The interaction of hard base chelating agents (LH) such as 8-quinolinol (oxH), salicylaldehyde (salH) and N-nitrosophenylhydroxylamine (NphaH) with ammonium tetrathiomolybdate(VI), (1), in aqueous solution yields disulphidomolybdenum(VI) complexes, [MoS2L2]. The i.r. absorptions at 540 and 510 cm-1 are assigned to v(Mo-S), which indicate that the two sulphido-groups are cis to each other, The interaction of (1) with soft base chelating agents (SS), such as dithiocarbamates (dtc), dithiophosphate (dtp) and xanthates (xan) yields dimeric sulphido-bridged sulphidomolybdenum(V) complexes, [Mo2S4(SS)2] and with potassium ethylthioxanthate (KEttxan) yields a molybdenum(IV) complex, [Mo(Ettxan)4]. The [Mo2S4(R2dtc)2] complexes were also obtained by the interaction of [MoO2(R2dtc)2] with phosphorus pentasulphide in xylene. It was concluded that tetrathiomolybdate(VI) undergoes facile reduction when compared with tetraoxomolybdate(VI). © 1986 VCH Verlagsgesellschaft mbH.