Irradiation of a THF solution containing Mn-2(CO)(10)and 3,6-di-tert-butyl-1,2-benzoquinone (3,6-DBBQ) have dark green crystals of Mn(thf)(2)(3,6-DBQ)(2). Structural characterization has shown that the inner coordination geometry about the metal has the axially elongated trans structure that appears characteristically for d4 Mn(III). trans-Mn-III(thf)(2)(3,6-DBSQ)(3,6-DBCat) has a solid-state electronic spectrum that resembles spectra of related Mc(III) complexes containing nitrogen-donor ligands. Crystallographically imposed mmm symmetry averages structural differences between the SQ (semioquinoate) and Cat (catecholate) ligands. When the related reaction betwenn Mn-2(CO)(10) and 3,6-DBBQ was carried out in the presence of 2,2′-bithiophene, Mc(3,6-DBQ)(3) was obtained. Crystallographic characterization has shown that the molecule is octahedral with relatively short Mn-O bond lengths (1.863(5)-1.927(5)A), and without the structural distortion that might be indicative of Mn(III). This result, with the spectral and magnetic properties of the complex, points to the Mn-IV(3,6-DBSQ)(2)(3,6-DBCat) charge distribution at room temperature in the solid state. An intense transition at 2300 nm has been assigned as an intervalence transfer (IT) band between SQ and Cat ligands of the Mn(IV) redox isomer. This band decreases in intensity with increasing temperature. Similar behavior has been observed for Mn-IV(N-N)(DBCat)(2) species as an indication of valence tautomerism. In this case the equilibrium is between Mn-IV(3,6-DBSQ)(2)(3,6-DBCat) and Mn-III(3,6-DBSQ)(3) redox isomers.