New nickel (II) complexes of 5-methyl-3-formylpyrazole-3- piperidinylthiosemicarbazone (HMPz3Pi), [Ni(HMPz3Pi) 2]X2 • 2H2O (X = Cl, Br, ClO4, BF4 and NO3) have been synthesised and physico-chemically characterised by elemental analyses, magnetic susceptibility and conductivity measurements at RT, electronic and IR spectra. All the reported species are 1:2 electrolytic cationic complexes. Electronic spectra in the solid state and in MeOH solutions suggest a six-coordinate distorted octahedral geometry for all the reported complexes. IR spectral data (4000-200 cm-1) are indicative of a neutral NNS tridentate function of HMPz3Pi coordinating to the central Ni(II) via the pyrazolyl (tertiary) ring nitrogen, azomethine nitrogen and thio-keto sulfur. X-ray crystallography of [Ni(HMP z3Pi)2]Cl2 • 2H2O (P1̄, triclinic) has authenticated a NiN4S2 octahedral coordination and the pair of coordinating ligands (HMPz3Pi) are almost orthogonal. A close scrutiny of the X-ray data of HMPz3Pi (Pbca, orthorhombic) and [Ni(HMPz3Pi)2]Cl2 • 2H2O (P1̄, triclinic) indicates an unusual rotation about the azomethine (CN) double bond during complexation with Ni(II). A possible mechanism for such a rotation has been proposed. © 2005 Elsevier Ltd. All rights reserved.