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Electrochemical evidence for metal polysulfide complexes: Tetrasulfide (S-4(2-)) reactions with Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+

Chadwell, SJ, Rickard, D and Luther, GW 2001. Electrochemical evidence for metal polysulfide complexes: Tetrasulfide (S-4(2-)) reactions with Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+. Electroanalysis 13 (1) , pp. 21-29. 10.1002/1521-4109(200101)13:1<21::AID-ELAN21>3.0.CO;2-P

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Pure sodium tetrasulfide (Na2S4) was prepared and reacted with the divalent cations of Mn, Fe, Co, Ni, Cu and Zn in aqueous solution at 25 °C and at concentrations below 10 micromolar of S42–. A series of S42– complexes of these metals were found to exist by voltammetric methods. At higher concentrations, S42– reacted to give MS and S8. Tetrasulfide complexes with compositions assigned as [M(η1‐S4)] and [M2(μ‐S4)]2+ or [M2<(η1‐S4)] occur for Mn, Fe, Co and Ni where only one terminal S atom in the polysulfide binds to one metal (η1: unidentate ligand or M–S–S–S–S; μ: ligand bridging two metal centers or M–S–S–S–S–M). Conditional stability constants are similar for all four metals: log β1 between 5.6 and 6.0 and log β2 between 11.0 and 11.7. The constants for these tetrasulfide complexes are approximately 0.8 log units higher than for comparable bisulfide complexes [M(SH)]+ as expected based on the higher nucleophilicity of S42– compared to HS−. Voltammetric results indicate that these are labile complexes. Zn2+ and Cu2+ are chemically distinct from the other metals. Zn2+ reacts with tetrasulfide to form a stable monomeric tetrasulfide chelate, [Zn(η1‐S4)], with log β=8.4. Cu2+ reacts with tetrasulfide to form a complex with the probable stoichiometry [Cu(S4)]2 with log β estimated to be 17.8. Discrete voltammetric peaks are observed for these complexes and indicate they are electrochemically inert to dissociation. Reactions of Zn2+ and Cu2+ also lead to some breakup of the tetrasulfide even at these low concentrations. The relative strength of the complexes is Cu>Zn>Mn, Fe, Co, Ni. Cu displaces Zn from [Zn(η1‐S4)]. Both Cu and Zn displace Mn, Fe, Co and Ni from their tetrasulfide complexes. The differences in metal tetrasulfide chemistry can be explained by a consideration of molecular orbital stabilization energies.

Item Type: Article
Status: Published
Schools: Earth and Ocean Sciences
Publisher: Wiley-VCH Verlag
ISSN: 1040-0397
Last Modified: 28 Feb 2019 15:30

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