SYSTEMATIC KINETICS OF HIGH NUCLEARITY METAL-CARBONYL CLUSTERS - ASSOCIATIVE SUBSTITUTION-REACTIONS OF RU6C(CO)17 WITH P-DONOR NUCLEOPHILES

The high nuclearity carbonyl cluster (HNCC) Ru6C(CO)17 has been shown to react with a wide variety of P-donor nucleophiles exclusively by associative processes. The rate constants, k2, can be subdivided into electronic and steric contributions which depend on the sigma-donicity (pK(a)' values) and size (Tolman cone angles, theta) of the P-donor nucleophiles. The data fit well to the equation log k2 = alpha + beta(pK(a)' + 4) + gamma(theta - theta(th))lambda. The standard reactivity for a weak and small hypothetical nucleophile of pK(a)' = -4 is defined by log k2-degrees = log k2 - beta(pK(a)' + 4) and shows an exceptionally high value of 1.51 +/- 0.26. The electronic sensitivity (beta = 0.41 +/- 0.04) is also very high and indicates a high degree of bond making in the transition state. The steric threshold, theta(th), below which no steric effects are apparent (switching function (lambda) = 0) is 119-degrees, showing that the transition states are quite congested as expected from the high degree of bond making. These transition states are proposed to contain a well-defined Ru6C(CO)17 moiety that is isomeric with the ground-state cluster and which is opened up to an extent that it can accommodate all nucleophiles of various sizes up to the cone angle 119-degrees, defined by theta(th), without any steric repulsions. Above the steric threshold, when lambda = 1, the flexibility of the transition state is found to be exceptionally low as indicated by the value gamma = -0.20 +/- 0.01 deg-1 which means that the rate of the reaction will decrease by 37% for each degree increase in the cone angle of the incoming nucleophile when electronic effects are absent or constant. This steric effect leads to values of k2-degrees that vary by 10(10) over the theta-range 119-170-degrees. Values Of DELTA-H-2 double dagger and DELTA-S2 double dagger are low (7 to 13 kcal mol-1) and very negative (-10 to -36 cal K-1 mol-1), respectively, again showing the exceptionally high degree of bond making in the transition state. These kinetic parameters are compared with those found for smaller clusters and a mononuclear carbonyl.