The electronic properties and i.r. spectra of a series of bis(ethylenediamine)copper(II) complexes are reported. From a gaussian analysis of the electronic spectra, measured at room temperature and at the temperature of liquid nitrogen, of the seven complexes of known crystal structure, and three transitions 2A1gâ†�2B1g, 2B2gâ†�2B1g, and 2Egâ†�2B1g in D4h symmetry are tentatively assigned. From the e.s.r. spectra of polycrystalline samples of the complexes, which yield tetragonal spectra, the values of (g‖– 2)/(g⊥– 2) have been determined. Where the ratio is 4·0 the observed g-values are considered to reflect approximately the local copper(II) ion g-values. In the latter case the g-values have been combined with the appropriate electronic transitions and the orbital reduction factors k⊥ and k‖ evaluated. From the i.r. spectra of the polyanions, an infrared criterion is established for recognising the presence of weakly co-ordinated polyanions in the long tetragonal positions above and below the [Cu en2]2+ cation. The term semi-co-ordination is introduced to describe this structural situation.