Redox behavior of edge-shared bioctahedral [Re-2(mu-NCS)(2)(NCS)(8)](3-) and its relationship to the direct metal-metal-bonded ion [Re-2(NCS)(8)](2-): A spectroelectrochemical study

Electrochemical and spectroelectrochemical studies have shown that the edge-shared bioctahedral [Re-2(mu-NCS)(2)(NCS)(8)](3-) anion can be reversibly oxidized and reduced (in one-electron steps) with retention of structure to give the even electron species [Re-2(mu-NCS)(2)(NCS)(8)](2-) and [Re-2(mu-NCS)(2)(NCS)(8)](4-). A second oxidation of [Re-2(mu-NCS)(2)(NCS)(8)](3-) is however only partially reversible, the [Re-2(mu-NCS)(2)(NCS)(8)](1-) ion being of limited stability. Further irreversible oxidation of this latter species can be achieved in what appears to be a ligand-based process yielding cyanide-containing species, An irreversible two-electron reduction of [Re-2(mu-NCS)(2)(NCS)(8)](4-) culminates in structural rearrangement to the direct metal-metal-bonded ion [Re-2(NCS)(8)(NCS)(2)](6-), as shown by voltammetric and spectroelectrochemical measurements. Re-oxidation of [Re-2(NCS)(8)(NCS)(2)](6-) results in the quantitative reformation of the edge-shared structure [Re-2(mu-NCS)(2)(NCS)(8)](4-). This reversible structural interconversion occurs in both noncoordinating and coordinating solvents, as well as in the presence of a high concentration of free Cl-. The redox behavior of [Re-2(NCS)(8)](2-) has also been reinvestigated, and the relationship between the direct metal-metal-bonded and edge-shared bioctahedral structures established, Oxidation of [Re-2(NCS)(8)](2-) at room temperature results in the formation of [Re-2(mu-NCS)(2)(NCS)(8)](2-) and NCS--depleted products. [Re-2(NCS)(8)](2-) reacts immediately with 2 mol equiv of NCS- to give [Re-2(mu-NCS)(2)(NCS)(8)](4-), which is susceptible to aerial oxidation. At low temperature, however, the one-electron oxidation and reduction of [Re-2(NCS)(8)](2-) become reversible on the time scale of the IR spectroelectrochemical experiment, such that [Re-2(NCS)(8)](1-) and [Re-2(NCS)(8)](3-) have been characterized in situ. Further reduction of [Re-2(NCS)(8)](3-) to [Re-2(NCS)(8)](4-) is partially reversible under these conditions thus permitting characterization of the latter species.