1: 1 equilibrium constants for the Lewis acid-base interaction between acetylacetonato- (N, N, N’, N’-tetramethylethylenediamine)copper(II) tetraphenylborate and tetrahydrofuran, pyridine, ethylacetate and N, N-dimethylformamide were obtained at 298 K in 1, 2-dichloroethane by u.v.-visible spectrophotometry. The optical absorption maxima, v, of the mixed-chelate copper(II) complex in various donor solvents which decrease as the axial interaction increases were also measured and it was found that no linear correlation exists between v (cm-1) and the solvent donor number. Isotropic hyperfine coupling constants of 63’65Cu, Aiso, and isotropic g values, giso, were obtained in various solvents. In general, Aiso decreases but giso increases as the axial interaction increases. The coefficient, a, of the dx2-y2 orbital in the ground state molecular orbital (B1g) was found to decrease with increasing axial interaction. The frozen solution e.s.r. spectra also gave a values similar to those obtained from the solution spectra. The relationship gƖ > g⊥was found in all solvents, indicating the presence of elongated tetragonal symmetry.