BTBI(1,2,4,5-tetrakis(benzimidazol-2-yl) benzene) acts as a tetradentate, dinucleating ligand, and on reaction with copper(II) salts forms both 2:1 (metal:ligand) and 2:2 derivatives, depending on reaction conditions. Initial reactions between BTBI and a large excess of copper salts produce "open" 2:1 derivatives, which rearrange on recrystallization and disproportionate to form 2:2 "closed" metallacyclic complexes or, with copper halides, species containing both types of cations. For example, the complex [Cu-2(BTBI)Cl-4(DMF)(2).DMF.H2O (I) produces [Cu-2(BTBI)(2)Cl-2][Cu-2(BTBI)Cl-2(DMF)Cl-4.12DMF (II) on recrystallization from DMF/EtOH. With cobalt salts (CoX(2); X = Cl, Br, NCS) "open" 2:1 species are formed as the major products : [Co-2(BTBI)X(4)].2DMF nH(2)O (X = Cl, n = 0.5 (x); x = Br, n = 0 (XI); X = NCS, n = 3 (XII)). The crystal and molecular structures of compounds II and XI are reported. II crystallized in the triclinic system, space group P $$($) over bar 1, with a = 15.538(4) Angstrom, b = 17.044(2) Angstrom, c = 15.186(4) Angstrom, alpha = 92.11(2)degrees, beta = 98.13(2)degrees, gamma = 92.06(2)degrees, V = 3975(2) Angstrom 3, and Z = 2. Refinement by full-matrix least-squares procedures gave final residuals of R = 0.066 and Rw( )= 0.063. Two square-pyramidal copper(II) centers are separated by 8.082(3) Angstrom in the 2:2 centrosymmetric "closed" metallacyclic cation and by 8.805(3) Angstrom in the 2:1 centrosymmetric "open" cation, which involves two coordinated DMF molecules per metal center. XI crystallized in the monoclinic system, space group P2(1)/n, with a = 10.021(2) Angstrom, b = 25.569(4) Angstrom, c = 11.518(2) Angstrom, beta = 111.78(1)degrees, V = 2700.4(9) Angstrom 3, and Z = 2. Refinement by full-matrix least-squares techniques gave final residuals of R = 0.066 and R(w) = 0.072. Compound XI is centrosymmetric with the two tetrahedral cobalt(II) centers separated by 8.652(3) Angstrom and the DMF molecules hydrogen-bonded to the NH- benzimidazole moieties. The spectral and magnetic properties of these complexes are also discussed. Very weak antiferromagnetic exchange exists in most of these complexes.