Coordination abilities of polycyano anions in the solid state: coordination geometries and d-d transition energies of mixed-ligand solvatochromic copper(ii) complexes with B(CN)4, C(CN)3, and N(CN)2 anions.

B(CN)4-, C(CN)3-, and N(CN)2- are highly versatile polycyano anions that produce various functional compounds. To investigate the coordination abilities of these anions in the solid state quantitatively, we synthesized mixed-ligand Cu(ii) complexes: [Cu(R-acac)(tmen)X] (X = polycyano anion, R-acac = acetylacetonate or butyl-acetylacetonate, tmen = tetramethylethylenediamine). The coordination abilities of the anions, increasing in the order B(CN)4- < C(CN)3- < N(CN)2-, result in a decrease in the d-d transition energies of the complexes and the shortening of the axial coordination distance. The influence of crystal packing on the coordination geometries and d-d transition energies of the complexes was also demonstrated. The donor numbers of the anions were determined from the d-d transition energies in solution.