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It has been long known that certain transition metal sulfides dissolve in aqueous ammonium polysulfide. Although it was assumed that they thereby formed metal polysulfides, attention was first paid to such compounds only a few years ago. In recent years a host of new complexes with polysulfido chelate ligands have been isolated and characterized. The complexes are of interest not only regarding their structure and reactivity but also in view of their potential uses; they can be used for the directed preparation of sulfur rings of a given size, and there are indications that they will find applications in catalysis.

for the opportunity offered to R.W. in 1983 to discuss deeply the specific experimental and theoretical facets of 119Sn NMR.

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The synthesis of the new dithiolene complexes, [{(MeOOC),C,S2}2Mo(2-S)]; and [OMo(S2C2(COOMe)2)2]2-, is reported. These complexes are obtained by the reaction of dicarbomethoxyacetylene (DMA) with either [(S~)MO(S)(~~-S)~MO(S)(S~)]~-or [(CS~)MO(S)(~~-S)~MO(S)(CS~)]~-and [OMO(S~)~]~-, respectively. The reaction of [(S4)Mo(0)(~2-S)2Mo(O)(S2)]2-with DMA results in the new dithiolene complex [{(MeOOC)2C2S2}M~(0)(~2-S)]~, which is the isomeric form of the vinyl disulfide complex obtained in the reaction of the [(S2)Mo(0)(~2-S)2Mo(O)(S2)]2-complex with DMA. The difference in reactivity between the two complexes that contain the same [Mo202S212+ core is attributed to the intrinsically different reactivity characteristics of the Sg-and S:-ligands. As a result of Mo-S d,-p, bonding an alternation in the S-S bond lengths is observed in virtually all of the structurally characterized Mo-S4 units. The consequent weakening of the S-S bonds adjacent to the Mo-S bonds allows for the ready dissociation of $3 from the MO-coordinated Si-ligands. This weakening also accounts for the facile formation of dithiolenes in cycloaddition reactions of alkynes with the Mo-S, units. By comparison, the S-S bond in side-on MO-coordinated St-ligands is strengthened as a result of depopulation of the ligand x*-orbitals. Reactions of the latter with alkynes do not proceed by cycloaddition. Instead, insertion into the Mo-S bond has been reported for at least one such reaction. The importance of activated polysulfide ligands in the hydrodesulfurization reaction is discussed.