text: Some catalysts operate by "outer sphere mechanisms" such that the substrate never bonds directly to the metal but rather interacts with its ligands, which is often a metal hydride and a protic hydrogen on a ligand. As such, in most cases dihydrogen is split heterolytically, with the metal acting as a Lewis acid and either an external or internal base "deprotonating" the hydride. 
  For an example of this mechanism we can consider the BINAP-Ru-diamine system. The dihalide form of the catalyst is converted to the catalysts by reaction of H in the presence of base:
  :RuCl(BINAP)(diamine)  +  2 KOBu-t  +  2 H  →  RuH(BINAP)(diamine)  +  2 KCl  +  2 HOBu-t
  The resulting catalysts have three kinds of ligands: 
  *hydrides, which transfer to the unsaturated substrate
  *diamines, which interact with substrate and with base activator by the second coordination sphere 
  *diphosphine, which confers asymmetry.
  The "Noyori-class" of catalysts are often referred to as bifunctional catalysts to emphasize the fact that both the metal and the (amine) ligand are functional. 
  In the hydrogenation of C=O containing substates, the mechanism was long assumed to operate by a six membered pericyclic transition state/intermediate whereby the hydrido ruthenium hydride center (HRu-NH) interacts with the carbonyl substrate RC=O. More recent DFT and experimental studies have shown that this model is largely incorrect. Instead, the amine backbone interacts strongly with the base activator, which often is used in large excess. However in both cases, the substate does not bond directly with the metal centre, thus making it a great example of an outer sphere mechanism.