The Nephelauxetic Series

A GIVEN transition group complex is characterized by two sets of parameters: (1) the orbital energy differences in the partly filled shell (one value, Δ, in octahedral and tetrahedral symmetry, two quantities in linear, three in tetragonal, and four in lower symmetries), and (2) the parameters of interelectronic repulsion. This determines a two-dimensional classification of octahedral complexes, according to two variables : Δ in the spectrochemical series and β in the nephelauxetic series (see p. 138). β is the ratio between the value of a representative parameter of interelectronic repulsion, e.g. Β from eqn. (35), in the complex and in the corresponding gaseous ion. This definition is equivalent to the assumption of the expanded radial function model, conserving the separability of each orbital of the partly filled shell in the product of an arbitrary radial function and a linear combination of angular functions, hydrogenic and corresponding to a well-defined / = 2. As expected from the occurrence of nine different parameters in the M.O. theory of y 5 a 7 3 b without the assumption of / = 2 (see eqn. 75), the absorption spectra show that the assumption of a single value of β is an over-simplification. Therefore, the comparison of β in different metal ions is somewhat uncertain; this is also because not all the values of Β are known from atomic spectroscopy. On the other hand, the comparison of β in different octahedral complexes with varying ligands is much more certain for the same central ion.