Activation of molecular hydrogen

The metal-catalyzed homogeneous hydrogenation of unsaturated organic 

Mortreux and F. Petit (Eds.), Industrial Applications of Homogeneous Catalysis, 65—92. 19SS by D. Reidel Publishing Company. All Rights Reserved. 

The reductions are typically multistep processes involving discrete hydrido- and organometallic intermediates. Basically, three main mechanistic pathways have been evidenced for the addition of H2 to unsaturated molecules [1]  

The list is by no means comprehensive, nor are the above mechanisms mutually exclusive. For example, a further variation has been proposed recently for reductions catalyzed by early transition metals and actinides, namely hydrogenolysis of metal—alkyl bonds via a four-center transition state. 

The mechanism encompasses a sequence of oxidative addition, migratory insertion, and reductive elimination steps. Actually, what appears at a schematic level as a fairly simple mechanistic scheme corresponds to an extremely complex reaction system involving a large number of species in simultaneous and consecutive reaction steps [2]. 

Quantum theory led in 1927 to the assertion that the hydrogen molecule could exist in two distinct and stable forms, parahydrogen and orthohydrogen. In 1929 parahydrogen was isolated and methods of analysis of mixtures were developed u 12). In 1931 the heavy hydrogen isotope deuterium was isolated, and the catalytic chemist could now study reactions which he considered to be of ideal simplicity. The reactions are  

We would classify reaction (1) in Class C. Reaction (2) on the other hand may be of a polar nature (Class A), if the H—H bond is ruptured heterolytically  

The theoretical treatment of reaction (1), when catalyzed by paramagnetic centres, was developed by Wigner, 3 , and the reaction was shown to occur, using oxygen or nitric oxide in the gas phase, or oxygen, paramagnetic ions or free radicals in solution 14 1S . 

The interesting aspect of reactions (1) and (2) is that on ortho/para conversion at a paramagnetic centre no H—H bond is broken, and consequently no H/D exchange occurs. On the other hand, when H/D exchange does occur, a hydrogen to hydrogen bond has been broken in the process, and O/P concer-sion occurs as a consequence. Therefore comparison of the rate of the two processes provides valuable information. 

As activated charcoal, metallo-phthalocyanines and solid free radicals catalyze these reactions, they form the oldest example of heterogeneous catalysis by organic and metallo-organic compounds. 

---

Activation of Molecular Hydrogen by Homogeneous Catalysts S. W. Weller and G. A. Mills Catalytic Syntheses of Ketones... 

Corma, A., Boronat, M., Conzalez, S. and Illas, F. (2007) On the activation of molecular hydrogen by gold a theoretical approximation to the nature of potential active sites. Chemical Communications, (23), 3371-3373. 

It is often suggested that the activation of molecular hydrogen may take place via the formation of a molecular hydrogen complex [M(H2)] [75-77] which may further undergo either oxidative addition giving a metal... 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

Recall that the concentration (and activity) of molecular hydrogen in the gas phase can be represented by its partial pressure, Phj, and the concentration of dissolved hydrogen is represented as ch (either ci or C2) in Figure 4.56, so that the equilibrium constant... 

---