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.
Read more about this topic: Lewis Acids And Bases
Famous quotes containing the words hard and, hard and/or soft:
“Machinery that gives us abundance has left us in want. Our knowledge has made us cynical, our cleverness hard and unkind. We think too much and feel too little. More than machinery, we need humanity. More than cleverness, we need kindness and gentleness. Without these qualities, life will be violent, and all will be lost.”
—Charlie Chaplin (1889–1977)
“No man is quite sane; each has a vein of folly in his composition, a slight determination of blood to the head, to make sure of holding him hard to some one point which nature has taken to heart.”
—Ralph Waldo Emerson (1803–1882)
“Since time immemorial, one the dry earth, scraped to the bone, of this immeasurable country, a few men travelled ceaselessly, they owned nothing, but they served no one, free and wretched lords in a strange kingdom. Janine did not know why this idea filled her with a sadness so soft and so vast that she closed her eyes. She only knew that this kingdom, which had always been promised to her would never be her, never again, except at this moment.”
—Albert Camus 1013–1960, French-Algerian novelist, dramatist, philosopher. Janine in Algeria, in The Fall, p. 27, Gallimard (9157)