Interelectronic repulsion parameters, decrease

The overall separation between [Xe]4 /Y/ of M"+ and [Xe]4/ l of M(n l)+ is primarily determined by the interelectronic repulsion parameter A (4/, 4/) since the coefficient changes by (q — 1). The value of A (4/, 4f) varies with q in a non-uniform manner which may be explained by the fact that the baricentre of all the states having the maximum S value is located below, the baricentre of the configuration of 4fq for q between 2 and 12. In practice the fraction of all having 5max decreases sharply near the half-filled shell. 

The shifts in the baricenters of the peaks in the spectra of rare earth complexes relative to the aquo ion are caused by what has been termed the nephelauxetic effect by J0rgensen (1971) and are related to the decrease in the interelectronic repulsion parameters in the complex. Numerous attempts have been made to relate this effect to weak covalency effects and these are considered in ch. 23 (Theoretical Chemistry of Rare Earths). 

In the infra-red, most of /-level energy difference is due to spin-orbit coupling 191). Though the Lande parameter should be decreased half as much (proportional to the squared amplitude) as the parameters of interelectronic repulsion in a pure L.C.A.O. description, much evidence2140 is available that the Lande parameter varies less than half of (1 — p) from one compound to another. The difference (22007 — 6415 = 15592 cm-1) between the two /-levels 3P2 and 3F3 of Pr+3 does not depend on the Lande parameter and is decreased2151 to 14954cm-1 (p = 0.959 for... 

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