol^-1 in TΔS‡2, so the relative rates with different nucleophiles are controlled largely by entropic factors. This causes the relative rates to converge as temperatures decrease, and the implications of this type of behavior in the study of linear free energy effects can be serious. Quite unexpected effects of temperature on the separate sensitivity of the rates to the electronic and steric properties of the nucleophiles are revealed. Thus, the small favorable electronic effect is almost temperature independent and, therefore, based mainly on entropic factors, and the much larger and unfavorable steric effects are due to favorable enthalpic contributions that are overcome by unfavorable entropic contributions. Some suggestions regarding the geometries of the reaction paths are offered.