Homolytic Splitting of Coordinated Dihydrogen

However, a cmcial question arose when H2 complexes were first discoveaed Does coordinated H2 directly transfer H atoms to unsaturated coUgands such as olefins without prior OA of (he H-H bond, as in Eq. (9.3) Ironically this is still 

It is remarkable that Eq. (9.1) is observed as a tautomeric equilibrium in solution in about a dozen types of Hj complexes, including the first complex, W(CO)3(PRj)2(H2) (1). Structure 2 exists in about 20% concentration in Eq. (9.4) 

Solution IR of CO-containing complexes such as 1 also indicate the simultaneous existence of Hj and dihydride forms by showing distinct v o absorptions for each tautomer Eq. (9.4)], even for unstable photogenerated CpNb(CO)3(H2) and CpNb(H)2(CO)3 in liquid Xe, where NMR studies would be difficult. The Hj complex is thought to have a square-based-piano-stool geometry but the geometry of the dihydride is unknown. More recently a similar cationic system [Cp Os(CO)2(H2)] (7) prepared by protonation of the corresponding monohydride showed equilibrium exclusively with the trans dihydride isomer (8) (87% at -80°C).  

Elimination of H2 from dihydrides is the microscopic reverse of H2 splitting [Eq. (9.1)], and proceeds via a M-H2 intermediate. The ease of H2 loss is highly variable, and although MoH2(CO)(R2PC2H4PR2)2 (R = Et, Bu) (9) contains hydrides well separated by phosphines, it slowly dissociates H2 in vacuo via 10 to form agostic 11. Well-known unsaturated systems that form a cis dihydride rather 

Several well-known examples of complexes, particularly those of cobalt, split Hj gas in an intermolecular homolytic fashion to give monohydrides  

In many cases an unobserved H2 complex is an intermediate in dihydrogen-dihydride interconversions where only the reactant or the product is observed as in Eq. (9.8) (R = Cy).19 The scrambling of H atoms in [M(H2)H(L2)2]+, L2 = dppe, 

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