Basic Mechanisms of Dihydrogen Activation

Obviously, there is a great deal of analogy between the mechanisms of hydrogen activation and hydrogenations in aqueous and non-aqueous systems. In principle, H2 may react with a suitable transition metal complex, [MLJ, in several ways (Eqs. 1-4). 

The chemistry of transition metal dihydrogen complexes, such as formed in Eq. (1) is in the focus of very intensive studies [13, 14] and such complexes were shown to be involved in the mechanism of several hyrogenation processes [15]. Despite this fact, their role in aqueous-phase hydrogenations is largely unexplored, although examples of water-soluble (or water-stable) dihydrogen complexes are known [16, 17]. Reaction (1) does not involve the change of the oxidation state of 

Oxidative addition of H2 to low-valent metal centers, such as Rh(I) or Ir(I), results in formation of transition metal dihydrides (Eq. 2) well-characterized dihydrides are known [16, 17] and play important role in hydrogenation cycles [15]. 

Equation (3) depicts direct formation of a monohydrido complex from H2 and MLn, although the reaction is likely to proceed via deprotonation of a dihydrogen complex or that of a transition metal dihydride. Such deprotonations are obviously facilitated by the presence of sufficiently strong bases (either H20 itself, or the bases dissolved in it). 

The hydrides of radical nature, formed for example in reaction (4), are less frequently encountered in aqueous-phase catalyzed hydrogenations, despite the fact that water is an ideal solvent for free radicals in terms of its unreactivity [24]. The best-known organometallic hydrogenation catalyst acting via this route in aqueous solutions is [Co(CN)5]3-. 

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