The two-phase reaction of NaL[sub OMe](L[sub OMe[sup [minus]]]) = [([eta][sup 5]-C[sub 5]H[sub 5])Co[l brace]P(O)(OCH[sub 3])[sub 2][r brace][sub 3]][sup [minus]]) in 1% H[sub 2]SO[sub 4] with RuO[sub 4] in CCl[sub 4] affords the edge-sharing octahedral dimer [(L[sub OMe])(HO)Ru[sup IV]([mu]-O)[sub 2]Ru[sup IV](OH)(L[sub OMe])] which is diprotonated by aqueous CF[sub 3]SO[sub 3]H to the corresponding salt [(L[sub OMe](H[sub 2]O)Ru[sup IV]([mu]-O)[sub 2]Ru[sup IV](OH[sub 2])(L[sub OMe])][CF[sub 3]SO[sub 3]][sub 2]. This dimer salt is oxidized by C[sub 6]H[sub 5]IO in CH[sub 3]CN to [(O[sub OMe])(O)Ru[sup V]([mu]-O)[sub 2]Ru[sup V](O)(L[sub OMe])] and reduced by alcohols, aldehydes, or triphenylphosphine in CH[sub 3]CN to [(L[sub OMe])(CH[sub 3]CN)Ru[sup III]([mu]-OH)[sub 2]Ru[sup III](NCCH[sub 3])(L[sub OMe])][CF[sub 3]SO[sub 3]][sub 2]. In buffered aqueous solution, the diprotonated Ru[sup IV]-Ru[sup IV] dimer reacts with formaldehyde to afford a triply bridged dimer, [(L[sub OMe])Ru[sup IV]([mu]-O)[sub 2]([mu]-HCOO)Ru[sup III](L[sub OMe])][CF[sub 3]SO[sub 3]]. The latter reaction appears to be autocatalytic via an inner-sphere mechanism. The Ru[sup III]-Ru[sup III] formate adduct is oxidized by AgCF[sub 3]SO[sub 3] to the Ru[sup IV]-Ru[sup IV] analog [(L[sub OMe])Ru[sup IV]([mu]-O)[sub 2]([mu]-HCOO)Ru[sup IV](L[sub OMe])][CF[sub 3]SO[sub 3]], which slowly reacts with aqueous formaldehyde to regenerate the Ru[sup III]-Ru[sup III] adduct and free formate. The dimeric system in aqueous solution functions as an electrocatalyst for the oxidation of formaldehydemore » at low anodic potentials (near 0.0 V versus SCE at pH 8.5). Both Ru[sup III]-Ru[sup III] dimers have been characterized by X-ray crystallography. 27 refs., 8 tabs., 6 figs.« less