Philip L. W. Tregenna-Piggott, Stephen P. Best, Hans U. Güdel, Høgni Weihe and Chick C. Wilson
Influence of the Mode of Water Coordination on the Electronic Structure of the [V(OH₂)₆]³⁺ Cation
J. Solid State Chem. 145, 460-470 (1999)                 

Abstract: The correlation between the stereochemistry and electronic structure of the [V(OH₂)₆]³⁺] cation has been examined using a variety of physical techniques in conjunction with angular overlap model calculations. The experimental data includes the first reported high-field, high-frequency EPR study of a vanadium(III) complex which enables a precise determination of the ground term spin-Hamiltonian parameters. The electronic structure and vibrational spectrum of the [V(OH₂)₆]³⁺ cation was studied in samples formed from the co-crystallization of RbV(SO₄)₂·12H₂O and RbGa(SO₄)₂·12H₂O. The structural modifications of these salts differ in terms of the orientation of the water molecules about the tervalent cation while the M^IIIO₆ framework remains approximately octahedral. The electronic structure of the [V(OH₂)₆]³⁺ cation in samples of Rb[Ga:V](SO₄)₂·12H₂O is found to depend greatly on the relative proportions of gallium(III) and vanadium(III). The experimental data are in accordance with predictions based on the angular overlap model when the p-bonding normal to the plane of the water molecule is dominant over the in-plane interaction (e_p  - e_p || ca. 930 cm^–1 for trigonal planar water coordination). The p-anisotropy results in a large trigonal field splitting of the ³T_1g (O_h) ground term in RbV(SO₄)₂·12H₂O (1930 cm^–1) which diminishes almost to zero when [V(OH₂)₆]³⁺ is doped into RbGa(SO₄)₂·12H₂O, on account of the change in the orientation of the water molecules imposed by hydrogen bonding constraints. This work demonstrates the strong correlation between the stereochemistry and electronic structure of the [V(OH₂)₆]³⁺ cation and accounts for the structural abnormalities reported for vanadium(III) salts of this type.