S42- Rings, Disulfides, and Sulfides in Transition-Metal Complexes: The Subtle Interplay of Oxidation and Structure

摘要

A sulfur atom, with its six valence electrons, may reach an octet by association into Sn rings. Two-electron reduction of the rings usually leads to polysulfide chains Sn2-. Small, neutral Sn rings are rare; witness S4, for which there is scant evidence[1] and continuing theoretical uncertainty about its structure.[2] As is the case for S3, the S4 ring's instability is presumably due to lone-pair repulsion. On the other hand, coordinated S42- most certainly exists, and as we will see, not only as a simple chain (of which Na2S4 is an example[3]). This paper moves toward an understanding of the variety of complexed S42- structural types and in the process shows that some compounds which hitherto have been considered as disulfide complexes may be profitably seen as containing D2h-distorted S42- rings. Their formation is attributable to a coupling redox process induced by either an external oxidant or inner oxidation of coordinated metal centers. The analysis leads us to the problem of tuning the S22-/2 S2- interconversion as well. The electronic structure of some compounds in the literature is reinterpreted, and, we believe, there emerges the outline of a consistent way of thinking about redox coupling of sulfides and disulfides.