Adducts

Adducts often form between Lewis acids and Lewis bases. A good example is the formation of adducts between the Lewis acid borane and the oxygen atom in the Lewis bases, tetrahydrofuran (THF): BH₃·O(CH₂)₄ or diethyl ether: BH₃·O(CH₃CH₂)₂. Many Lewis acids and Lewis bases reacting in the gas phase or in non-aqueous solvents to form adducts have been examined in the ECW model. Trimethylborane, trimethyltin chloride and bis(hexafluoroacetylacetonato)copper(II) are examples of Lewis acids that form adducts which exhibit steric effects. For example: trimethyltin chloride, when reacting with diethyl ether, exhibits steric repulsion between the methyl groups on the tin and the ethyl groups on oxygen. But when the Lewis base is tetrahydrofuran, steric repulsion is reduced. The ECW model can provide a measure of these steric effects.