Frustrated Lewis Pairs (FLPs) are combinations of bulky Lewis acids and bases that can carry out small-molecule activation and catalysis. Mechanistically, the reaction of the acid, base and substrate involves the collision of three distinct molecules, and so the pre-association of the acid and base to form an encounter complex has been proposed. This article will examine the evidence for the formation of this encounter complex, focusing on the archetypal main-group combinations P(tBu)3/B(C6F5)3 and PMes3/B(C6F5)3 (Mes = mesityl), and includes quantum chemical calculations, molecular dynamics simulations, NMR spectroscopic measurements and neutron scattering. Furthermore, the recent discovery that the associated acid and base can absorb a photon to promote single-electron transfer has enabled the encounter complex to also be studied by UV-Vis spectroscopy, EPR spectroscopy, transient absorption spectroscopy, and resonance Raman spectroscopy. These data all support the notion that the encounter complex is only weakly held together and in low concentration in solution. The insights that these studies provide underpin the exciting transformations that can be promoted by FLPs. Finally, some observations and unanswered questions are provided to prompt further study in this field.
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A. R. J. would like to thank his mentors and colleagues for all their support in his career so far, and particularly to Prof. Doug Stephan, Dr Tim Johnstone, and Prof. Chris Slootweg for the stimulating discussions about FLPs and their mode of reactivity that have prompted this article. A. R. J. would also like to thank the Royal Society (URF\R1\201636) and the School of Chemistry at the University of Birmingham for funding.
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