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 x 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.