HSAB Analysis

In the 1960s, Ralph Pearson suggested that Lewis acids and Lewis bases should be classified as hard, borderline or soft, with the observation that: "Hard [Lewis] acids prefer to complex with hard [Lewis] bases and soft [Lewis] acids with soft [Lewis] bases", the HSAB principle.

The original HSAB analysis is very limited, but it regains its promise and power when applied after Lewis acids and Lewis bases are first classified by their frontier molecular orbital (FMO) topology. The analysis now be usefully applied to describes the richness of bonding interactions.