H2 is a weak Lewis base that can bond to strong electrophiles, but transition metals are particularly exceptional in stabilising H2 complexes by back donating electrons from a filled metal d orbital to the s* orbital of H2, an interaction that is used to distinguish the use of transitional metal as a metal centre with other main group metal atoms like in group 1 or group 2 metals. Backdonation is of vital importance in this process, for sustaining the bonding and splitting the H-H bond. If it is too strong, the H-H bond is broken to form a dihydride complex for the excess electrons in the H2 s* orbital. Importantly, although the activation energy required for the disassociation between H2 and metal centre can be as high as 15-20 kcal/mol, the H2 binding is often be seen as completely reversible here, i.e., H2 can be simply removed by the exposure of hydrogen to vacuum and re-supplied as many times as at mild conditions such as room temperature.