B TECH
COMPUTER SCIENCE ENGINEERING
CHEMISTRY(BTCH101-23)
HSAB THEORY (HARD SOFT ACID BASES)
The HSAB (Hard-Soft Acid-Base) Principle was developed by Ralph Pearson. It is utilized to
ascertain the stability of complexes.
According to this principle, a stable complex forms when a hard acid combines with a hard
base , or when a soft acid combines with a soft base.
Characteristics of Hard and Soft Acids and Bases:
● Hard Acids and Bases: These species are typically characterized by a small size and
high charge. They are considered to have high charge density.
○ Hard Acids: These are small cations with high charge, for example, Co3+, Al3+, and
H+.
○ Hard Bases: These are small anions with high charge, such as F−, OH−, and O2−.
● Soft Acids and Bases: These are generally large species with low charge.
○ Soft Acids: These are large cations with low charge, such as Ag+ and Cu+.
○ Soft Bases: These are large anions with low charge, such as I− and S2−.
The HSAB theory predicts reactivity and complex formation by classifying acids and
bases as hard or soft based on their size, charge, and polarizability. Hard acids and
bases tend to interact strongly with each other, as do soft acids and bases. This
principle helps predict reaction outcomes and complex stability in coordination
chemistry.
