Heterolytic Cleavage and Acidity of Coordinated Dihydrogen

Intemolecular hetsTolytic cleavage involves proton transfer to an external base B to give a metal hydride (H fragment) and the conjugate acid of the base, This 

These reactions are also extremely important in H2 activation and catalysis as will be shown. 

Free Hj is an extremely weak acid with a pK 49 in THF, and heterolytic splitting of rj -H2 in relatively electron-rich neutr complexes is usually achieved only by strong bases. For example, copper alkoxides deprotonate W(CO)3(PR3)2(H, and FeH2(Hj)(PR3)2 to give heterobimetallic species with bridging hydrides.  

However, when is bound to a highly electrophilic cationic metal center, the acidity of H2 gas can be increased spectacularly, up to 40 orders of magnitude. The PK, of H2 can become as low as -6 and thus the acidity of rj -H2 becomes as strong as that in sulfuric or triflic acid. 

Crabtree first demonstrated heterolytic cleavage of / -H2 by isotopic labeling studies to show that the H2 in [IrH(H2)(bq)(PPh3)2] is deprotonated by LiR in preference to the hydride ligand. A milder base, NEtj, was shown by Chinn and Heinekey to specifically deprotonate the j -H2 tautomo- in the equilibrium mixture (84 14 ratio of to dihydride form in Eq. (9.29)  

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