Abstract The synthesis, structure and reaction products of an eight-coordinate Mn(II) complex are described. The reaction of Mn(CIO 4) 2·6H 20 with the tripodal amine ligand tris(2-methylpyridyl)amine (TMPA) yields the eight-coordinate complex [Mn(TMPA) 2](CIO 4) 2 ( 1) which crystallizes in the monoclinic space group P2 1/ n with a=9.7063(3), b=10.929(3), c=18.378(7) Å, β=101.96(3)°. The X-ray structure shows an eight-coordinate Mn(II) ion with coordination to six pyridyl and two amine nitrogens which form a bicapped trigonal antiprism geometry. The oxidation of the Mn(II) complex with H 2O 2 produces the dioxo-bridged dimeric Mn(III,IV) complex [Mn 2(μ- O) 2(TMPA) 2](CIO 4) 3 which shows antiferromagnetic coupling between the two valence trapped Mn ions leading to S=1/2 ground state in a 16-1ine EPR spectrum typical of such dioxo-bridged mixed valent Mn complexes and similar to the EPR spectrum exhibited by the S 2 state Photosystem II. While the Mn(II) complex is stable to oxidation by O 2, in the presence of catechol in methanol solution, it is readily oxidized by O 2 to give the catecholate complex. The oxidation of 3, 5-di-t-butylcatechol to the quinone by O 2 is catalyzed by 1.