The mechanism of synergistic selective extraction of Am(III) over Eu(III) from aqueous nitric acid solutions using di(chlorophenyl)dithiophosphinic acid [(ClΦ)₂PSSH] and neutral O-bearing organophosphorus co-extractants (B) has been investigated. (ClΦ)₂PSSH forms weaker complexes with Am(III) than R₂PSSH (where R = alkyl group). However, the Am(III)/(ClΦ)₂PSSH complexes are stronger than the corresponding Eu(III)/(ClΦ)₂PSSH complexes, although the difference is not as large as in the case of the corresponding complexes formed with R₂PSSH. The donor properties of co-extractants are also discussed. Chemical shifts in the ³¹P NMR spectra of the organophosphorus co-extractants correlate qualitatively with the extractability of Am(III) and Eu(III) in (ClΦ)₂PSSH + B synergistic mixtures. The synergistic effect determines the selectivity not only qualitatively but also quantitatively. The high selectivity observed using tri-2-ethylhexyl phosphate (T2EHP) is caused by a synergistic bonding effect for the Am(III) complex and an antagonistic effect for Eu(III).

A donor–acceptor mechanism is shown to promote Am(III) extraction for almost all B co-extractants. A contribution of entropy terms for extracted Eu(III) extraction is assumed to be significant. Interatomic distances R_Eu–S in the Eu(III) complexes are calculated.

The softness of S-bearing extractants XYPSSH (with X and Y = R, RO or ClC₆H₄) is discussed from the point of view of the effective charge carried by the S atoms. The hardness of Eu(III) and Am(III) is evaluated.