Sigwick Theory
 |
| Sigwick Theory |
This theory explains the subordinating agents in coordination
sphere complexes. It is based upon electronic theory.
L: → M
(metal)
Lewis
Base Lewis acid
Ligand donate e- pair to the central metal atom. It was
proposed in 1923. Cobalt and platinum complexes were under consideration at
that time.
Why transition metal made bond / coordinate compounds?
 |
| [Co(NH3)6]Cl3 |
 |
| [Co(NH3)5Cl]Cl2 |
To attain the configuration of noble gases, present at the end of
its period. Metals accept the e-s from the ligands to attain the
noble gas configuration. Sigwick gives the concept of charge sphere.
Effective atomic number (EAN):
EAN
rule also called effective electron rule. It is used to determine that how much
electrons are required by an atom.
EAN
= Atomic number – electron lost/required
+ electron gain
For
example: [Co(NH3)6]Cl3
EAN =
27 – 3+ + 12
EAN =
36 (Krypton)
Here
27 = atomic number of cobalt.
3+
is electron lost or oxidation state of cobalt
12 is
electron gain by ligands which are 6 pair because 6 ligands are attached each
donating 1 lone pair of electron. Hence the molecule [Co(NH3)6]Cl3
will try to gain electronic configuration of Krypton.
Limitations of Sigwick theory
1) It
was unable to explain the geometry of complexes.
2) Central metal atom cannot gain so many electrons as electron repulsion will occur and
thus electron gaining was out of comprehension.
3) Many
metals don't follow EAN rule in their complexes
Click Here To Read More Articles
