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

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 O=ML4, ore-complexes [M=Re(v) and Tc(v)] react easily with oxygenated ligands of different basicities (H2O, RO-, ArO-, RCOO- etc.) to give quasi-octahedral O=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 v(M=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 O=ML4-OR --> O=ML4 + OR of the complex associating rather similar O=ML4 accepters with a series of OR Ligands having quite different donor properties. Assuming that the ligand pK(a) (or related Delta G degrees) 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, a posteriori, against experimental values of energies, bond distances and quadratic vibrational force constants.