Potential Energy Surfaces in Transition States for Associative Reactions of Metal Carbonyl Clusters:  Reactions of Rh4(CO)12 with P-Donor Nucleophiles

The metal carbonyl cluster Rh4(CO)12 reacts with a wide variety of P-donor nucleophiles solely by an associative mechanism, and the rate constants can be analyzed quantitatively according to the electronic and steric properties of the nucleophiles by the QALE method. An unexpected outcome of this analysis is that inclusion of what has become known as the “aryl effect” is clearly necessary, together with positive contributions to the rates of effects due to the π-acidity of the phosphite nucleophiles. A simple way of representing the individual contributions of the various effects to the overall rates is described, and the general σ-basicity and steric effects can be represented graphically by a three-dimensional “log k 2 surface” upon which the aryl and π-acidity effects can separately be superimposed as additional peaks. A free energy surface can be obtained by a simple scale change.