A new tridentate N-3 ligand (TMG(3)tach) consisting of cis,cis-1,3,5-triaminocydohexane (tach) and three N,N,N',N'-tetramethylguanidino (TMG) groups has been developed to prepare copper complexes with a tetrahedral geometry and a labile coordination site. Treatment of the ligand with (CuX2)-X-II (X = Cl and Br) gave copper(II)-halide complexes, [Cu-II(TMG(3)tach)Cl](+) (2(Cl)) and [Cu-II(TMG(3)tach)Br](+) (2(Br)), the structures of which have been determined by X-ray crystallographic analysis. The complexes exhibit a four-coordinate structure with C-3 symmetry, where the labile halide ligand (X) occupies a position on the trigonal axis. 2(Br) was converted to a methoxido-copper(II) complex [Cu-II(TMG(3)tach)(OMe)]-(OTO (2(OMe)), also having a similar four-coordinate geometry, by treating it with an equimolar amount of tetrabutylammonium hydroxide in methanol. The methoxido-complex 2(OMe) was further converted to the corresponding phenolato-copper(II) (2(Ar)) and thiophenolato-copper(II) (2(SAr)) complexes by ligand exchange reactions with the neutral phenol and thiophenol derivatives, respectively. The electronic structures of the copper(II) complexes with different axial ligands are discussed on the basis of EPR spectroscopy and DFT calculations.