Valence Bond Theory (VBT)
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| Valence Bond Theory (VBT) |
Valence
bond theory for d-block dements explains the atomic structures, geometry of
compounds, chemical bonding, electronic structure and magnetic properties. It
was proposed by Pauling and modified by Huggins.
Postulates of valence bond theory:
VBT
for coordination compounds has following postulates:
1)
Central metal atom/ion must have available number of empty orbitals (s, p, d)
equal to the coordination number (C.N) of metal. If CN = 6 then, empty orbitals
= 6. If CN = 4 then, empty orbitals = 4.
2)
Empty orbitals undergo hybridization and made hybrid orbitals of same shapes
and energy.
3)
Suitable number of atomic orbital of central atom or ions hybridize to produce
empty hybrid orbitals.
4)
L: (lone pair) any specie. Ligand have lone pair and orbitals. [at least one
empty orbital containing electron pair].
5)
Empty hybrid orbital of metal overlapped with filled orbital of ligand to form
"ligand to metal bond". This type of covalent bond shows characteristics
of both orbitals.
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6) Non-bonding electrons is of metal atom or ion are rearranged in multiple orbitals which do not take part in forming hybrid orbitals. This rearrangement follows Hund’s rule. For example, cobalt has 6 electrons.
After rearrangement
7) Bonding electrons arrange themselves at maximum distance. Hybrid orbitals participate in bonding and are directed towards ligands position according to the geometry of complex ligands fix themselves.
8)
For a metal having electronic configuration M =1s2, 2s2,
2p6, 3s2, 3p6, 4s2, 3d, 4p, 5s, 4d.
There are two possibilities for incoming electrons. The electron may either go to 3d to 4d.
When
inner d-orbitals are involved in hybridization then the complexes are called
inner orbital complexes. When outer d orbitals are involved then they are known
as outer d orbital complexes.
9) Complexes may paramagnetic or diamagnetic in nature depending upon the number of unpaired electrons. Number of unpaired electrons tells the geometry (inner or outer).
Types of ligands according to VBT
According
to VBT, ligands are of two types
1) Strong Field ligands:
Ligands
that can pair up the electrons present in d orbital of metals. E.g. CN-,
Co, en, NH3, H₂O, NO, Py.
2) Weak Field ligands:
Ligands
that cannot pair up electrons present in d orbital of metals e.g. halogen
family, C2O4-, OH-, NH3,
H2O etc.
[Note: NH3, H2O depends upon nature of metal].
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Bloomstrand Jorgenson Chain Theory
Nomenclature of Coordination Compounds
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