The 'solvatochromic' effects of a vanadyl salen complex [VOIV(salen)] in frozen solution was studied by ENDOR and DFT calculations. In a non-coordinating solvent (dichloromethane), both ENDOR and DFT were in excellent agreement on the expected square pyramidal structure, where V-IV=O is positioned out of the equatorial ligand xy-plane (as determined from calculated V...H distances). In a coordinating solvent (dimethylformamide), a subtle perturbation from the square pyramidal structure was observed, suggesting that DMF coordinates trans to the vanadyl oxo-ligand, pulling V-IV=O back into the ligand plane. The axial coordination of DMF was confirmed by ENDOR and in the DFT optimised [VOIV(salen)]-DMF complex. (C) 2003 Elsevier B.V. All rights reserved.