Nickel(II) and nickel(III) thiosemicarbazone and hydrazone complexes: An unexpected journey

In this study, the properties of nickel(II) thiosemicarbazone (complex 1) and nickel(III) (complex 2) hydrazone complexes were investigated using single crystal X-ray diffraction analysis, electron paramagnetic resonance, infrared spectroscopy, UV-Vis spectroscopy, molar conductivity and Density Functional Theory (DFT) calculations. The large difference in magnetic moments led us to suspect that we had obtained nickel complexes of different oxidation states (3.5 μB and 2.1 μB). This was verified after recording the Electron Paramagnetic Resonance (EPR) spectra for complex 2. The g value of 2.018 is consistent with a low spin (S = 1/2) Ni(III) species. DFT calculations are in agreement with the EPR data and show a doublet spin state of complex 2 with the unpaired electron practically completely located in the first coordination sphere (76.5%). All the findings show that the studied complex have different structural and electronic properties as a function of the oxidized state of nickel. In general, nickel(III) complexes are more difficult to obtain than nickel(II) complexes due to the stability of the nickel(II) oxidation state. Considering that this is the first Ni(III) hydrazone complex to be synthesised without an oxidising agent, its importance lies in providing insight into the fundamental properties and structure of Ni(III) hydrazone complexes. This could help to improve their synthesis and further investigate their applications.