Enthalpies of oxidative addition of PhE-EPh (E = S, Se, Te) to the M(0) complexes M(PiPr(3))(2-) (CO)(3) (M = Mo, W) to form stable complexes M((EPh)-E-.)(PiPr(3))(2)(CO)(3) are reported and compared to analogous data for addition to the Mo(III) complexes Mo(N[Bu-t]Ar)(3) (Ar = 3,5-C6H3Me2) to form diamagnetic Mo(IV) phenyl chalcogenide complexes Mo(N[Bu-t]Ar)(3)(EPh). Reactions are increasingly exothermic based on metal complex, Mo((PPr3)-Pr-i)(2)(CO)(3) < W ((PPr3)-Pr-i)(2)(CO)(3) < Mo(N[Bu-t]Ar)(3), and in terms of chalcogenide, PhTe-TePh < PhSe-SePh < PhS-SPh. These data are used to calculate LnM-EPh bond strengths, which are used to estimate the energetics of production of a free (EPh)-E-. radical when a dichalcogenide interacts with a specific metal complex. To test these data, reactions of Mo(N[Bu-t]Ar)(3) and Mo((PPr3)-Pr-i)(2)(CO)(3) with PhSe-SePh were studied by stopped-flow kinetics. First- and second-order dependence on metal ion concentration was determined for these two complexes, respectively, in keeping with predictions based on thermochemical data. ESR data are reported for the full set of bound chalcogenyl radical complexes (PhE.)M((PPr3)-Pr-i)(2)(CO)(3); g values increase on going from S to Se, to Te, and from Mo to W. Calculations of electron densities of the SOMO show increasing electron density on the chalcogen atom on going from S to Se to Te. The crystal structure of W((TePh)-Te-.)((PPr3)-Pr-i)(2)(CO)(3) is reported.