The biheterocycle, 1,1′-dimethyl,2,2′-biimidazole (DMBIm) forms a number of mononuclear complexes of the type [UO 2(DMBIm)X 2] (X = Cl, NCS, NO 3, CH 3COO, 0.5 SO 4); UO 2(DMBIm) 2I 2 and Th(DMBIm) 2X 4 (X = I, NCS, NO 3). UO 2(H 2acacen)Cl 2 reacts with DMBIm to form the mixed ligand complex [UO 2(H 2acacen)(DMBIm)Cl 2]. Further, the reaction of UO 2(DMBIm)Cl 2 with HgCl 2 yields the hetero-bimetallic complex [UO 2(DMBIm)HgCl 4]. Elemental analyses, conductivity measurements, thermogravimetric studies and IR, electronic, mass and NMR ( 1H & 13C) spectral studies characterized the complexes. The 1H NMR of the free ligand has two singlets at δ7.27 and 7.03 ppm due to the inequivalence of the protons at C-4(4′) and 5(5′), respectively, and a more intense signal at δ 3.94 ppm for the methyl protons (—CH 3). These singlets further split into two indicating the ligand to be present in the trans-form. In the complexes, this inequivalence is also confirmed from the appearance of two singlets at ca 7.26 and 7.49 ppm and the methyl proton signal appears as a singlet at 3.95 ppm. The coordination results in a downfield shift of the ring protons. The 13C NMR signals of the free ligand appear at 138.05, 127.253, 123.109 and 34.82 ppm representing the C-2(2′), C-4(4′), C-5(5′) and methyl carbons, respectively, which shift slightly downfield in complexes. DMBIm is coordinated in the cis-form. The EIMS of DMBIm ligand and FABMS of the complex reveal the molecular mass and the fragmentation pattern.