Steric effects can influence the ability of a Lewis acid or base to form adducts by introducing: front strain (F-strain) whereby bulky groups make it difficult for the Lewis acid and Lewis base centers to approach and interact. back strain (B-strain) associated with steric interactions that do not directly impede the Lewis acid and base centers from interacting but instead occur as the acid and base rearrange upon adduct formation. For instance, when trivalent boron compounds form adducts with amines, the boron center changes from a more open trigonal geometry to a more hindered tetrahedral one: internal strain (I-strain) is also associated with the geometry changes incident on adduct formation. However, while B-strain involves direct steric clashes that occur on adduct formation, I-strain is the strain involved in deforming bond and torsional angles away from more stable local geometries. Thus it is more important for Lewis base centers embedded in rings or clusters. References 1. Alder, R. W., Strain effects on amine basicities. Chemical Reviews 1989, 89 (5), 1215-1223.