Hydrides and Dihydrogen Activation

The formation of hydrides by activation of molecular hydrogen implies the transformation of H2 into MH , MH or MH. The hydrogen activation can be either homolytic or heterolytic, and can occur at single or multiple metal centres. Examples of homolytic activation are given in Equations (1) and (2). 

Equation (3) shows an example of heterolytic aetivation. In this case the intermediacy of a non-classical molecular dihydrogen (ri -H2) species has been proposed. This reaction is also an example of a a-bond metathesis. 

Both modes of hydrogen activation, heterolytic and homolytic, may be achieved either directly or through a two-step mechanism involving the intermediate formation of ri -H2 complexes. 

The heterolytic activation of hydrogen involves no change overall in either the formal oxidation state or the coordination number of the metal. The hydride (H ) has replaced the coordinated anion (X ) and release of the proton (ET ) can be promoted by the presence of a base such as triethylamine. 

2 The Reversible Addition of M-H to C=X Bonds on Model Complexes Olefin Isomerization Reactions 

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