Thioated Oxyacids

A series of acids based on hydrogen, sulphur and trivalent phosphorus are possible in principle, although in practice they are only encountered in the form of their esters. Like their oxy analogues (Chapter 6) they exist in equilibrium with tetrahedral forms (9.412) through (9.414). 

Sulphur adds readily to secondary phosphines to give secondary phosphine sulphides (6.79), which exist in thiono rather than thiolo form as phosphine sulphides (9.415). On the other hand, sulphur does not in general add directly to primary phosphines to give primary phosphine sulphides or their pyramidal equivalent (9.416). (One derivative, PhP(S)H2, does appear to have been made, however.) 

Secondary phosphine sulphides can be obtained in a variety of reactions including the action of caustic soda on aliphatic diphosphine disulphides, or hydrogen sulphide on phosphinous amides. Secondary phosphine sulphides readily add more sulphur to give phosphino dithioic acids (6.79) -an excess produces diphosphine derivatives (below). 

Oxidation with hydrogen peroxide or iodine will give rise to a mixture of phosphinodithioic and phosphinic acids (9.418). With carbon tetrachloride in the presence of triethylamine a vigorous exothermic reaction occurs, giving the chloride (9.419). 

The only acid known to exist in thiol form is bis(trifluoromethyl)phosphinothious acid, which can be made in 50% yield by the action of hydrogen sulphide on trifluoromethylphosphinous chloro-phosphine. This acid will add sulphur to form bis(trifluoromethyl)phosphinodithioic acid. 

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