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

Abstract

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.

The Chemistry and Biochemistry of Vanadium and the Biological Activities Exerted by Vanadium Compounds

“…Spectroscopic, 307,382,383 theoretical, 96,384 and solid-state model studies 87,355,363,364,376,377 were key in confirming that the vanadium atom was fivecoordinate. 307,383 Although this ruled out suggestions of four-and six-coordinate derivatives, three fivecoordinate structures remained.…”

“…93 A theoretical evaluation of the effects of ligands on vanadium coordination geometry in these simple systems suggested that electronic effects did not induce the coordination differences, but that subtle ligand effects such as the gem-dimethyl effect were invoked. 96 Of the numerous complexes structurally characterized, the system with the cyclopentanol ligand resulted in the vanadium atom showing differential V-O bond lengths (up to about 0.3 Å) (16) depending on whether the alkoxide group was directly bound to the vanadium or merely associated with its dinuclear partner. 88 This structure clearly reflects the favorable alkoxide association even with cyclopentanol, a sterically bulky ligand.…”

Fifteen years of dancing with vanadium

“…The first structurally characterized model systems showed the tendency of vanadium(V) to form five-coordinate alkoxides [71][72][73][74][75][76]. A vanadium(V) alkoxide formed from the reaction of pinacol with VOCl 3 provided this structural precedent for the transition-state structure used in biological studies (4) [72], modeling the vanadate-ribonuclease complex [77,78].…”

“…Indeed this replacement used by the Zubieta group provided the first structural example of a monomeric vanadium(V) alkoxide complex with four-coordinate vanadium [81]. A dinuclear complex from ethylenediol demonstrated that four-coordinate vanadium(V) was a very stable alternative to the five-coordinate form [76,82]. The vanadium(V) alkoxides associate in solution [83], but the high reactivity could be decreased if bulky ligands were employed, and a variety of such systems have been reported (see below for more details) [75,84].…”

Synthesis and characterization of some oxovanadium(V) complexes with internally functionallized oximes. Crystal and molecular structure of heptacoordinated [VOCl{ON=C(CH3) (C4H3S-2)}2] · CH3OH

“…Subtle variations in the vicinity of the small V +5 ion may result in large changes in the stability and reactivity of oxovanadium(V) complexes [1,6,[20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Oximes, including internally functionallized oximes, constitute an important class of N/O type donor ligands [34][35][36][37][38][39] because of their varying coordination abilities with the metal centers.…”