Electrochemical Evidence for Pentasulfide Complexes with Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+

Research paper by Steven J. Chadwell, David Rickard, George W. Luther

Indexed on: 01 Mar '99Published on: 01 Mar '99Published in: Aquatic Geochemistry


A series of stable pentasulfide complexes of the common base metals, Mn, Fe, Co, Ni, Cu and Zn exist in aqueous solutions at ambient temperatures. Pure sodium pentasulfide was prepared and reacted with the divalent cations of Mn, Fe, Co, Ni, Cu and Zn in aqueous solution at ambient temperature. The S52- complexes were found to exist as determined by voltammetric methods.Pentasulfide complexes with compositions assigned as [M(η1-S5)] and [M2(μ- S5)]2+ occur for Mn, Fe, Co and Ni where only one terminal S atom in the S52- binds to one metal (η1 = mono-dentate ligand or M-S-S-S-S-S, μ = ligand bridging two metal centers or M-S-S-S-S-S-M). Conditional stability constants are similar for all four metals with log β1 between 5.3 and 5.7 and log β2 between 11.0 and 11.6. The constants for these pentasulfide complexes are similar to the tetrasulfide complexes and are approximately 0.4–0.8 log units higher than for comparable bisulfide complexes [M(SH)]+ as expected based on the higher nucleophilicity of S52- compared to HS-. Voltammetric results indicate that these are labile complexes.As with the bisulfide and tetrasulfide complexes, Zn(II) and Cu(II) are chemically distinct from the other metals. Zn(II) reacts with pentasulfide to form a stable monomeric pentasulfide chelate, [Zn(η1-S5)] with log β = 8.7. Cu(II) reacts with pentasulfide to form a complex with the probable stoichiometry [Cu(S5)]2 with log β estimated to be 20.2. As with the other four metals, these complexes are comparable with the tetrasulfide complexes. Discrete voltammetric peaks are observed for these complexes and indicate they are electrochemically inert to dissociation. Reactions of Zn(II) and Cu(II) also lead to significant breakup of the polysulfide.The relative strength of the complexes is Cu > Zn > Mn, Fe, Co, Ni. Cu displaces Zn from [Zn(η1- S5)] and both Cu and Zn displace Mn, Fe, Co and Ni from their pentasulfide complexes.