Solid-state absorption and circular-dichroism spectra of five-co-ordinate trigonal copper(II) complexes: anisotropic contributions to the d–d transition probabilities

The d–d dipole strengths of trigonal five-co-ordinated copper(II) complexes in microcrystalline form have been determined, together with the corresponding rotational strengths of the tris(2-aminoethyl)amine (tren) complexes [Cu(tren)(NCS)][SCN], and, as a single crystal, [Cu(tren)(NH3)][ClO4]2. The results are compared with the corresponding theoretical transition probabilities based upon the dynamic ligand-polarization mechanism. A consideration of the polarizability anisotropy of the chelate-ring bonds is required to account for the optical activity, and an overall negative d–d circular dichroism is found to characterize the δ conformation of the chelate rings in five-co-ordinate (tren) complexes of copper(II).