Per cent 'covalency' and the nephelauxetic effect in lanthanide complexes

The use of the nephelauxetic ratio for the calculation of percent covalency is examined. Values of F k and E k obtained from four parameter fits (e.g. F 2 , F 4 , F 6 and ζ 4f ) neglecting configuration interaction for Pr 3+ in various host lattices are compared to those of the free ion. By including the effects of 4f-ligand overlap one can account for the empirically obtained order of sensitivity to the environment for F k : F 2 > F 4 > F 6 and for E k : E 2 > E 3 > E 1 . The results further indicate that by using the nephelauxetic effect and the relationship b = (1 - β)/2 one is likely to overestimate the degree of covalency of a complex.