Spectroscopic investigation of mixed-ligand nickel(II) chelates containing β-ketoenols and 1,2-diamines

Abstract

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 (β-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 β-dionato moiety and the counterion present. Whenever the substituents and bulky counteranions fail to stabilise the [Ni(β)enR] + entity, a second diamine molecule confers coordination saturation and the octahedral [Ni(enR) 2β] + 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(β)enR] + entity, the NiN 2O 2 chromophore is square-planar and the diamine nitrogens are coplanar with the resonance stabilised β-dionato ring. Nickel(II) in the [Ni(β)enR] + entity interacts covalently with basic solvents, halides, pseudohalides, and other polyatomic anions. While unidentate ligands preserve the planarity of the NiN 2O 2 chromophore, the spatial requirements and restrictions of bidentate ligands induce rearrangements and octahedral coordination.