The title anions were prepared as (DBU-H)(+) salts upon reaction of the oxophosphinidene complex (H-DBU)[MoCp(CO)2{P(O)R*}] with either dimethyldioxirane or elemental sulphur (R* = 2,4,6-C6H2(t)Bu3; Cp = η(5)-C5H5, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). The dioxophosphorane complex failed to react with MeI at room temperature, but reacted readily with (Me3O)BF4 to give the phosphonite complex [MoCp{O,P-OP(OMe)R*}(CO)2]. In contrast, the thiooxophosphorane complex reacted with MeI to give the thiolophosphinide derivative [MoCp{S,P-(MeS)P(O)R*}(CO)2], whereas its reaction with (Me3O)BF4 gave a mixture of the latter complex and the phosphonothiolate isomer [MoCp{S,P-SP(OMe)R*}(CO)2] in similar amounts. Other electrophiles were added selectively to the terminal O atom of the R*POS ligand. Thus the thiooxophosphorane complex reacted with ClC(O)C2H3, [NH4]PF6, ClSiMe3, ClSnMe3 and [ZrCp2Cl2] to give the corresponding derivatives [MoCp{S,P-SP(OX)R*}(CO)2] (X = C(O)C2H3, H, SiMe3, SnMe3, ZrCp2Cl). The structure of two of these products (X = C(O)C2H3, SiMe3) was determined by single-crystal X-ray diffraction studies. Density functional theory (DFT) calculations of the title anions and some of their derivatives indicated that attachment of an external electrophile to the terminal O atom of the thiooxophosphorane ligand is favoured under the conditions of charge control, while the sulphur atom is the favoured site under the conditions of orbital control, although it leads to less stable products.