AllChemicals — The Online Chemical Glossary | https://allchemicals.info/chemical/742-soft-base
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Quick Reference
Also Known As
Class B Lewis base, polarizable Lewis base, HSAB soft base
What is Soft Base?
In HSAB theory, a Lewis base that is large, highly polarizable, and of low electronegativity. Soft bases prefer to bond with soft acids. Examples include S²⁻, RS⁻, CO, CN⁻, and phosphines. They are easily oxidized and have high electron density in diffuse orbitals.
Properties & Characteristics
A soft base (in HSAB theory) is a Lewis base that is large, highly polarizable, and has low electronegativity. Soft bases have loosely held electrons that are easily distorted. Examples include iodide (I⁻), thiols (RS⁻), phosphines (R₃P), and carbon monoxide. Soft bases preferentially bind to soft acids through covalent interactions.
Uses & Applications
Soft bases serve as ligands in coordination complexes used in homogeneous catalysis. Phosphine ligands (soft bases) stabilise transition metal catalysts in organic synthesis. Antidotes for heavy metal poisoning (chelation therapy) work by acting as soft bases to sequester soft acid metal ions.
Safety Information
Some soft bases are toxic, such as CO (carbon monoxide) and cyanide. Thiol compounds often have strong, unpleasant odours and may be toxic. Appropriate fume hood use is essential.
Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.
SynonymsClass B Lewis base, polarizable Lewis base, HSAB soft base
Frequently Asked Questions
In HSAB theory, a Lewis base that is large, highly polarizable, and of low electronegativity. Soft bases prefer to bond with soft acids. Examples include S²⁻, RS⁻, CO, CN⁻, and phosphines. They are easily oxidized and have high electron density in diffuse orbitals.
Soft bases serve as ligands in coordination complexes used in homogeneous catalysis. Phosphine ligands (soft bases) stabilise transition metal catalysts in organic synthesis. Antidotes for heavy metal poisoning (chelation therapy) work by acting as soft bases to sequester soft acid metal ions.
Some soft bases are toxic, such as CO (carbon monoxide) and cyanide. Thiol compounds often have strong, unpleasant odours and may be toxic. Appropriate fume hood use is essential.
Editorial standards: Chemical data is sourced from peer-reviewed literature,
CAS Registry, NIST WebBook, and PubChem. Safety information reflects guidance from OSHA, ECHA,
and IAEA. For educational purposes only — always consult official SDS documentation and qualified
professionals before handling chemicals.
