The common feature of these elements is the existence of a partially filled valance shell. Thus, transition metals possess some interesting attributes which may affect the hydrogenation and the dihydrogen activation process. Among them, the presence of various transition metal complexes in which the metal is bonded with electron-donating ligands, can be perceived as the most important thing to note when we are considering the possible mechanism of the activation process. Generally, the bonding between the metal and the ligands is weaker than that of a full covalent bond between typical non-metals in terms of bonding enthalpy but not so weak as to be classed as a secondary interaction. These complexes may retain their structures when dissolved in solution, but in such circumstances, one ligand can readily displace another, since the activation energy of substitution of ligands is usually low. 12