New zinc(II), cobalt(II) and copper(II) complexes with tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (L) have been prepared. The tris(benzimidazole) donors provided by the ligand simulate the environment of the zinc(II) site of carbonic anhydrase. The structural properties of the complexes deduced on the basis of their spectroscopic characteristics correlate with those of the metal derivatives of the enzyme, at least in the solid state, while the alkyl chains connecting the donor groups of L allow a more flexible co-ordination environment of the metal sites in solution, with the adoption of five-co-ordinate structures. The complexes bind the inhibitor acetazolamide {N-[5-(aminosulfonyl)-1,3,4-thiadiazol-2-yl]acetamide} in its monoanionic form through the deprotonated sulfonamide group and these ternary complexes model the carbonic anhydrase–acetazolamide adduct recently characterized by X-ray crystallography. In the case of copper(II) it was impossible to isolate the pure ternary complex, probably because of the ease of formation of a dinuclear Cu^II–L–OH^– complex under the reaction conditions, but the related ternary complexes with acetazolamide and diethylene- or dipropylene-triamine were readily obtained.