Normal and inverted redox potentials and structural changes tuned by medium effects in [M2Mo(η5-C5Me5)2(S2C6H4)2(CO)2] (M: Co, Rh)

Redox potential inversion was observed during the two-electron (2e(-)) reduction of two novel heterometal trinuclear complexes, [Co2Mo(eta(5)-C5Me5)(2)(S2C6H4)(2)(CO)(2)] (1) and [Rh2Mo(eta(5)-C5Me5)(2)(S2C6H4)(2)(CO)(2)] (2), the metalladithiolenes of which were found to communicate electronically via a Mo(CO)(2) moiety through two Co/Rh-Mo bonds. The redox properties and structural changes associated with each complex were investigated. 1 showed both two-step one-electron (1e(-)) reductions with a normal-order redox potential difference and a one-step 2e(-) reduction with a redox potential inversion that depended on the solvent and electrolyte. The size of the counter cation was found to significantly influence the reduction behaviour of 1. In contrast, 2 showed only a one-step 2e(-) reduction that didn't depend on the solvent-electrolyte conditions. The CO coordination mode in each reduction underwent a reversible structural reorganization upon reduction, from a semibridging configuration between the Co/Rh atoms and the Mo atom in the neutral form to a bridging configuration in the reduced form. The local coordination structure of the metal atoms in the dianion 2(2-) was experimentally determined for the first time. The interactions between Co and CO in 1 were weaker than the interactions between Rh and CO in 2, and differences in the interaction strengths, together with the influence of the solvent-electrolyte medium, were found to determine the redox processes available to the complexes.