The coordinating properties of the title ligand, 5-methyl-3-formyl pyrazole 3-hexamethyleneiminyl thiosemicarbazone (HMPz3Hex), synthesized and characterized (elemental analysis, MS, IR, 1H and 13C NMR and X-ray crystallography) for the first time, are reported by solid-state isolation of cobalt(III) complexes, [Co(MPz3Hex)2]X·nH 2O (X=Cl, Br, NO3, ClO4 or BF4; n=1to2). The reported complex species have been spectroscopically characterized with X-ray structural identification of [Co(MPz3Hex)2] Br·H2O. IR and NMR data (1H and 13C) for the free ligand and its CoIII complexes confirm that the ligand, HMPz3Hex, behaves as a uninegative anion with NNS tridentate function via the pyrazolyl tert-nitrogen, azomethine nitrogen and thiol sulfur. Electronic spectral data recommend a distorted octahedral environment for the six-coordinate CoIII species. X-ray crystallography of [Co(MPz3Hex)2]Br·H2O (C2/c, monoclinic), has shown unambiguously that the crystallographic asymmetric unit consists of a [Co(MPz3Hex)2]+ cation and one Br- anion with one H2O molecule as solvent of crystallization. The two NNS ligands coordinate orthogonally to the central CoIII ion with mer-configuration. A critical scrutiny of the X-ray data of free HMPz3Hex (Pna21, orthorhombic) and [Co(MPz3Hex)2]Br·H 2O indicates that an unusual rotation about the azomethine (C=N) double bond occurs during complexation with CoIII. A plausible reaction mechanism for the same has been proposed. Sulfur is involved in a C(7)-H(7)⋯S(1) intramolecular contact restricting the conformation of the ligand both in free and its complexed form. © 2003 Elsevier Ltd. All rights reserved.