Hydrogenation of Alkenes or Alkynes

The use as hydrogenation catalysts of neutral or cationic complexes of rhodium with one or two chelating diphosphine ligands is well estab-lished. The mechanisms of reaction have been intensely studied 

The structures of binuclear, diphosphine-bridged complexes of various metals, including rhodium, are discussed in more detail in Chapter 5. 

Results, using cluster complexes of nickel or iridium, support the proposal that hydrogenation of triple-bonded substrates can be more readily achieved by the use of polynuclear homogeneous catalysts than by use of mononuclear catalysts.The requirement for multinuclear centers to heterogeneously catalyze the hydrogenation of CO has also been proposed. Consequently, complexes of transition metals in which the metal atoms are constrained to be proximate are potentially of interest as catalysts. 

However, the only neutral complexes of structure (V) that are catalyti-cally active are the cyano-complex and the diarsine analog (see Table I), which are formed in situ by loss of CO from the tetracarbonyl precursor, (VI).  

The formulation of the catalytically active species may differ from that of the catalyst orginally dissolved in methanol under an atmosphere of dihydrogen. 

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Alkanes are prepared simply by catalytic hydrogenation of alkenes or alkynes (see Section 5.3.1). 

The existence of a step similar to Reaction 2.4 in the hydrogenation of alkenes or alkynes has not been proven, or even considered, for most... 

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