Nickel(II) complexes of chiral tripodal N,O,S-ligands: Square-planar vs pseudo-octahedral coordination in the solid state and in solution, metal-induced racemization of the ligand

The complexation behavior of three chiral, tripodal N,O,S-ligands with nickel(II) salts was examined. It was found that the coordination mode of the ligands (bidentate N,O and N,S; tridentate N,O,S) and of the central nickel ion (square-planar vs. pseudo-octahedral) is not simply an intrinsic property of the ligand-metal combination, but also a function of the counter ions and of the solvent of crystallization. In the case of one ligand, a diamagnetic square-planar and a paramagnetic pseudo-octahedral form could be switched back and forth, by just changing the solvent of crystallization from methanol to ethanol. By using an enantiomerically pure tripodal N,O,S-ligand, a metal induced racemization was unveiled which would have otherwise remained undetected.