Definition of Hard Acid

Hard acids are Lewis acids that are only weakly polarizable.

Other things being approximately equal:

• hard acids react faster with hard bases and form stronger bonds with them
• soft acids react faster with soft bases and form stronger bonds with them

These observations are sometimes called the "HSAB" rule: the hard-soft acid-base rule. The 'rule' is a qualitative guide to reactions; it is not a quantitative, numerical method.

An example of HSAB is that hard acids, such as alkali metal ions, prefer to bond to hard bases, such as O^- or N^-, rather than their softer S^- or P^- analogues.

Hard - Soft Acid Ions

Hard - Soft Base Ions

Further Examples of Hard Acids: BF₃, BCl₃, CO₂, RCO⁺, SO₃, RMgX, VO₂⁺, AlCl₃

Further Examples of Soft Acids: BH₃, Br₂, I₂, RO⁺

Further Examples of Hard Bases: NH₃, ROH, H₂O, OH^-, SO₄^2-

Further Examples of Soft Bases: H^-, R^-, CO, PR₃, C₆H₆, SCN^-