The reaction of 9,10-phenanthrenequinone (PQ) with [M-II(H)(CO)(X)(PPh3)(3)] in boiling toluene leads to the homolytic cleavage of the M-II-H bond, affording the paramagnetic trans-[M(PQ)(PPh3)(2)(CO)X] (M = Ru, X = Cl, 1; M = Os, X = Br, 3) and cis-[M(PQ)(PPh3)(2)(CO)X] (M = Ru, X = Cl, 2; M = Os, X = Br, 4) complexes. Single-crystal X-ray structure determinations of 1, 2 center dot toluene, and 4 center dot CH2Cl2, EPR spectra, and density functional theory (DFT) calculations have substantiated that 1-4 are 9,10-phenanthrenesemiquinone radical (PQ(center dot-)) complexes of ruthenium(II) and osmium(II) and are defined as trans-[Ru-II(PQ(center dot-))(PPh3)(2)(CO)Cl] (1), cis-[Ru-II(PQ(center dot-))(PPh3)(2)(CO)Cl] (2), trans-[Os-II(PQ(center dot-))(PPh3)(2)(CO) Br] (3), and cis- [Os-II(PQ(center dot-)(PPh3)(2)(CO)Br] (4). Two comparatively longer C-O [average lengths: 1, 1.291(3) angstrom; 2 center dot toluene, 1.281(5) angstrom; 4 center dot CH2Cl2, 1.300(8) angstrom] and shorter C-C lengths [1, 1.418(5) angstrom; 2 center dot toluene, 1.439(6) angstrom; 4 center dot CH2Cl2, 1.434(9) angstrom] of the OO chelates are consistent with the presence of a reduced PQ(center dot-) ligand in 1-4. A minor contribution of the alternate resonance form, trans- or cis-[M-I(PQ)(PPh3)(2)(CO)X], of 1-4 has been predicted by the anisotropic X- and Q-band electron paramagnetic resonance spectra of the frozen glasses of the complexes at 25 K and unrestricted DFT calculations on 1, trans- [Ru(PQ)(PMe3)(2)(CO)Cl] (5), cis-[Ru(PQ)(PMe3)(2)(CO)Cl] (6), and cis-[Os(PQ)(PMe3)(2)(CO)Br] (7). However, no thermodynamic equilibria between [M-II(PQ(center dot-))(PPh3)(2)(CO)X] and [M-I(PQ)(PPh3)(2)(CO)X] tautomers have been detected. 1-4 undergo one-electron oxidation at -0.06, -0.05, 0.03, and -0.03 V versus a ferrocenium/ferrocene, Fc(+)/Fc, couple because of the formation of PQ complexes as trans-[Ru-II(PQ)(PPh3)(2)(CO)Cl](+) (1(+)), cis-[Ru-II(PQ)(PPh3)(2)(CO)Cl](-) (2(+)), trans- [Os-II(PQ)(PPh3)(2)(CO)Br](+) (3(+)), and cis-[Os-II(PQ)(PPh3)(2)(CO)Br](+) (4(+)). The trans isomers 1 and 3 also undergo one-electron reduction at -1.11 and -0.96 V, forming PQ(2-) complexes trans-[Ru-II(PQ(2-)(PPh3)(2)(CO)Cl](-) (1(-)) and trans- [Os-II(PQ(2-))(PPh3)(2)(CO)Br](-) (3(-)). Oxidation of 1 by I-2 affords diamagnetic 1(+)I(3)(-) in low yields. Bond parameters of 1(+)I(3)(-) [C-O, 1.256(3) and 1.258(3) angstrom; C-C, 1.482(3) angstrom] are consistent with ligand oxidation, yielding at coordinated PQ ligand. Origins of UV-vis/near-IR absorption features of 1-4 and the electrogenerated species have been investigated by spectroelectrochemical measurements and time-dependent DFT calculations on 5, 6, 5(+), and 5(-).