Lewis Acids and Bases - Hard and Soft Classification

Hard and Soft Classification

Lewis acids and bases are commonly classified according to their hardness or softness. In this context hard implies small and nonpolarizable and soft indicates larger atoms that are more polarizable.

• typical hard acids: H+, alkali/alkaline earth metal cations, boranes, Zn2+
• typical soft acids: Ag+, Mo(0), Ni(0), Pt2+
• typical hard bases: ammonia and amines, water, carboxylates, fluoride and chloride
• typical soft bases: organophosphines, thioethers, carbon monoxide, iodide

For example, an amine will displace phosphine from the adduct with the acid BF₃. In the same way, bases could be classified. For example, bases donating a lone pair from an oxygen atom are harder than bases donating through a nitrogen atom. Although the classification was never quantified it proved to be very useful in predicting the strength of adduct formation, using the key concepts

• hard acid — hard base interactions are stronger than hard acid — soft base or soft acid — hard base interactions.
• soft acid — soft base interactions are stronger than soft acid — hard base or hard acid — soft base interactions.

Later investigation of the thermodynamics of the interaction suggested that hard—hard interactions are enthalpy favored, whereas soft—soft are entropy favored.