Definition of Hard Acid

What is a 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

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Hard - Soft Base Ions

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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-, SO42-

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

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