REACTION OF MOO4(2-) AND WO4(2-) WITH AQUEOUS POLYSULFIDES - SYNTHESIS, STRUCTURE, AND ELECTROCHEMISTRY OF ETA-2-POLYSULFIDO COMPLEXES CONTAINING A BRIDGING S,S (M2O2S2)2+ (M = MO, W) CORE

Aqueous molybdate and polysulfide solution when cooked together for 3 h affords R2[Mo2O2S2(S2)(S4)], where R = Me4N (3a), Et4N (3b), and Ph4P (3c) cations. Tungstate solution however needs to be reduced to oxotungsten(V) before polysulfide treatment to get the compounds R'2[W2O2S2(S2)(S4)] [R' = Me4N (4)] and R"2[W2O2S2(S4)2] [R" = Et4N (5a), Ph4P (5b)]. A nu-S-S band above 500 cm-1 in the Raman spectrum is diagnostic for complexes containing the S2(2-) ligand, while two such bands, one at approximately 420 cm-1 and the other at approximately 490 cm-1, are diagnostic for the S4(2-) ligand. The M-M interaction in the compounds is evident from their observed diamagnetism and is further supported by Mo-Mo and W-W distances of ca. 2.83 angstrom in 3b (crystal data: a = 14.993 (4) angstrom, b = 12.397 (3) angstrom, c = 15.825 (4) angstrom, beta = 92.29 (3)-degrees, monoclinic P2(1)/a, Z = 4, R = 0.045) and 4. The complex anion in 3b contains a discrete {Mo2O2S2}2+ core coordinated by eta-2-S2 and eta-2-S4 ligands, and the coordination geometry about the Mo atoms can be described as that of two square pyramids sharing an edge containing bridging S,S ligands. In the electronic spectrum, a band above 400 nm has been found to be characteristic of an M(S2) three-membered-ring system. The position of Mo 3d5/2,3/2 and W 4f7/2,5/2 is typical for pentavalent Mo and W, respectively, in a sulfur environment. An interesting aspect of the electrochemical study of the {M2O2S2}2+ core in DMF is the observation of reversible or quasireversible cyclic responses showing step by step reduction (for M = Mo) of two Mo(V) centers to Mo(IV), the reverse scan also showing corresponding oxidation steps. In the case of M = W, unless at slow scan, the reduction is a 2-electron single-step process, the reverse sweep exhibiting two-step single-electron oxidative responses.