Simple and mixed-ligand copper(II) complexes were synthesized and characterized by crystallographic and spectroscopic techniques. The N,S-and O-donor chelating agents used include the following: l,5-bis(benzimidazol-2-yl)-3-thiapentane, BBES; l,5-bis(iV-methylbenzimidazol-2-yl)-3-oxopentane, Me2BBEO; 2,5,8-trimethyl-2,5,8-triazanonane, pmeta; pentane-2,4-dionate anion, acac"; 2,2'-bipyridyl, bpy; 2,2'-biimidazole, biim. The perchlorate of Cu(BBES) crystallizes from methanol as the salt [Cu(BBES)(Me0H)(0H2)](C104)2, with the symmetry of the space group Pi, Z = 2, a = 9.306(3) k,b= 10.072 (5) A, c = 14.194 (5) A, a = 92.23 (2)°, ß = 96.63 (4)°, 7 = 108.56 (3)°, and Pcalcd = 1.66 g-cm"3 456.The structure was solved with the Patterson method, incorporating 3088 of the 3884 independent reflections, to final R values of 5.6% and 6.1% (J?w). The complex cation has a distorted tetragonal pentacoordinate structure ( = 0.27) with the BBES occupying three equatorial positions, with a Cu-S linkage of 2.297 A, and the methanol bound axially. This contrasts with the perchlorate salt of a similar ligand's complex, obtained from ethanol, which has a perchlorate coordinated at this axial position. [Cu(BBES)(acac)]PF6-H20-Et0H was recrystallized from ethyl acetate to give the ester solvate, [Cu(BBES)(acac)]PF6*C4H802. This complex also crystallizes with symmetry Pi, and Z = 2, a = 13.961 (3) A, b = 12.892(3) A,c = 10.154 (3) A, a = 101.57 (4)°, ß = 104.99 (4)°, 7 = 104.34 (4)°, and Pcalcd = 1.160 g-cm"3. The structure was solved by the Patterson method, using 2939 of the 3243 independent reflections, to final R values of 6.3% and 6.1% CRyi). The complex cation is of regularly tetragonal pyramidal geometry ( = 0.02) with an equatorial N202-donor set and the thioether bound axially at 2.696 A, in contrast to the Cu(BBES) perchlorates. There are hydrogen-bonding interactions with benzimidazole and water protons in the lattices of the complexes. ESR spectroscopy shows that the Cu(BBES)2"1" and Cu(Me2BBEO)2+ complexes retain their distorted structures in solution; both give single-line ESR spectra in solution at ambient temperatures. The [Cu(BBES)(acac)]+ salts exhibit pairwise dipolar coupling in their crystalline states. Substitution of ether by thioether as donor causes absorptivity enhancement throughout the visible-near-UV spectrum, and difference spectroscopy has been used to assign sulfur-to-copper charge-transfer transitions. Unlike [Cu(bpy)(acac)]+ and [Cu-(BBES)(acac)]+, the complex cations [Cu(Me2BBEO)(acac)]+ and [Cu(biim)(acac)]+ are unstable toward coordinative disproportionation in the solvents used. The Cu(BBES)2"1" cation in solution is in equilibrium between the/ac-BBES and rwer-BBES isomers, the latter being dominant.