Dihydrogen complexes homolytic cleavage

The third major mechanism is based on homolytic cleavage of the dihydrogen molecule by metal-metal (Co) bonded species or by a paramagnetic complex (equations 3 and 4)15. 

Another well-documented way of activating hydrogen on metal complexes is the homolytic cleavage of dihydrogen on two adjacent metal centers which yields also monohydrides, as exemplified for CojCCOjs in Eq. 1.7. 

Scheme 20.4 shows a set of dynamics involving the binding and splitting of H2 that essentially represent the reaction coordinate for homolytic H-H bond cleavage or in the reverse process, the formation of H2 from a dihydride species. Remarkably these can be equilibrium processes in certain cases. Solutions of the complex W(CO)3(PTr3)2(H2) were observed by NMR spectroscopy to contain about a 4 1 ratio of dihydrogen to dihydride complex, proving that side-on bonded H2 complexes were the first step in formation of hydride complexes (Eq. (20.1) P = PiPrj) [lb, 2]. 

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