The syntheses and characterization of five-coordinate Cu(II) complexes of the pentadentate ligands trenimpy (4-[4-(2-pyridyl)-3-aza-3-butenyl]-1,4,7-,triazaheptane), trenpy (4-[4-(2-pyridyl)-3-azabutyl]-1,4,7-triazaheptane) and dmptacn (1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) are described. X-ray diffraction studies have established the structures of [Cu(trenimpy)](ClO4)(2) and [Cu(trenpy)](ClO4)(2). [Cu(trenimpy)](ClO4)(2) crystallizes in the monoclinic space group P2(1)/n; with a = 12.940(5) Angstrom, b = 15.418(2) Angstrom, c = 9.853(3) Angstrom, beta = 96.59(2)degrees, and Z = 4. Refinement gave final R and R(W) values of 0.066 and 0.070, respectively, for 2170 observed reflections. [Cu(trenpy)](ClO4)(2) crystallizes in the monoclinic space group P2(1), with a = 16.367(2) Angstrom, b = 15.191(1) Angstrom, c = 8.132(1) Angstrom, beta = 96.21(2)degrees, and Z = 4. Refinement gave final R and R(W) values of 0.065 and 0.063, respectively, for 2796 observed reflections. Two slightly different cations were present in the unit cell. The geometry of Cu(II) in the [Cu(trenimpy)](2+) cation is close to square pyramidal while that of Cu(II) in [Cu(trenpy)](2+) is intermediate between square pyramidal (SP) and trigonal bipyramidal (TBP). Visible spectra recorded in Nujol mulls were consistent with these geometries but the spectra recorded in various solvent indicated a shift in stereochemistry toward TBP for both complexes. This was not apparent in the case of [Cu(dmptacn)](ClO4)(2) were the mull and solution spectra were very similar and indicated a geometry close to SP. Frozen solution ESR spectra indicate that the geometry is close to SP for all three complexes. Significant changes in stereochemistry are induced by changes in the environment around the cations. Cyclic voltammetry of the complexes in acetonitrile solution revealed a quasi-reversible redox wave, corresponding to the Cu(II)-Cu(I) couple, at -0.80, -0.94, and -0.81 V for [Cu(trenimpy)](2+), [Cu(trenpy)](2+), and [Cu(dmptacn)](2+), respectively. Thus, some stabilization of the Cu(I) state results from the greater ability of trenimpy and dmptacn (cf. trenpy) to accept electron density from Cu(I).