Use of QALE to generate potential energy surfaces in transition states: associative reactions of Ru3(CO)12 with P-donor nucleophiles

Abstract

Previous studies of associative reactions of Ru₃(CO)₁₂ have been greatly extended to include a total of 23 P-donor nucleophiles with widely differing electronic and steric properties. Application of standard QALE methodology (QALE = quantitative analysis of ligand effects) enabled the rate constants to be analyzed according to the electronic and steric properties of the nucleophiles. It was unexpectedly found necessary to include what has become known as the aryl effect in this analysis, together with a positive contribution to the rates due to the π-acidity of phosphite nucleophiles. Also the analysis was found to be quite sensitive to the cone angle taken for P(n-Bu)₃ and the value 136°, favoured by Giering and Prock, was found to be superior to Tolman's value of 132°. The individual contributions of the various effects to the rate constants for each ligand are presented as absolute contributions to ΔG^‡ or fractional contributions to the rate constants. σ-Basicity and steric effects are represented graphically by a three-dimensional ‘logk₂ surface’ to which the aryl and π-acidity effects can be added and appear as peaks. This logk₂ surface can be converted into a free energy or potential energy surface by a simple scale change for the y-axis. However, a free energy surface would be difficult to represent, unless minus ΔG^‡ was plotted on the y-axis, because ‘negative spikes’ would represent a graphical problem.