Four- and Five-Coordinate Oxovanadium(V) Alkoxides: Do Steric Effects or Electronic Properties Dictate the Geometry?

Sterically hindered ligands generally form metal complexes with lower coordination numbers than less hindered ligands. In contrast to dogma, the solid state vanadium(V) complexes with ethylene glycol and pinacol contain four- and five-coordinate vanadium atoms, respectively. Ab initio and electrostatic potential distribution calculations were conducted on both experimental and optimized geometries of the four- and five-coordinate oxovanadium chloroalkoxides. Ab initio energy calculations favor the five-coordinate species for all ligands at all levels of theory examined. No significant differences in the electrostatic properties of the vanadium atoms in the two types of molecules were observed by using electrostatic potential distribution analysis. Methyl group substitutions on the ligand did not change the electronics of the vanadium atoms sufficiently to be a factor. Thus, we conclude that neither electronics nor sterics at the metal center explains the experimental geometries. In the absence of a significant observable electronic effect at the vanadium, we note that the experimental observations can be attributed to ligand geometric effects such as the Thorpe-Ingold and/or gem-dialkyl effects.