Application Of VBT For Four-Coordinated Complexes
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| Application Of VBT For Four-Coordinated Complexes |
Four coordinated
complexes are of two types.
i)
Tetrahedral: They are usually
paramagnetic and have high spin. For example, [MnCl4]2-,
[NiCl4]2-
ii)
Square plannar: They are usually
diamagnetic and have low spin. For example, [NiCN4]2-,
[Cu(NH3)4]2+
Example of tetrahedral complex [NiCl4]2-
i) As we solved for
octahedral complexes here too the first step is to know the O.S. and electronic
configuration. Electronic configuration of Nickel is
Ni
= 1s2, 2s2, 2p6, 3s2, 3p6,
4s2, 3d8, 4p
ii) At ground
state
iii) At
excited state
Nickel will lose
two electrons.
iv) Chloride is a
weak field ligand so it will not pair up electrons and the next four orbitals
will be filled up by the electrons donated by specific ligand. The
hybridization will be sp3.
v) Now we can see
here four sp3 hybrid orbitals will be formed, each having a lone
pair of electron donated by Cl-. The final diagram for [NiCl4]2-
complex will be
Example of squareplannar complex [NiCN4]2-
i) Just like
above example the O.S and electronic configuration will be found. Then:
At ground state:
ii) At excited state:
iii)
CN is a strong field ligand so it will pair up electrons and the next four
orbitals are taken up by ligands. The hybridization will be dsp2.
iv)
Now we have four dsp2 hybrid orbitals, each having an electron pair
donated by CN-. The final diagram for [Ni(CN)4]2-
complex will be
Limitations of VBT:
1)
It cannot account for different shapes for the same coordination number in
metal complexes.
2)
it was unable to explain the magnetic moments of some complicated completes.
3)
it was unable to explain the absorption of spectra of all the complexes.
4)
Nature of copper complexes cannot be explained by VBT.
For
dsp2 one d orbital geometry should be empty. Due to energy movement
can't be explained by VBT.
5)
It can't explain relative rates of analogues of complexes.
6)
In this theory, too much emphasize is given on the metal and nature of ligand
is not explained in detail.
7)
VBT does not explains why different metal complexes of same metal have
different colors e.g. Ni give different colors with cyano and chlorine.
8)
It does not explain the reasons for high spin low spin complexes on the basis
of nature of ligand.
To
overcome the limitations of valence bond theory, Crystal Field Theory was
proposed which is explained next.
