Substituent effects of beta-diketiminate ligands on the structure and physicochemical properties of the copper(II) complexes have been systematically investigated by using 3-iminopropenylamine derivatives (RLH)-L-1-H-R3, R-3-N=CH-C(R-1)=CH-NH-R-3, where R-1 is Me, H, CN, or NO2, and R-3 is Ph, Mes (mesityl), Dep (2,6-diethylphenyl), Dipp (2,6-diisopropylphenyl), or Dtbp (3,5-di-tert-butylphenyl). When the ligands with R-3 = Ph or Dtbp were treated with Cu-II(OAc)(2), bis(beta-diketiminate) copper(II) complexes exhibiting distorted tetrahedral geometries were obtained, the crystal structures of which were nearly the same as each other regardless of the alpha-substituent (R-1); dihedral angles between the two beta-diketiminate coordination planes are 62.5 +/- 1.2degrees, and the Cu-N bond lengths are 1.959 +/- 0.008 Angstrom. The distorted tetrahedral structures are maintained in solution, but the spectroscopic features, especially g(I) values of the ESR spectra and the d-d bands of the absorption spectra, as well as the electrochemical behaviors of the complexes, are significantly affected by the electronic nature of R-1. The ligands with R-3 = Mes and Dep, on the other hand, gave di(mu-hydroxo)dicopper(II) complexes, and their crystal structures as well as spectroscopic and electrochemical features have also been explored. Furthermore, the ligand with the more sterically encumbered aromatic substituent (Dipp) provided a mononuclear four-coordinate square planar copper(II) complex supported by one beta-diketiminate ligand and one didentate acetate ion. Thus, the beta-diketiminate ligands with a variety of substituents (R-1 and R-3) have been explored to provide coordinatively unsaturated (four-coordinate) mononuclear and dinuclear copper(II) complexes with significantly different coordination geometry and properties.