Two members of a homologous series of bis(triazacyclononane) Ligands, 1,3-bis(1,4,7-triaza-1-cyclononyl)propane, EM3, and 1,4-bis(1,4,7-triaza-1-cyclononyl)butane, EM4, form stable 1:1 and 2:1 Cu(II)-L complexes. Dissociation constants for the acid salts of the compounds and equilibrium studies with copper(II) in aqueous 0.1 M KNO3 at 25 degrees C an reported. Cu(EM3)(2+) is more stable than Cu(EM4)(2+) while Cu-2(EM4)(4+) is slightly more stable than Cu-2(EM3)(4+). Probable reasons for this behavior are discussed in light of the 1:1 complexes existing in solution as monomeric species. Binuclear copper(II) complexes of each ligand have been isolated and their structures determined by X-ray crystallography. Cu-2(EM3)Cl-4 . 2H(2)O crystallizes in space group P2/c with unit cell dimensions a = 12.607(3) Angstrom, b = 7.589(2) Angstrom c = 12.948 Angstrom, and beta = 93.71(3)degrees. Cu-2(EM4)Cl-4 crystallizes in space group P2(1)/c with unit cell dimensions a = 11.919(2) Angstrom, b = 8.468(2) Angstrom, c = 11.508(2) Angstrom, and beta = 99.06(3)degrees. In both complexes the copper(II) is 5-coordinate with two secondary amine N atoms of a [9]aneN(3) group and two Cl- occupying sites at the base and the tertiary N atom of the same moiety at the apex of a square pyramid. The structures have somewhat different conformations. The pair of Cl- attached to the two Cu(II) are approximately syn for Cu-2(EM3)Cl-4 . 2H(2)O and anti for Cu-2(EM4)Cl-4.