text: The so-called inner sphere mechanism entails coordination of the alkene to the metal center. Other characteristics of this mechanism include a tendency for a homolytic splitting of dihydrogen when more electron-rich, low-valent metals are present while electron-poor, high valent metals normally exhibit a heterolytic cleavage of dihydrogen assisted by a base. 
  The diagram below depicts purposed mechanisms for catalytic hydrogenation with rhodium complexes which are inner sphere mechanisms. In the unsaturated mechanism, the chiral product formed will have the opposite mode compared to the catalyst used. While the thermodynamically favoured complex between the catalyst and the substrate is unable to undergo hydrogenation, the unstable, unfavoured complex undergoes hydrogenation rapidly. The dihydride mechanism on the other hand sees the complex initially hydrogenated to the dihydride form. This subsequently allows for the coordination of the double bond on the non-hindered side. Through insertion and reductive elimination, the product's chirality matches that of the ligand.
  The preference for producing one enantiomer instead of another in these reactions is often explained in terms of steric interactions between the ligand and the prochiral substrate.  Consideration of these interactions has led to the development of quadrant diagrams where "blocked" areas are denoted with a shaded box, while "open" areas are left unfilled. In the modeled reaction, large groups on an incoming olefin will tend to orient to fill the open areas of the diagram, while smaller groups will be directed to the blocked areas and hydrogen delivery will then occur to the back face of the olefin, fixing the stereochemistry. Note that only part of the chiral phosphine ligand is shown for the sake of clarity.