We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)(2)Os-center dot and (eta(5)-(Pr4C5H)-Pr-i)(CO)(2)Os-center dot radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 X 10(5) M-1 s(-1) to 1.7 X 10(7) M-1 s(-1) at 25 degrees C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 +/- 1.0 and 16.8 +/- 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)(2)OsH and (eta(5)-(Pr4C5H)-Pr-i)(CO)(2)OsH, respectively, at 25 degrees C. Temperature-dependent measurements of the kinetic isotope effects over a 60 degrees C temperature range were carried out to obtain the difference in activation energies (E-D - E-H) and the pre-exponential factor ratio (A(H)/A(D)). For hydrogen atom transfer from xanthene to (eta(5)-(Pr4C5H)-Pr-i)(CO)(2)Os-center dot, the (E-D - E-H) = 3.3 +/- 0.2 kcal mol(-1) and A(H)/A(D) = 0.06 +/- 0.02 values suggest a quantum mechanical tunneling mechanism.