The attachment of a very unreactive H2 molecule to the organometallic complexes, such as M(CO)3(PR3)2(H2) (M = Mo or W), is an excellent typical example of activation of hydrogen molecules by transition metal complexes through oxidative addition mechanism. The H2 is strongly connected at its side to the central metal primarily, donating two electrons in s orbital to an unoccupied d orbital to construct a stable dihydrogen complex eventually (figure , part B) instead of forming a dihydride complex (figure , part A), through oxidative addition. Sometimes, it could be classified as homolytic fission of H2. 17 The 3-centre metal-H2 interconnection (figure , part A) complies with Werner's theory of the coordination complexes where a ligand transfers its electron density through its lone electron pairs and π-complexes in which electrons are donated from π-electrons.