Thiophosphonic dihalides

The tetrachloroaluminate complexes are also derivable from halogenophosphoranes, RPCI4 (in turn preparable, in principle, from RPCI2 and Cl2 ) and AICI3. The complex from EtPCl4 reacts very slowly at 90-100 °C with H2S with the liberation of HCl and the ultimate formation of the species EtPS2 , but the stepwise process can be interrupted to allow the isolation of EtP(S)Cl2. More conveniently, a treatment of the complexes with sulphur, EtSH or KCNS, particularly in the presence of KCl (to remove the AICI3), leads to the thiophosphonic dihalides, RP(S)Cl2 a similar reaction with elemental selenium... 

Although free thiophosphonic acids may be obtained by the hydrolysis of the corresponding dihalides, a much more convenient procedure, which exploits milder conditions, consists in the conversion of a phosphonous acid into its bis(trimethylsilyl) ester, followed sequentially by the addition of sulphur (and presumably that of selenium would be equally feasible) and hydrolytic removal of the trimethylsilyl groups the procedure has been used for the preparation of both alkylphosphonothioic acids and their aryl analogues ... 

Thiohalides of the type RP(S)X2 and R2P(S)X can be made by heating sulphur with the corresponding phosphonous or phosphinous halides. Phosphonothioic (thiophosphonic) halides can be obtained by the action of hydrogen sulphide on tetrachlorophosphoranes (9.425), or phosphonous halide-aluminium trichloride complex (Chapter 6), or P4SJ0 on the corresponding phosphonic dihalide (9.426), or by reaction (9.427) in which PSCI3 acts as a sulphur donor. Monophenyl phosphine and thionyl chloride produce phenyl phosphonothionic dichloride, which can also be obtained by thermal isomerisation (9.428). 

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