SPECTROSCOPIC INVESTIGATION OF MIXED-LIGAND NICKEL(II) CHELATES CONTAINING BETA-KETOENOLS AND 1,2-DIAMINES

The infrared and electronic excitation spectra of a series of new nickel(II) chelates containing N-substituted 1,2-diaminoethane derivatives (enR) and the anion of a 1,3-ketoenol (beta-dione) were obtained in the solid state and in solution. The composition and the overall structure of the new chelates depend on ligand concentration, on the substituents within the beta-dionato moiety and the counterion present. Whenever the substituents and bulky counteranions fail to stabilise the [Ni(beta)enR]+ entity, a second diamine molecule confers coordination saturation and the octahedral [Ni(enR)2beta]+ species are obtained. The infrared spectra disclose that the 1,2-diamines adopt the gauche conformation. The ligand-field excitation spectra and solvatochromic shifts suggest that in the [Ni(beta)enR]+ entity, the NiN2O2 chromophore is square-planar and the diamine nitrogens are coplanar with the resonance stabilised beta-dionato ring. Nickel(II) in the [Ni(beta)enR]+ entity interacts covalently with basic solvents, halides, pseudohalides, and other polyatomic anions. While unidentate ligands preserve the planarity of the NiN2O2 chromophore, the spatial requirements and restrictions of bidentate ligands induce rearrangements and octahedral coordination.