HARD SOFT ACIDS BASES POWER POINT PRESENTATION

Hard and soft acid baseS
Dr M.Prashanthi
Assistant Professor of Chemistry
Government Degree College for Women
Karimnagar

To explain and to predict the stability of a complex
formed between metal ions and ligands,Pearson
proposed a rule called Pearson’s HSAB Principle.
Hard and Soft Acids and Bases

Ralph Pearson introduced the hard-soft-acid-base (HSAB) principle.
Hard acids prefer to coordinate with hard bases and soft acids with soft bases”
1983 – the qualitative definition of HSAB was converted to a quantitative one by
using the idea of polarizability.
A less polarizable atom or ion is “hard” and a more easily polarized atom or ion
is “soft”
Hard and Soft Acids and Bases

Classification of metal ions
Metal ions are classified in to two classes based on preferential
bonding.
Class one – (a)- metal ions :
Ions of Alkali metals : H+, Li+, Na+, K+,Rb+, and Cs+,
Ions of alkaline earth metals : Be2+,Mg2+,Ca2+, Sr2+,and
Ba2+,
 Lighter transition metals of higher oxidation states : Ti4+, Cr3+,
Fe3+,and Co2+.

Class one- (b)- metal ions:
Heavier transition metals of lower oxidation states
Cu+
, Ag+
, Hg+
, Hg2+, Pd2+, and Pt2+.

Classification of ligands
Class two –(a) ligands :
These have tendency to complex with class (a) metal ions.
N >> P > As > Sb
O >> S > Se > Te
F > Cl > Br > I

Class two-(b)- Ligands:
These have tendency to complex with class (b) metal ions
N << P < As < S
O << S < Se < Te
F < Cl < Br < I

Class –a-metals are hard acids.
Class –b-metals are soft acids.
Class –a-ligands are hard bases.
Class –b-ligands are soft bases.

According to HSAB principle-
Hard acids prefer to form bond with hard bases
and
Soft acids prefer to form bond with soft bases.
Based on this principle,Pearson classified hard and soft acids
and bases as follows-
Hard acids
Hard bases
Soft acids
Soft bases

The metal ions or cations with high positive charge,small size
and low polarisability are hard acids.
They form stable complexes with small ligands.
H+
, Li+
, Na+
, K+
,Rb+
, and Cs+
Mg2+
,Ca2+
,Sr2+
,Ba2+
.
Ti4+
, Cr 3+
, Fe3+
and Co2+
Hard acids

Ligands with small size and higher E.N donor atom are hard
bases.They form stable complexes with smaller metal
ions.They have low polarizability.
OH-
,F-
,Cl-
,NO3-
,SO4
-2
,CO3
-2
,PO4
-3
Hard Bases

Soft acids
The metal ions or cations with low
positivecharge,
large size and highly polarisability are soft acids.
They form stable complexes with larger ligands.
Cu+
, Ag+
, Hg+
, Hg2+
,Pd2+
and Pt2+

The ligands which have large size,high polarizability and
low electronegative donor atom are soft bases.They form
stable complexes with larger metal ions.
I-
,H-
,CN-
,R3P,CO,C2H4,R-S-R etc.
Soft bases

A greater covalent character resulting from a soft-soft
interaction is related with lower solubility, color and short
interionic distances.
whereas hard-hard interactions result in colorless and highly
soluble compounds

Applications of HSAB Principle
1.Stability of Complexes.
Pearson explained the stability of complexes on the
basis of HSAB principle.
Ex.Co+3
+ 6F-
-------->[CoF6]-3
Co+3
+ 6I-
--------> [CoI6]-3

2.Feasibility of a reaction
HSAB principle is used to predict feasibility of a
reaction.
Ex.Li-I + Cs-F ---> Li-F + Cs-I

3.Occurance of minerals
The kind of mineral ores found in the earth’s crust can be
found by using HSAB principle.
4.Poisoning of metal catalysts.
5.Heterogeneous catalysis.
6.Behaviour of trivalent B-atom

HARD SOFT ACIDS BASES POWER POINT PRESENTATION

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HARD SOFT ACIDS BASES POWER POINT PRESENTATION