With support by macrocyclic tertiary amine ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 tacn), a number of mononuclear metal–ligand multiple bonded complexes have been isolated. Starting with a brief summary of these complexes, the present review focuses on ruthenium-oxo and -imido complexes of Me 3 tacn. A family of monooxoruthenium(IV) complexes [Ru IV (Me 3 tacn)O(N–N)] 2+ (N–N=2,2′-bipyridines) and a cis-dioxoruthenium(VI) complex cis-[Ru VI (Me 3 tacn)O 2 (CF 3 CO 2 )] + have been isolated, and the structures of [Ru IV (Me 3 tacn)O(bpy)](ClO 4 ) 2 (bpy=2,2′-bipyridine) and cis-[Ru VI (Me 3 tacn)O 2 (CF 3 CO 2 )]ClO 4 have been determined by X-ray crystallography. Oxidation of [Ru III (Me 3 tacn)(NHTs) 2 (OH)] (Ts=p-toluenesulfonyl) with Ag + and electrochemical oxidation of [Ru III (Me 3 tacn)(H 2 L)](ClO 4 ) 2 (H 3 L=α-(1-amino-1-methylethyl)-2-pyridinemethanol) are likely to generate ruthenium-imido complexes supported by Me 3 tacn. DFT calculations on cis-[Ru VI (Me 3 tacn)O 2 (CF 3 CO 2 )] + and proposed ruthenium-imido complexes have been performed. Complexes [Ru IV (Me 3 tacn)O(N–N)] 2+ are reactive toward alkene epoxidation, and cis-[Ru VI (Me 3 tacn)O 2 (CF 3 CO 2 )] + efficiently oxidizes various organic substrates including concerted [3+2] cycloaddition reactions with alkynes and alkenes to selectively afford α,β-diketones, cis-diols, or CC bond cleavage products. Related oxidation reactions catalyzed by ruthenium Me 3 tacn complexes include epoxidation of alkenes, cis-dihydroxylation of alkenes, oxidation of alkanes, alcohols, aldehydes, and arenes, and oxidative cleavage of CC, CC, and C–C bonds, all of which exhibit high selectivity. Ruthenium Me 3 tacn complexes are also active catalysts for amination of saturated C–H bonds.