Hard and Soft Acid-Base Behavior in Aqueous Solution: Steric Effects Make Some Metal Ions Hard: A Quantitative Scale of Hardness-Softness for Acids and Bases

The idea of hard and Soft Acids and Bases (HSAB), developed by R. G. Pearson some 30 years ago, has been used as a unifying principle in several texts. In spite of this, HSAB has remained qualitative and largely intuitive. In this article, the behaviour of HSAB in aqueous solution is addressed. A quantitative scale of hardness-softness for acids and bases is presented also.