The copper(I) and copper(II) complexes with the nitrogen donor ligands bis[(1-methylbenzimidazol-2-yl)methyl]amine (1-BB), bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB), N-acetyl-2-BB (AcBB), and tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (TB) have been studied as models for copper nitrite reductase. The copper(II) complexes form adducts with nitrite and azide that have been isolated and characterized. The Cu(II)-(1-BB) and Cu(II)-AcBB complexes are basically four-coordinated with weak axial interaction by solvent or counterion molecules, whereas the Cu(II)-(2-BB) and Cu(II)-TB complexes prefer to assume five-coordinate structures. A series of solid state structures of Cu(II)-(1-BB) and -(2-BB) complexes have been determined. [Cu(1-BB)(DMSO-O)(2)](ClO(4))(2): triclinic, P&onemacr; (No. 2), a = 9.400(1) Å, b = 10.494(2) Å, c = 16.760(2) Å, alpha = 96.67(1) degrees, beta = 97.10(1) degrees, gamma = 108.45(1) degrees, V = 1534.8(5) Å(3), Z = 2, number of unique data [I >/= 3sigma(I)] = 4438, number of refined parameters = 388, R = 0.058. [Cu(1-BB)(DMSO-O)(2)](BF(4))(2): triclinic, P&onemacr; (No. 2), a = 9.304(5) Å, b = 10.428(4) Å, c = 16.834(8) Å, alpha = 96.85(3) degrees, beta = 97.25(3) degrees, gamma = 108.21(2) degrees, V = 1517(1) Å(3), Z = 2, number of unique data [I >/= 2sigma(I)] = 3388, number of refined parameters = 397, R = 0.075. [Cu(1-BB)(DMSO-O)(NO(2))](ClO(4)): triclinic, P&onemacr; (No. 2), a = 7.533(2) Å, b = 8.936(1) Å, c = 19.168(2) Å, alpha = 97.66(1) degrees, beta = 98.62(1) degrees, gamma = 101.06(1) degrees, V = 1234.4(7) Å(3), Z = 2, number of unique data [I >/= 2sigma(I)] = 3426, number of refined parameters = 325, R = 0.081. [Cu(2-BB)(MeOH)(ClO(4))](ClO(4)): triclinic, P&onemacr; (No. 2), a = 8.493(3) Å, b = 10.846(7) Å, c = 14.484(5) Å, alpha = 93.71(4) degrees, beta = 103.13(3) degrees, gamma = 100.61(4) degrees, V = 1270(1) Å(3), Z = 2, number of unique data [I>/= 2sigma(I)] = 2612, number of refined parameters = 352, R = 0.073. [Cu(2-BB)(N(3))](ClO(4)): monoclinic, P2(1)/n (No. 14), a = 12.024(3) Å, b = 12.588(5) Å, c = 15.408(2) Å, beta = 101,90(2) degrees, V = 2282(1) Å(3), Z = 4, number of unique data [I >/= 2sigma(I)] = 2620, number of refined parameters = 311, R = 0.075. [Cu(2-BB)(NO(2))](ClO(4))(MeCN): triclinic, P&onemacr; (No. 2), a = 7.402(2) Å, b = 12.500(1) Å, c = 14.660(2) Å, alpha = 68.14(1) degrees, beta = 88.02(2) degrees, gamma = 78.61(1) degrees, V = 1233.0(4) Å(3), Z = 2, number of unique data [I>/= 2sigma(I)] = 2088, number of refined parameters = 319, R = 0.070. In all the complexes the 1-BB or 2-BB ligands coordinate the Cu(II) cations through their three donor atoms. The complexes with 2-BB appear to be more flexible than those with 1-BB. The nitrito ligand is bidentate in [Cu(2-BB)(NO(2))](ClO(4))(MeCN) and essentially monodentate in [Cu(1-BB)(DMSO-O)(NO(2))](ClO(4)). The copper(I) complexes exhibit nitrite reductase activity and react rapidly with NO(2)(-) in the presence of stoichiometric amounts of acid to give NO and the corresponding copper(II) complexes. Under the same conditions the reactions between the copper(I) complexes and NO(+) yield the same amount of NO, indicating that protonation and dehydration of bound nitrite are faster than its reduction. The NO evolved from the solution was detected and quantitated as the [Fe(EDTA)(NO)] complex. The order of reactivity of the Cu(I) complexes in the nitrite reduction process is [Cu(2-BB)](+) > [Cu(1-BB)](+) > [Cu(TB)](+) > [Cu(AcBB)](+).