The syntheses, crystal structures, and electronic absorption spectra of the copper(I) and copper(II) complexes of 2,9-diphenyl-1,10-phenanthroline (dpp) are reported. The complex [Cu(dpp)(2)](PF6) (1) crystallizes in space group P2(1)/c with a = 11.081(4) Angstrom, b = 25.491(8) Angstrom, c = 14.263(5) Angstrom, beta = 92.84(3)degrees, Z = 4, and V = 4024(2) Angstrom(3). For 4813 unique data with F > 4.0 sigma(F), R = 5.41% and R-w = 6.43%. The coordination geometry about the copper(I) center in [Cu(dpp)(2)](+) is best described as distorted tetrahedral with approximate C-2 symmetry. The structure of [Cu(dpp)(2)](+) is largely determined by interligand pi-stacking interactions that occur between the phenyl groups of one ligand and the phenanthroline moiety of the other ligand. Solution-state absorption and H-1 NMR spectra indicate that the [Cu(dpp)(2)](+) complex is fluxional in solution, rocking between two enantiomeric structures of C-2 molecular symmetry through an intermediate of C-s symmetry. The complex [Cu(dpp)(2)](ClO4)(2) (2) crystallizes in space group P (1) over bar with a = 7.809(3) Angstrom, b = 13.027(6) Angstrom, c = 20.344(10) Angstrom, alpha = 87.68(4)degrees, beta = 89.16(4)degrees, gamma = 79.26(4)degrees, Z = 2, and V = 2032(1) Angstrom(3). For 4943 unique data with F > 4.0 sigma(F), R = 5.22% and R-w = 5.37%. The coordination geometry about the copper(II) center in [Cu(dpp)(2)](2+) is best described as flattened tetrahedral with approximate D-2 symmetry. There are no interligand pi-stacking interactions in the structure of [Cu(dpp)(2)](2+). The four-coordinate geometry in [Cu(dpp)(2)](2+) persists in solution on the basis of solution-state and solid-state absorption spectroscopy. Structural distortion in the metal-to-ligand charge-transfer excited state of [Cu(dpp)(2)](+) is discussed on the basis of the structures of 1 and 2.