COUNTERION EFFECTS IN THE SPECTRA AND STRUCTURE OF SOLVATOCHROMIC COPPER(II) CHELATES CONTAINING 1,2-DIAMINES AND BETA-KETOENOLS

The contemporaneous reaction of copper(II) 'salts' with certain beta-d iones and N-substituted 1,2-diaminoethanes (enR) is reported. The infl uence of gamma-position derivatives of 2,4-pentanedione, X-acacH (X=H, CN, NO2), is described and the contribution of anions to the stabilit y, the stereochemistry and the electronic structure of the resulting m ixed-ligand chelates is discussed. Spectroscopic observations disclose that the bidentate ligands form chelate rings with copper(II) as comm on vertex. Whenever the [Cu(beta-dionato)enR]+ entity pertains, the IR and the electronic excitation spectra, in conjunction with other phys icochemical measurements, suggest that the CuN2O2 chromophore attains a virtually square-planar structure. Copper(II) in the [Cu(beta-dionat o)enR]+ entity lacks coordination saturation and, in addition to elect rostatic interactions that prevail when bulky polyatomic anions counte rbalance the positive charge, it is capable of forming covalent bonds with neutral molecules and charged species such as the halides and pse udohalides. The basal CuN202 plane is distorted upon coordination of u nidentate ligands residing on the apex of the resulting square-pyramid al structure. Further increase in the coordination number of copper(II ) by bidentate anions or neutral molecules leads to tetragonally disto rted octahedral structures. This change in the symmetry of the field i nduced on copper(II) is revealed by spectral shifts that also disclose covalent interactions in the encounters of the CuN202 chromophore wit h polar or polarisable molecules. These interactions are enhanced with increasing ability of the attacking species to act as an electron pai r donor. Linear dependence of the ligand field excitation maxima on th e donicity of the attacking species is established. Rearrangement invo lving rapture of the Cu-0 bond and formation of the CuN4 chromophore a ccompanies the addition of NN-dimethylethylenediamine, Me2en, to Cu(CN -acac)2. The charge in the resulting [Cu(Me2en)2]2+ entity is neutrali sed by the anion of the 3-cyano-2,4-pentanedione, CN-acac-, that binds to copper(II) through the nitrogen atom coordinating as nitrile. The coordination of a beta-ketoenol as unidentate ligand through an atom o ther than oxygen or carbon has no precedence.