Synthesis and spectroscopic characterisation of new nickel (II) complexes with 5-methyl-3-formylpyrazole-3-piperidinylthiosemicarbazone (HMPz3Pi): X-ray structures of HMPz3Pi and [Ni(HMPz3Pi)2]Cl2 · 2H2O with indication for unusual rotation about the a

The synthesis and spectroscopic characterization of 5-methyl-3-formylpyrazole-3-piperidinylthiosemicarbazone, (HMPz3Pi) and its nickel (II) complexes, [Ni(HMPz3Pi)2]X2 · 2H2O (X = Cl, Br, ClO4, BF4 and NO3) have been reported. X-ray crystallography of [Ni(HMPz3Pi)2]Cl2 · 2H2O, as a representative species, has authenticated a NiN4S2 octahedral coordination with neutral NNS tridentate function of the primary ligand system with two azomethine nitrogen atoms in trans-positions and the pyrazolyl nitrogen atoms and thio-keto sulfurs in cis-positions. On comparing the crystal structure of the free ligand, HMPz3Pi, with that of its Ni(II) complex, [Ni(HMPz3Pi)2]Cl2 · 2H2O, it is observed that an unusual rotation about the azomethine double bond occurs during complex formation. A possible mechanism for this phenomenon has been proposed.