Nickel(II) N2X2 Schiff-base complexes incorporating pyrazole (X = NH, O or S):: syntheses and characterization

A series of four-co-ordinate nickel(II) N2X2 complexes (X = NH, O or S) with tetradentate Schiff-base ligands incorporating pyrazole has been prepared in order to compare the influence on the nickel(II) centre of the donor atoms in N-2(NH)(2) and N2S2 complexes versus N2O2 complexes In each case,two identical parts of the ligand are linked by a two-or three-carbon aliphatic chain (n = 2 or 3). Crystal structures have been determined for a n = 3 N-2(NH)(2) complex, for two n = 3 N2S2 complexes and for a n = 2 N2O2 complex. All of the complexes have been investigated in solution by spectroscopic methods (UV/VIS/NIR, H-1 NMR). The n = 2 and 3 N-2(NH)(2) complexes and the n = 2 N2S2 complexes are fully diamagnetic. An extension of the chain linking the two halves of the ligand from two to three carbon atoms induces a spin-equilibrium process (S = 0 reversible arrow S = 1) for the resulting N2S2 complex (Delta G = 19-26 kJ mol(-1) at 50 degrees C). Ligand-field parameters have been derived from the electronic spectra and the electrochemical properties of the N2S2 complexes have been investigated by cyclic voltammetry. From these results it is clear that low-spin character is emphasized in N-2(NH)(2) and N2S2 complexes of nickel(II) when compared with the corresponding N2O2 complexes.