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 - t Bu-C 6 H 4 ) 3 C • ( 1 ) has been measured from −40 to +60 °C in toluene; the isotope effect for transfer from H 2 Os(CO) 4 /D 2 Os(CO) 4 has been measured from 30 to 60 °C. Iterative kinetic simulation has been used to correct measured D • rate constants for the effects of incomplete deuteration. The isotope effect k H / k D at 25 °C is 3.18(3) for HMn(CO) 5 and 4.3(3) for H 2 Os(CO) 4 . The larger isotope effect in the osmium case may be due to the difference in thermochemistry; the H−Os bond is stronger than the H−Mn one, and H • transfer from H 2 Os(CO) 4 to 1 is approximately thermoneutral and in that sense “symmetric”.