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

摘要

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 degreesC and at concentrations below 10 micromolar of S-4(2-). A series of S-4(2-) complexes of these metals were found to exist by voltammetric methods. At higher concentrations, S-4(2-) reacted to give MS and S-8. Tetrasulfide complexes with compositions assigned as [M(eta (1)-S-4)] and [M-2(mu -S-4)](2+) or [M-2 < ((1)-S-4)] occur for Mn, Fe, Co and Ni where only one terminal S atom in the polysulfide binds to one metal (eta (1): unidentate ligand or M-S-S-S-S; mu: ligand bridging two metal centers or M-S-S-S-S-M). Conditional stability constants are similar for all four metals: log beta (1) between 5.6 and 6.0 and log beta (2) 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 oil the higher nucleophilicity of S-4(2-) 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(eta (1)-S-4)], with log beta =8.4. Cu2+ reacts with tetrasulfide to form a complex with the probable stoichiometry [Cu(S-4)](2) with log beta 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(eta (1)-S-4)]. 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. [References: 46]