Rhenium and manganese complexes of the 3,5-di-tert-butyl-1,2-benzosemiquinone (DBSQ) ligand, [M(CO)(4)(DBSQ)], fac-[M(Co>(3)(L)(DBSQ)], and cis,trans-[M(CO)(2)(L)(2)(DBSQ)], with a widely varied nature of co-ligand(s) (L = THF, Me(2)CO, MeC(O)Ph, py, NEt(3), Ph(3)PO, SbPh(3), AsPh(3), PCy(3), P(OPh)(3), PPh(3), dppe-p, PPh(2)Et, P(OEt)(3), PEt(3)) were generated in solution and characterized as valence-localized molecules containing the radical-anionic DBSQ ligand bound to Re-1 or Mn-1 metal atoms. This is evidenced by the following. (i) Carbonyl stretching frequencies v(C drop O) and average force constants k(av) are typical for Mn-1 or Re-1 carbonyls. (ii) Frequencies of the intra-dioxolene C=O bond stretching vibration, v(C=O), lie within the 1400-1450 cm(-1) range which is diagnostic for coordinated semiquinones. (iii) EPR spectra indicate a very small spin density on the metal atom(0.2% < a(M)/A(iso) > 2.6%). (iv) Absorption spectra show Re-1 --> DBSQ MLCT electronic transitions characterized by a resonant enhancement of the Raman peaks due to the v(C drop O) and intra-DBSQ v(C=O) vibrations. (iv) Finally, the electrochemical pattern consists of DBSQ/DBQ and DBSQ/DBCat ligand-localized redox couples. All these properties are, in a limited range, dependent on the nature and, especially, the number of co-ligands L, indicating a small delocalization of the singly occupied MO of the DBSQ ligand over the metal atom. The extent of this delocalization may be finely tuned by changing the co-ligands, although in absolute terms, it remains rather limited, and the DBSQ ligand behaves toward Re-1 and Mn-1 as a very weak pi-acceptor only. The changes of the electronic properties of the metal center induced by the co-ligands are mostly compensated by more flexible M --> CO pi back-bonding as is manifested by large variations of the average C drop O stretching force constant.