Frustrated Lewis pair

A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit activation.

The discovery that some FLPs split H₂ triggered a rapid growth of research into FLPs. Because of their "unquenched" reactivity, such systems are reactive toward substrates that can undergo heterolysis. For example, many FLPs split hydrogen molecules. Thus, a mixture of tricyclohexylphosphine (PCy₃) and tris(pentafluorophenyl)borane reacts with hydrogen to give the respective phosphonium and borate ions:

${\displaystyle {\ce {PCy3 + B(C6F5)3 + H2 -> [HPCy3]+ [HB(C6F5)3]-}}}$

This reactivity has been exploited to produce FLPs which catalyse hydrogenation reactions.