Frustrated Lewis pair
In chemistry, a frustrated Lewis pair is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form an adduct. Because of their "unquenched" reactivity, such systems are very reactive and are able to split the hydrogen molecule heterolytically, which makes them potentially useful as metal-free catalysts for hydrogenation reactions.
In the scheme below, compound 1 has a frustrated Lewis pair: the lone pair on the phosphorus atoms cannot be donated to the boron atoms because of the large substituents on both atoms (Mes = mesityl). However, when exposed to hydrogen at 1 atm, the zwitterionic compound 2 is formed. This hydrogen addition is reversible, and it is possible to transfer the activated hydrogen to a sterically hindered imine, which results in a catalytic cycle that produces the hydrogenated product, using only 0.05 equivalents of 1 as a catalyst.