Extraction by solvating molecules:

At this concluding stages of the discussion on the extraction by solvating molecules, it may be important to make a brief comment on some other extractants with phosphorus - sulphur bond. This group contains sulphur analogues of the previous group e.g. alkylthiophosphoric,  alkylthiophosphonic  and alkylthiophosphinic esters as well as phosphine sulphides like the Cyanamid reagent, trisobutylphosphine  sulphide (TIBPS), Cyanex 471 X. The importance of having analogues of oxyphosphorus reagents with sulphur, is of course, the difference between the oxygen and sulphur affinities for metals. The extractants with oxygen donor are classified as hard bases while with sulphur donor are soft bases. It is, therefore, expected that hard bases will associate with hard acids and soft bases with soft acids. For metal ions like Fe³⁺, La³⁺, Th⁴⁺ etc. (hard acids), extractants with oxygen donors are a better choice than a nitrogen or sulphur donor. Metals such as Cu²⁺, Ag⁺, Cd²⁺, Pd²⁺, etc. are soft acids, hence, prefer sulphur donor extractants. Cyanex 471 X has been successfully developed as an extractant for Ag⁺, Pd²⁺ and Hg²⁺.

Another class of extractants which could be of considerable interest is the amide type extractant (A 101). It is a weakly basic extractant with carbonyl being the functional group. The reagent has been used for the separation of Nb and Ta and Mo and Re. Subsequent to development of A 101 in China another reagent N 503, which is N, N - dimethylheptyl acetamide has been used for the separation of Nb and Ta.