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 nick el(II) chelates containing N-substituted 1,2-diaminoethane derivatives (enR) and the anion of a 1,3-ketoenol (beta-dione) were obtained in t he solid state and in solution. The composition and the overall struct ure of the new chelates depend on ligand concentration, on the substit uents within the beta-dionato moiety and the counterion present. Whene ver the substituents and bulky counteranions fail to stabilise the [Ni (beta)enR]+ entity, a second diamine molecule confers coordination sat uration and the octahedral [Ni(enR)2beta]+ species are obtained. The i nfrared spectra disclose that the 1,2-diamines adopt the gauche confor mation. The ligand-field excitation spectra and solvatochromic shifts suggest that in the [Ni(beta)enR]+ entity, the NiN2O2 chromophore is s quare-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 planar ity of the NiN2O2 chromophore, the spatial requirements and restrictio ns of bidentate ligands induce rearrangements and octahedral coordinat ion.