Pearson principle of hard and soft acid and base

Pearson introduced a simple yet powerful concept to predict the stability of chemical complexes based on the nature of acids and bases involved.

Hard acids prefer to combine with hard bases, and soft acids prefer to combine with soft bases.

If both the acid and base in a complex (AB) are either hard or soft, the resulting compound tends to be more stable. This principle provides a qualitative tool for predicting the relative stability of coordination compounds.

The stability arises because:

• Hard acid + hard base: High electronegativity difference → Strong ionic attraction.
• Soft acid + soft base: Similar electronegativity → More covalent character.

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