Isotope effects on hydrogen atom transfer from transition metals to carbon

The isotope effect for transfer of a hydrogen / deuterium atom from HMn(CO) (5)/DMn(CO)(5) to the monomeric trityl radical (p-Bu-t-C6H4)(3)C-. (1) has been measured from -40 to +60 degrees C in toluene; the isotope effect for transfer from H2Os(CO)(4)/D2Os(CO)(4) has been measured from 30 to 60 degre es C. Iterative Kinetic simulation has been used to correct measured D-. ra te constants for the effects of incomplete deuteration. The isotope effect k(H)/k(D) at 25 degrees C is 3.18(3) for HMn(CO)(5) and 4.3(3) for H2Os(CO) (4). The larger isotope effect in, the osmium case may be due to the differ ence in, thermochemistry; the H-Os bond is stronger than the H-Mn one, and H-. transfer from H2Os(CO)(4) to 1 is approximately thermoneutral and in th at sense "symmetric".