Two new mixed-chelate dinuclear copper(II) complexes, [Cu2(tmen)2(TAP)]X2 where tmen = N,N,N′,N′-tetramethylethylenediamine, TAP= tetraacetylpropane, and X=ClO4 or BPh 4 , were prepared and characterized by physicochemical and spectral (IR, UV–vis) data. The X-ray diffraction study of [{Cu(tmen) (CH3CN)}{Cu(tmen)(ClO4)}(TAP)](ClO4) demonstrated that coordination geometry around the copper centers is square pyramidal where axial position of one copper is occupied by a ClO4 and the second copper with acetonitrile. However, in solution, the resulting complexes display affinity for axial ligation so that the apical ligands are driven out by solvent molecules. The solvatochromism of the complexes was investigated in various organic solvents and was compared with that of the corresponding mononuclear complex [Cu(tmen)(CH3-acac)]ClO4. A multi-parametric equation has been utilized to explain the solvent effect on the d–d transition of the complexes using SPSS/PC software. To explore the mechanism of the interaction between the solvent molecules and the complexes, different solvent parameters such as DN, AN, α, ET(30), π⁄, and β using stepwise multiple linear regression method were employed. In pyridine, the original color of the solution changed over time due to removal of tmen chelates and substitution by pyridine in two successive steps.
