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Electrochemical Evidence for Metal Polysulfide Complexes: Tetrasulfide (S 2– 4 ) Reactions with Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+

Electrochemical Evidence for Metal Polysulfide Complexes: Tetrasulfide (S 2– 4 ) Reactions with... 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Electroanalysis Wiley

Electrochemical Evidence for Metal Polysulfide Complexes: Tetrasulfide (S 2– 4 ) Reactions with Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , and Zn 2+

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References (34)

Publisher
Wiley
Copyright
Copyright © 2001 WILEY‐VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany
ISSN
1040-0397
eISSN
1521-4109
DOI
10.1002/1521-4109(200101)13:1<21::AID-ELAN21>3.0.CO;2-P
Publisher site
See Article on Publisher Site

Abstract

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.

Journal

Electroanalysis – Wiley

Published: Jan 1, 2001

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