Absorption Spectra of Co-ordination Compounds. II

Abstract (1) The absorption spectra of co-ordination compounds consist of the first, the second and the third bands as well as those due to ligands themselves. Some of the compounds may lack the first, the third, or both the bands, but none of them the second. (2) Postulating that by absorption of a quantum hν3 (ν3 = the frequency of the third band) a kind of neutralization takes place between the central ion and one of the co-ordinated anion to produce an excited ionogen in the original seat of co-ordination, the following relation has been derived. hν_3=S+P+E-J, where E= electron affinity of the anion, J= ionization energy of the central ion, S= co-ordination energy, and P= work of approach of the anion. The ultra-violet absorption band of solid alkali halide has been explained as a special case of the third band. (3) Besides the numbered bands, a co-ordination compound has generally some more bands due to ligands themselves. Examples of these special bands are given. The end absorption has also been explained. (4) The co-ordination energy, S(=hv2), may be taken as the measure of stability of the co-ordination compound. The order of hypsochromic effect of ligands on the second band gives the order of strength of the co-ordinate linkages between the central ion and the ligands. The order of stability has been determined experimentally. NH_3,NO_2^-, ONO^-, H_2O, NCS^-, OH^-, NO_3^-, Cl^-, CO_3^=, Br^-. (5) Whareas the stability of co-ordination compounds may be defined by S as a rule, the photochemical stability, however, should be given by P–R, R being the activation energy. Both the stabilities were compared with illustrations. (6) The substitution of ligands which proceeds naturally, takes place in the direction, in which the value S increases, or in other words, the second band is displaced towards the shorter wave-lengths. This rule and the spectrochemical series in (4) give the means of explaining as well as finding methods of preparation of various co-ordination compounds.