The composition of the complexes is established based on the data obtained from the elemental analysis and molecular weight determinations. <...> The structure of the complexes is discussed in the light of data obtained from molar conductance, magnetic moment, electronic, EPR and IR spectroscopic studies. <...> INTRODUCTION Molybdenum occurs in nature in combination with copper in the unique heterobimetallic enzyme carbon monoxide dehydrogenase [ 1 ]. <...> The molybdenum ion shows an antagonistic function with regard to copper in humans and animals [ 2 ]. <...> Molybdenum is present in the heterobimetallic enzyme nitrogenase along with iron which is present in free living and symbiotic microorganisms catalyses the reduction of atmospheric dinitrogen to ammonia [ 4 ]. <...> The chemistry of molybdenum attracts attention not only in view of its role in the biosphere as an essential element, but also due to importance in catalysis [ 6—8]. <...> In catalysis, a heterobimetallic complex containing an electron deficient metal atom and an electron rich metal atom presents the possibility of Lewis acid activation of a substrate molecule bound to the electron rich metal centre due to the electronic interaction between metals and cooperative activation of the substrate [ 9, 10 ]. <...> The dihydrazone ligand is a multifunctional ligand with as many as eight bonding sites, naphtholic —OH, azomethine, secondary —NH, and carbonyl oxygen in duplicate in its molecular skeleton capable of giving rise to mononuclear and polynuclear homo- and heterometal complexes [ 15, 16 ]. © Kumar A., Chanu O.B., Koch A., Lal R.A., 2013 658 A. KUMAR, O.B. CHANU, A. KOCH, R.A. LAL Fig. 1. <...> Structure of bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (H4L) ligands A survey of the literature reveals that although works on metal complexes of monohydrazide based ligands and their Schiff bases have been carried out in some details [17—19 ], but those on metal complexes of dihydrazones containing active methylene functions are quite meagre. <...> In view of the above importance of the heterobimetallic complexes of molybdenum and the virtual absence of work on heterometallic complexes of dihydrazones, the present paper aims to describe the synthesis of some heterobimetallic complexes of molybdenum in its 6+ oxidation state, derived from the polyfunctional dihydrazone (H4L) ligand (Fig. 1). <...> Accordingly, the paper describes the synthesis of heterobimetallic complexes of molybdenum and some first row transition metal ions derived from the title <...>