The mixed-chelate copper(II) complexes [Cu(Ph-acac)(diamine)]X, where Ph-acac is 1-phenyl-1,3-butanedione, diamine is N,N-dimethyl,N'-benzyl-1,2-diaminoethane and X = ClO4-, PF6- and BF4-) have been synthesized and characterized spectroscopically (by IR and UV–Vis) and structurally (by single-crystal X-ray diffraction). The X-ray structure of [Cu(Ph-acac)(diamine)]ClO4 demonstrated that the central copper atom is placed in a square planar geometry made by Ph-acac and diamine chelates. Solvatochromism behavior of the compounds was studied in 14 organic solvents, which showed a positive trend with increasing donor power of the solvents. Multi-parametric equation has been utilized to explain the solvent effect on the d–d transition of the complexes using SPSS/PC software. The stepwise multiple linear regression (SMLR) method demonstrated that the donor power of the solvent plays the most important role in the solvatochromism of the compounds. To study the electronic structure of the complex and to confirm the mechanism of the observed solvatochromism, a computation analysis was accomplished on [Cu(Ph-acac)(diamine)]+ by the density functional theory (DFT) and time-dependent DFT calculations. DFT computational results buttressed the experimental observations and indicated that the solvatochromism phenomenon is due to coordination of the solvent molecules on the above and below of the molecular plane, which causes change in geometry of the complex from square planer to octahedron. In this process the d–d transition band of the complex moves to the red with increasing the DN of the solvent.
