There is a remarkable redox chemistry of higher oxidation state M(IV)-M(VI) polypyridyl complexes of Ru and Os. They are accessible by proton loss and formation of oxo or nitrido ligands, examples being cis-[Ru-IV(bpy)(2)(py)(O)](2+) (Ru-IV=O2+, bpy = 2,2'-bipyridine, and py = pyridine) and trans-[Os-VI(tpy)(Cl)(2)(N)](+) (tpy = 2,2':6',2"-terpyridine). Metal-oxo or metal-nitrido multiple bonding stabilizes the higher oxidation states and greatly influences reactivity. O-atom transfer, hydricle transfer, epoxiclation, C-H insertion, and proton-coupled electron-transfer mechanisms have been identified in the oxidation of organics by Ru-IV=O2+. The Ru-O multiple bond inhibits electron transfer and promotes complex mechanisms. Both O atoms can be used for O-atom transfer by trans-[Ru-VI(tpy)(O)(2)(S)](2+) (S = CH3CN or H2O). Four-electron, four-proton oxidation of cis, cis-[(bpy)(2)(H2O)Ru-III-O-Ru-III(H2O)(bpy)(2)](4+) occurs to give cis,cis-[(bpy)(2)(O)Ru-V-O-Ru-V(O)(bpy)(2)](4+) which rapidly evolves O-2. Oxidation of NH3 in trans-[Os-II(tpy)(Cl)(2)(NH3)] gives trans-[Os-VI(tpy)(Cl)(2)(N)](+) through a series of one-electron intermediates. It and related nitrido complexes undergo formal N- transfer analogous to O-atom transfer by Ru-IV=O2+. With secondary amines, the products are the hydrazido complexes, cis- and trans-[Os-V(L-3)(Cl)(2)(NNR2)](+) (L-3 = tpy or tpm and NR2- = morpholicle, piperidide, or diethylamide). Reactions with aryl thiols and secondary phosphines give the analogous adducts cis- and trans[Os-IV(tpy)(Cl)(2)(NS(H)(C6H4Me))](+) and fac-[Os-IV(Tp)(Cl)(2)(NP(H)(Et-2))]. In dry CH3CN, all have an extensive multiple oxidation state chemistry based on couples from Os(VI/V) to Os(III/II). In acidic solution, the Os(IV) adducts are protonated, e.g., trans-[Os-IV(tpy)(Cl)(2)(N(H)N(CH2)(4)O)](+), and undergo proton-coupled electron transfer to quinone to give Os(V), e.g., trans-[Os-v(tpy)(Cl)(2)(NN(CH2)(4)O)](+) and hydroquinone. These reactions occur with giant H/D kinetic isotope effects of up to 421 based on O-H, N-H, S-H, or P-H bonds. Reaction with azide ion has provided the first example of the terminal N-4(2-) ligand in mer-[Os-IV(bpy)(Cl)(3)(N(alpha)N(beta)N(gamma)Ndelta)](-). With CN-, the adduct mer-[Os-IV(bpy)(Cl)(3)(NCN)](-) has an extensive, reversible redox chemistry and undergoes NCN2- transfer to PPh3 and olefins. Coordination to Os also promotes ligand-based reactivity. The sulfoximido complex trans-[Os-IV(tpy)(Cl)(2)(NS(O)-pC(6)H(4)Me)] undergoes loss Of O-2 with added acid and O-atom transfer to trans-stilbene and PPh3. There is a reversible two-electron/two-proton, ligand-based acetonitrilo/imino couple in cis-[Os-IV(tpy)(NCCH3)(Cl)(p-NSC6H4Me)](+).It undergoes reversible reactions with aldehydes and ketones to give the corresponding alcohols.