The hexaaza macrocyclic ligand 3,6,9,16,19,22-hexaaza-27,28-dioxatricyclo[(11,14)]octacosa-1(26),11,13,24-tetraene (BFBD), forms both mono- and dinuclear complexes, as well as several protonated and hydroxo chelates, with Cu(II) ions. These cationic species can bind inorganic and organic anions through coordination and hydrogen bonding. Stability constants of the mono- and dinuclear Cu(II) complexes of BFBD and their interaction with oxalate, malonate, and pyrophosphate anions have been measured potentiometrically. The nature of the bonding between the hosts and the guests is discussed. The crystal structures of two new dinuclear Cu(II) complexes, determined by X-ray crystallography, are also reported. [BFBDCu2(Cl)(3)]ClO4 . 0.5H(2)O crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.267(2) Angstrom, b = 12.155(5) Angstrom, c = 18.461 0 Angstrom, beta = 90.86(2)degrees, and Z = 4. Each Cu(II) ion is coordinated by three nitrogen atoms from the diethylenetriamine unit of the macrocyclic ligand and two chloride anions, forming a square pyramidal geometry. [BFBDCu2(Ox)](BF4)(1.8)Cl-0.2 crystallizes in the triclinic system, space group P1, with a = 6.772(1) Angstrom, b = 10.645(2) Angstrom, c = 11.517(2) Angstrom, alpha = 64.74(3)degrees, beta 79.79(3)degrees, gamma = 81.94(3)degrees, and Z = 1, The environment of each copper is intermediate between square pyramidal and trigonal pyramidal. The oxalate anion bridges in a bis-bidentate fashion between two Cu(II) ions.