The key difference between hard acid and soft acid is that hard acids are cations of electropositive metals, and they are relatively nonpolarizable with a higher charge-to-radius ratio, whereas soft acids are cations of less electropositive metals with a lower charge-to-radius ratio and are more polarizable.
Typically, the thermodynamic stability of a metal complex depends largely on the properties of the ligand and the metal ion and on the type of bonding. There are two types of Lewis acids as hard acids and soft acids.
CONTENTS
1. Overview and Key Difference
2. What is Hard Acid
3. What is Soft Acid
4. Hard Acid vs Soft Acid in Tabular Form
5. Summary – Hard Acid vs Soft Acid
What is a Hard Acid?
Hard acids are cations of electropositive metals, and they are relatively nonpolarizable with a higher charge-to-radius ratio. In other words, hard acids are Lewis acids that are only weakly polarizable. Some examples of hard acids include H+, Li+, Na+, K+, Be2+, Mg2+, Al3+, and Ti4+. Typically, hard acids can react with hard bases fast to form strong bonds with them.
Hard acids tend to bind with hard bases. Hard acids and hard bases are primarily electrostatic in nature. Therefore, the stability of complexes involving hard acids and hard bases increases when the positive charge on the metal increases and when the radius decreases.
What is a Soft Acid?
Soft acids are cations of less electropositive metals which a lower charge-to-radius ratio and are more polarizable. In other words, soft acids are normally larger ions that are polarizable. Some examples of soft acids include BF3, Al2Cl6, CO2, SO3, Cu+, Ag+, Pd2+, Pt2+, and GaCl3. Soft acids react fast with soft bases to form strong bonds with them.
The interaction between soft acids and soft bases can be described as a largely covalent interaction. Most soft acids have completely filled or nearly filled d atomic subshells. This suggests that metal-to-ligand pi bonding is important. Soft acids and soft bases form complexes that are much more stable than the predicted electrostatic arguments.
What is the Difference Between Hard Acid and Soft Acid?
Typically, the thermodynamic stability of a metal complex depends largely on the properties of the ligand and the metal ion and on the type of bonding. There are two types of Lewis bases: hard bases and soft bases. The key difference between hard acid and soft acid is that hard acids are cations of electropositive metals, and they are relatively nonpolarizable with a higher charge-to-radius ratio, whereas soft acids are cations of less electropositive metals with a lower charge-to-radius ratio and are more polarizable. Some examples of hard acids include H+, Li+, Na+, K+, Be2+, Mg2+, Al3+, and Ti4+, while some examples of soft acids include BF3, Al2Cl6, CO2, SO3, Cu+, Ag+, Pd2+, Pt2+, and GaCl3.
The below infographic presents the differences between hard acid and soft acid in tabular form for side by side comparison.
Summary – Hard Acid vs Soft Acid
Hard acids are cations of electropositive metals, and they are relatively nonpolarizable with a higher charge-to-radius ratio. Soft acids are cations of less electropositive metals with a lower charge-to-radius ratio; they are more polarizable. So, this is the key difference between hard acid and soft acid.
Reference:
1. “4.2: Hard and Soft Acids and Bases.” Chemistry LibreTexts, Libretexts, 14 Aug. 2021.
2. “Hard Soft Acid Base – – Theory, Definition, List and Examples.” Learn Chemistry | Primary Study Center.
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1. “Hard soft bases” By Tem5psu – Own work (CC BY-SA 3.0) via Commons Wikimedia
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