In 1923, G. N. Lewis described electron donors as bases and electron acceptors as acids. Such Lewis acids and bases can combine to share electrons, forming a Lewis acid–base adduct. This fact leads to an understanding of the formation of main-group adducts and transition-metal coordination compounds, and it offers a rationale for many reactions in organic chemistry. In 2006, D. W. Stephan and colleagues uncovered a perturbation to this long-standing principle. By combining electron-rich and electron-poor species when the molecular structures preclude direct interaction of the electron acceptor and donor, they demonstrated that such sterically “frustrated” Lewis pairs (FLPs) provide unquenched Lewis acidity and basicity that is available for new reactivity.