A series of new mixed copper(II) chelates with N,N,N′,N′-tetramethylethylenediamine (TMEN) and various β-diketonate ions of the general formula [Cu TMEN(β-dik)] X. nH2O(β-dik = aca, bzaca, dbm, taa, hfa or dpm; X = ClO4 - or NO3 -; n = 1 2 or 0) were prepared. The magnetic moments, infrared spectra, and reflection spectra of their solids and the visible absorption spectra and electric conductivities of their organic solutions were measured, and the effects of the substituent groups in the β-diketonate ions, and those of the anions and solvents, on the electronic state and strengths of the coordinate bonds in the chelate cations were studied. The obtained results indicate that the magnitudes of ligand field splitting in these chelates strongly depend on the nature of the substituent groups, being particularly small in the chelates with fluorinated diketones. Further they show that there is a very obvious relation between these magnitudes and coordination abilities of the solvents expressed in their donor numbers. In most of the cases studied, ionization and solvation of the chelate cation of the type [Cu TMEN(β-dik)]X + 2S ⇄ [Cu TMEN (β-dik)S2]+ + X- (S = solvent molecule) seem to occur in organic solutions. However, in a highly non-polar solvent like 1,2-dichloroethane, anion coordination seems to occur.