This study presents an experimental and computational study on ligand L (N,N-diisopropyl-N’-pyridine- 2-yl(methyl)ethane-1,2-diamine) and the mixed chelate copper(II) complexes of [Cu(L)(Cl-acac)](ClO4), 1 and [Cu(LH)(Cl-acac)(ClO4)]ClO4, 2 , in which Cl-acac stands for 3-chloroacetylacetone. The ligand L and complexes were characterized by elemental analysis, IR, molar conductivity measurements and single crystal X-ray analysis. The ligand L in [Cu(L)(Cl-acac)](ClO4), 1 acts as a tridentate ligand, while in 2 it serves as bidentate ligand due to protonation of one of the coordinating atoms. The geometry around the copper(II) is distorted trigonal bipyramidal in 1 but 2 is almost square pyramidal. The square pyramidal geometry of compound 2 made it a good solvatochromic probe with the color change range from green to blue in 14 organic solvents. Its solvatochromism was studied in 14 organic solvents, which showed a positive trend with increasing donor power of the solvents. A multi-parametric equation was used to describe the solvent effect on the d–d transition of the complex using SPSS/PC software. Statistical analysis using stepwise multiple linear regression (SMLR) showed that the solvent donor number (DN) model plays a significant role in the observed salvochromism. The positive solvatochromism is due to the coordination of the solvent molecules with different donor power to the axial positions of the copper(II) ion by replacing the weakly coordinated perchlorate ion. To study the electronic structure of the complex and examine the proposed mechanism the density functional theory (DFT) and time dependent DFT calculations were used, respectively. The computational results in three solvents with different donor numbers support the proposed solvatochromism mechanism
