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 ²A_1gâ†�²B_1g, ²B_2gâ†�²B_1g, and ²E_gâ†�²B_1g in D_4h 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, exchange coupling is considered to be present; where it 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.