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Valence Bond Theory
By Prof Yeap Guan Yeow
1
The knowledge on complex formation from the reaction between
the donor and acceptor properties was developed by Linus Pauling
to become valence bond theory (VBT)
-
This basic concept involves donation of electron pairs from the
donor atom to the empty orbital (d orbitals) of metallic ion.
-
This phenomenon is encouraged via the hybridization of
valence atomic orbitals within the metal atom leading to a set
of equivalent orbitals which receives the electron pairs from
the donor atom.
2
As a result, the structure for transition metal
complexes can be stabilized through different
types of orbital hybridization of metallic atom as
shown in the following structure .
Hybridization
sp, ds
sp2
sp3 , d3s
dsp2
dsp3, d2sp2 , sp3d
sp3d2, d2sp3
Structure
linear
trigonal planar
tetrahedral
square planar
trigonal bipyramidal
octahedral
3
(a) Ni(CO) 4
Ni(0)
Ni(CO)4
[Ar]3d84s2
xx
x x xx xx
sp3
4
The above suggest that all electrons are paired and that the
magnetic moment, µ = 0. It means that the Ni(CO)4 complex is
diamagnetic. The structure of molecule for the Ni(CO)4 complex
can be shown as follows:
O
C
Ni
C
C
C
C
O
O
5
(b) [Ti(H2O)6 ]3+
O O
H2 H2
- Paramagnetic property
- Orbital hybridization d 2sp3
(octahedral)
O
H2
Magnetic moment, ? =
Since, n = 1
therefore, ? =
B. M.
O O O
H2 H 2 H2
B. M.
Ti(O) 3d2 4s2
Ti(III) 3d1
6
3
In order to predict the actual molecular structure, the µ value is
needed.
To understand this fact, try to refer to the [Co(NH3)6]3+ and
[CoF6]3- complexes.
For [Co(NH3)6]3+
XX XX
XX
XX XX XX
d2sp3
µ=0
diamagnetic
7
[CoF6]3XX
XX XX XX
XX XX
sp3d2
µ = 5.9. B.M.
paramagnetic
8
The difference in above complexes can be explained by the outer
and inner orbital complex concept.
In this context, d2sp3 and sp3d2 are thought to have the same
energy. Every orbital is resulted from the hybridization of the
s, p and d orbitals leading to d2sp3 and sp3d2 respectively.
(i) For sp3d2 hybridization: 4s, 4p and 4d orbitals are close
in energy levels. (Outer orbital complex)
(ii) For d2sp3 hybridization: 3d, 4s and 4p orbitals are close
in energy levels. (Inner orbital complex)
9
The weakness of valence bond theory
(i)
The assumption that all 3d orbitals have the same
energy contradicts with other theories.
(ii) The use of 3d and 4s orbitals with different energy to
form the chemical bonding is unsatisfactory.
(iii) This theory does not explain the electronic spectra
which are caused by the transition of
electrons between the d orbital levels.
(iv) This theory cannot predict the electronic spectra
which are caused by the transition of electrons
between the orbital levels (degenerate).
10
For example,
Ni(CN)42
→
Ni(CN)3-
d8
d9
µ=0
µ=0
diamagnetic
diamagnetic
11
According to valence bond theory;
Ni(CN)4 2-
0
0
0
0
0
0
0
0
0
0
0
0
? = 0 ; diamagnetic
d sp2
Ni(CN) 3 -
0
0
sp 3
0
0
? =
=
= 1.7 B.M
Paramagnetic
12
Ni(CN)3- complex which is a monomer can adopts a
paramagnetic properties if based on the value of µ.
However, in practical it is found to have diamagnetic
properties.
This observation suggests that there are changes occurred
in that complex.
One of the probability is the dimer transformation as shown
in the following scheme
Ni(CN)3- → → Ni2(CN)62- .
This phenomenon can be stabilized by using dsp2 orbital.
13
Ni
Ni
Ni-Ni bond formation.
As a result, µ = 0 diamagnetic ;
Ni(CN)3- complex is diamagnetic.
14
dz2 and dx2-y2 orbitals have the charge density which is
directed to
(i) z axis ,
(ii)
x and y axis, respectively.
The dxy, dxz, and dyz orbitals will not direct themselves
accurately on the Cartes axes; that is to say, the lobes for
the three orbitals are placed 45° in between the axes.
15