Vanadium-51 NMR chemical shifts δV for a variety of VO3+ compounds have been found to range from —432 ppm (liquid VOBr3) to + 786 ppm (VOF3/CH3CN) relative to a liquid VOCl3 standard. The δV values and hence the shieldings of the vanadium nucleus are shown to increase in the order VOBr3 ≪ VOCl3 < VO(NEt2)3 < VO(OR)3 < VOF3 (1); VOCl3 < VOCl2(OEt) < VOCl(acac)2 < VOCl(OEt)2 < VO(OEt)3 (2); VOCl2(OMe) < VOCl2(OEt) < VOCl2(OPri) < VOCl2(OBut) (3); VO(OEt)3 < VO(OBun)3 < VO(OPrn)3 ≲ VO(OBui)3 < VO(OPri)3 (4). These trends are elucidated by considering the energy separation between highest occupied and lowest unoccupied molecular orbitals (1,2), inductive and π-transmittance of electron density (1, 2), and hindered σ-donation and expansion of the coordination sphere due to ligand bulkiness (3, 4). High solvent shifts in THF solution suggest complete or partial removal of the halogen from the VO3+ moiety for VOBr3 and VOCl2(OBut). For other compounds, solvent shifts of approximately 30 ppm are likely caused by solvation effects. In the case of VOCl3, an equilibrium between solvated VOCl3 and a species containing THF molecules directly bonded to VO3+ is proposed. VOCl3 · 2 RCN (R = Me, Ph) in THF show intact VOCl3 units : solvent-solute interaction is restricted to the second coordination sphere.
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Dieter Rehder (1977) studied this question.