The quadruply bonded metal-metal complex Mo2[O2P(OC6H5)2]4 reduces 1,2-dichloroalkanes and 1,2-dichloroalkenes upon its irradiation with visible light (lambda(exc) greater-than-or-equal-to 495 nm). The inorganic product of the photochemistry in all cases is the Mo(II)Mo(III) mixed-valence binuclear complex Mo2[O2P(OC6H5)2]4+, which has been structurally characterized. Green prismatic crystals of the tetrafluoroborate salt are triclinic with a P1BAR space group : a = 10.917(8), b = 11.793(4), c = 12.430(4) angstrom; alpha = 63.45(2), beta = 70.38(4), gamma = 70.64(5)degrees; V = 1316.8(12) angstrom3; Z = 1. The structure was refined to values of R = 0.036 and R(w) = 0.043. The Mo-Mo bond distance of 2.191 angstrom is 0.05 angstrom greater than its quadruple bond parent complex, Mo2[O2P(OC6H5)2]4.2THF. The mixed-valence dimer shows a vibrationally-structured delta --> delta* (2B1u <-- 2B2g) absorption band in the near-infrared spectral region (lambda(max) = 1469 nm, epsilon = 142 M-1 cm-1) with a 308-cm-1 energy spacing, which is consistent with a progression in the symmetric metal-metal stretching vibration. The mixed-valence complex is reversibly reduced and oxidized by one-electron (E1/2(Mo2(II,III/II,II) = +0.067 V and E1/2(Mo2(III,III/II,III) = +0.997 V vs SCE). The nature of the organic product resulting from Mo2[O2P(OC6H5)2]4 photochemistry depends on the organic substrate. For 1,2-dichloroalkanes, photoreaction is facile and affords the olefin (phi(p)(1,2-dichloroethane) = 0.029; phi(p)(1,2-trans-dichlorocyclohexane) = 0.040), whereas photoreaction of 1,2-dichloroalkenes yields monohalogenated alkenes (phi(p)-1,2-cis-dichloroethylene) = 5.5 X 10(-6); phi(p)(o-dichlorobenzene) = 1.4 X 10(-4)).