Abstract In order to probe the ability of the bis-bidentate ligand 4-isobutyl-3,5-di(2-pyridyl)-4 H-1,2,4-triazole (ibdpt) to form mononuclear complexes of the 1:2 type, [M II(ibdpt) 2(X) 2] y+ , a metal-to-ligand molar ratio of 1:2 has been employed in reactions with Co(ClO 4) 2 · 6H 2O, Ni(ClO 4) 2 · 6H 2O and Zn(ClO 4) 2 · 6H 2O. A high yield of the pure mononuclear complex [Zn II(ibdpt) 2(ClO 4) 2] is obtained when Zn(ClO 4) 2 · 6H 2O is employed. In contrast, when Co(ClO 4) 2 · 6H 2O is employed a mixture of mono- and dinuclear complexes, [Co II(ibdpt) 2(X) 2] y+ and [ Co 2 II (ibdpt) 2(X) 4] y+ , respectively, results. Even more dramatically, when Ni(ClO 4) 2 · 6H 2O is employed, no mononuclear [Ni II(ibdpt) 2(X) 2] y+ complex is obtained despite the 1:2 ratio. It is clear from these studies that stoichiometry alone does not control the outcome of such complexation reactions but that other factors, including the nature of the N 4 substituent and the relative solubility of the various possible products, also influenced by the N 4 substituent, are also important. The X-ray crystal structures of the mononuclear complexes [Co II(ibdpt) 2(H 2O) 2](ClO 4) 2 · Et 2O, [Zn II(ibdpt) 2(ClO 4) 2] and [Zn II(ibdpt) 2(H 2O) 2](ClO 4) 2 · 3MeCN and of the dinuclear complex [Zn 2(ibdpt) 2(H 2O) 4](ClO 4) 2 are presented. In all of these complexes the metal centres are octahedrally N 4O 2 coordinated by two in-plane ibdpt molecules and axial H 2O or ClO 4 - co-ligands. The trans-( N′, N 1) 2 coordination mode is adopted in all three mononuclear complexes and the trans-( N′, N 1, N 2, N″) 2 double bridging coordination mode is adopted in the dinuclear complex.