Synthesis of a new hydrazone based Schiff base ligand, 3-methylpyrazole-5-carbohydrazone of 2,3-butanedione monoxime (HL) is reported. The reaction of Cu(ClO₄)₂·6H₂O with HL in any ratio in ethanol affords the dinuclear complex [CuL(EtOH)]₂(ClO₄)₂·2H₂O (1). An unusual coordination mode of the ligand was observed, in which the ligand forms stable six and five membered chelate rings around the metal centres without enolization of the carbonyl group of the hydrazone moiety. The same coordination behavior of the ligand was observed in its cobalt(III) complex. The reaction of CoCl₂·6H₂O in methanol with the ligand HL affords the mononuclear complex [CoL₂]Cl (2). Both 1 and 2 were characterized by X-ray crystallography and various physicochemical techniques (elemental analyses, UV–Vis, IR, EPR spectroscopy etc.). In complex 1, the copper centres are bridged through the oxime N–O groups forming a dimer containing a crystallographic centre of symmetry. 1 is found to undergo an autoreduction transformation in solvents such as dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). The EPR spectrum of 1 in DMF shows the partial dissociation of the dinuclear complex into a mononuclear species. Magnetic studies of 1 show that the two Cu(II) centres are strongly antiferromagnetically coupled via the bridging N–O groups. The structure of 2 is a monomer with crystallographic C2 symmetry in which the metal is bonded to two tridentate ligand anions, L⁻, in mer configurations to give a distorted octahedral geometry.