Valence bond
theory explains the bonding in co-ordination compounds. VBT proposed by Pauling
and The main postulates of valence bond theory are:
(1) The central metal ion makes available a number of empty orbitals for
accommodating electrons donated by the ligands. The number of empty orbitals is
equal to the coordination number of the metal ion for the particular complex.
COORDINATION NUMBER
(2) These
empty atomic orbitals (s, p or d) of the metal ion hybridize to form hybrid
orbitals with definite directional properties and give a specific geometry. These hybrid
orbitals now axially overlap with the filled orbital of ligand orbitals to form
strong coordinate bonds and complex obtained a specific geometry.
HYBRIDASATION AND GEOMETRY
(3) The
d-orbitals involved in the hybridization may be either inner (n–1) d-orbitals or outer
ndorbitals. The
complexes formed in these two ways are referred to as low spin and high spin complexes, respectively.
(4) Each
ligand contains a lone pair of electrons.
(5) A covalent-bond is formed by the overlap of a vacant
hybridized metal orbital and a filled orbital of the ligand. The bond is also
sometimes called as a coordinate bond.
(6) If the complex contains unpaired electrons,
it is paramagnetic in nature, while if
it does not
contain unpaired electrons, it is diamagnetic in nature.
(7) The number of unpaired electrons in the
complex points out the geometry of the complex and vice-versa. In practice, the
number of unpaired electrons in a complex is found from magnetic moment
measurements as given as
Magnetic
moment (BM)
|
0
|
1.73
|
2.83
|
3.87
|
4.90
|
5.92
|
Number of
unpaired electron
|
0
|
1
|
2
|
3
|
4
|
5
|
Thus the knowledge of the magnetic moment
can be of great help in ascertaining the type of complex.
(8) Strong field
ligands affects electronic configuration of central metal my making unpaired electron to pair up, while
weak field ligands does not affect electronic configuration of central metal
atom i.e. they does not make unpaired electrons to pair up.
SPECTROCHEMICAL SERIES:
Under the influence of a strong
ligand, the electrons can be forced to pair up against the Hund’s rule of
maximum multiplicity.
Important Note:
Above Statement is valid for first
transition series however for second and third transition series unpaired
electrons, pair up irrespective of nature of ligands provide pairing
of electrons is allowed.
LIMITATION OF VALENCE BOND THEORY
