COVALENCE EFFECT ON SECOND MOMENTS OF ESR ABSORPTION CURVES . CASE OF NA4[CU11(NH3)4][CU1(S2O3)2]2

An extension of Van Vleck's theory of the second moments of ESR absorption curves is presented for the case of complex ions, in which the covalence effects are important. The model takes into account that, in a complex ion, the unpaired spin is localized not only at the central metal, but also at the ligands. The calculations, for a single crystal of Na4[CuII (NH3)4] [CuI(S2O3)2]2, yield a 30% smaller dipolar contribution to the second moments than the usual model, and this is in encouraging agreement with the experimental data. © 1969 The American Physical Society.