Why is it so Difficult to Activate

The H2 molecule is, in fact, so stable that it was used as an inerf gas in early air-free chemistry. The following physical properties of H2 combine to make it a very unreactive molecule (i) the H-H bond is remarkably strong, (ii) the H2 molecule is completely nonpolar, (iii) the frontier molecular orbitals of H2 do not permit most direct, concerted reactions between dihydrogen and other non-metals, and (iv) the molecule is a very poor acid. 

Bond type Average bond dissociation energy (kcal mol )  

The direct reaction of H2 with C2H4 is therefore expected to have a large activation barrier. 

The uncatalyzed heterolytic cleavage of the H-H bond (H2- H + H ) is also difficult. The strength of the H-H bond and its lack of polarity contribute to the poor kinetic and thermodynamic acidity of H2. The pfC values for a series of mono-protic acids , dissolved in either tetrahydrofuran (THF) or acetonitrile solvent, are given in Table 4.2. Dihydrogen with an estimated pfCa of 49 in THF solvent is among the weakest acids. 

---