The energy required to activate the H-H bond in the entire series of Cp(2)LnH complexes has been calculated by DFT (B3PW91) methods. The activation energies have been calculated to vary from 0.5 to 8.0 kcal.mol(-1), indicating an overall facile reaction. The electronegativity of the lanthanide in its most stable oxidation state is suggested to be a leading factor for interpreting the trends in activation energy. The geometry of the transition state is best viewed as an almost linear H-3(-) ligand with short H-H distances and strong M-H interaction, through the wingtip H centers, with Ln. The exchange reaction is thus established to be a sigma bond metathesis reaction.