Mapping of reaction pathways by structure correlation methods. A study of the ligand dissociation reaction in quasi-octahedral Re(V) and Tc(V) oxo-complexes.

Abstract

Intercorrelations among geometrical parameters of a molecular fragment as found in different crystal structures are called structure correlations. Such correlations are believed to represent possible reaction pathways mapping the course of chemical reactions. Pyramidal Oâ–·ML₄ oxo-complexes [MRe(V) and Tc(V)] react easily with oxygenated ligands of different basicities (H₂O, RO^-, ArO^-, RCOO^-etc.) to give quasi-octahedral Oâ–·ML₄OR addition compounds which are often observed in the crystalline state and a relatively large number of structural and spectroscopic data on such complexes are available. Coordination changes from square-pyramidal to quasi-octahedral caused by the approach of the sixth ligand are found to induce systematic variations in the polyhedron geometry which are found to correlate with IR ν(Mâ–·O) stretching frequencies and pK_a values of the entering ligands. According to structure correlation methods, each fragment geometry was assumed to represent a point along a single reaction pathway of the dissociation reaction Oâ–·ML₄–OR→Oâ–·ML₄+OR of the complex associating rather similar Oâ–·ML₄ acceptors with a series of OR ligands having quite different donor properties. Assuming that the ligand pK_a (or related ΔG°) values can be considered as a measure of the relative thermodynamic stabilities of the complexes, a mathematical model of the reaction pathway is proposed which, on the grounds of the Marcus rate-equilibrium theory, relates activation free energies, thermodynamic stabilities, and geometrical distances from the reaction transition state. The reliability of the model is tested, aposteriori, against experimental values of energies, bond distances and quadratic vibrational force constants.