Rational synthesis of monooxo thioanions from tetrakis(benzenethiolato)oxomolybdate(1-). Structural characterization of the [Ph4P]+ salts of the [(.eta.2-S2)MoO(.mu.-S)2MoS(.eta.2-S2)]2-, [(.eta.2-S2)MoO(MoS4)]2-, [(S4)MoX(.mu.-S)2MoSX(.eta.2-S2)]2- (X = S, O), and [(S4)MoO(MoS4)]2- anions