The energetics of hydrogen scrambling in the ethylene hydride complexes of two late transition metals [(C5R5)(L)M(H)(C2H4)](+) (R = H, Me; L = P(OMe)(3), PMe3; M = Co, Rh) were studied using density functional theory. Four potential nuclear (C/H) scrambling processes in these complexes are discussed; the computed energy barriers for three of them compare very well with activation energies from previous NMR studies, while the fourth process appears to be inactive for energetic reasons. The observed trends for the different metals M, ligands L, and substituents R are reproduced; the activation barriers are insensitive toward L, but somewhat higher for R = H than R = Me. The connection of these elementary processes (or lack thereof) to the beta-migratory insertion reaction in these complexes is discussed; the reported data from NMR experiments are not directly related to the elementary steps of the insertion/elimination reaction. Furthermore, for rhodium complexes with L = C2H4, the observed shift of the global energy minimum is correctly reproduced by our calculations.