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Chem 321
Prof. Saad M. Alshehri
Subject 6
(Lecture Notes)
Outlines of lecture
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King Saud University
Chem 321
Prof. Saad M. Alshehri
King Saud University
Valence Bond theory
 Linus Pauling mainly developed this approach of coordination
compounds. According to this approach, the formation of a
complex is considered to be a reaction between a Lewis base
(ligand) and a Lewis acid (metal or metal ion) with the
formation of a coordinate covalent bond between the ligand
and the metal.
 Examples:
 H2:
Overlap of two half-filled orbitals leads to the formation of
a covalent bond.
1s-1s overlap gives a H – H single bond
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Chem 321
Prof. Saad M. Alshehri
King Saud University
 HF:
The 1s-2p overlap gives a H – F single bond
Non-bonding electrons
 F2:
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Chem 321
Prof. Saad M. Alshehri
 O2:
NH3:
NH4+:
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King Saud University
Chem 321
Prof. Saad M. Alshehri
King Saud University
 Formation of Bonds:
 Formation of a Sigma bond: A sigma bond is occurs by side
by side overlapping:
Step 1:
Step 2:
Step 3:
Step 4:
 Formation of a π (pai) bond:
Step 1;
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Chem 321
Prof. Saad M. Alshehri
Step 2:
Step 3:
Step 4:
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King Saud University
Chem 321
Prof. Saad M. Alshehri
King Saud University
 Hyberdization:
Hybridization: the combining of solutions to the Schrodinger
wave function for atomic orbitals to produce hybrid orbitals.
Note: the total number of orbitals available for forming
bonds does not change—a new set is simply formed.
 Hybrid orbital: an orbital created by the combination of
atomic orbitals in the same atom.

Type of hybridization:
 Linear sp hybrids: the two hybrid orbitals formed by the
mixture of one s and one p orbital.
 Trigonal sp2 hybrids: the three hybrid orbitals formed by
the mixture of one s and two p orbitals.
 Tetrahedral sp3hybrids: the four hybrid orbitals formed by
the mixture of one s and three p orbitals.
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Chem 321
Prof. Saad M. Alshehri
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King Saud University
Chem 321
Prof. Saad M. Alshehri
King Saud University
 The metal atom or ion under the influence of ligands can use
(n-1)d, ns, np or ns, np, nd orbital for hybridization to
yield a set of equivalent orbitals of definite geometry such
as octahedral, tetrahedral, square planar and so on as given
below in table.
Number of orbitals and type of hybridizations
Coordination
4
4
5
6
6
Type of hybridization
sp3
dsp2
Distribution of hybrid
orbitals in space
Tetrahedral
Square planar
sp3d
sp3d2
Trigonal bipyramidal
Octahedral
Octahedral
d2sp3
 These hybridized orbitals are allowed to overlap with those
of ligand orbitals that can donate an electron pair (two
electrons) for bonding. Let us take some examples to
explain this approach.
 Magnetic Properties are Related to Electron “Spin”
 Electrons have a “spin” (up or down)
 Spin” is a Magnetic property
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Chem 321
Prof. Saad M. Alshehri
King Saud University
 Single electron (spin) will attract a magnetic field so called
paramagnetic.
 If multiple electrons have the SAME spin in a complex, the
complex will be MORE attracted to a magnetic field (more
paramagnetic)
 Paramagnetic due to the presence of unpaired electrons
 Two paired electrons (one up, one down) have no net “spin”
(they cancel out)
 NOT attracted to a magnetic field
 Diamagnetic due to the absence of unpaired electrons
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Chem 321
Prof. Saad M. Alshehri
King Saud University
 Valance bond theory some metal complexes:
[Co(NH3)6]3+:
1.
 In the octahedral complex, [Co(NH3)6]3+. The cobalt (III)
has the electronic configuration 3d6. The complex is
diamagnetic. The hybridization scheme can be given as :
2
2
6
2
6
2
7
Co27 = 1S , 2s , 2p , 3s , 3p , 4s , 3d
Co
3+
2
2
6
2
6
0
= 1S , 2s , 2p , 3s , 3p , 4s , 3d
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6
3+
 To form [Co(NH3)6] , need 6 empty orbitals to hold the
electron pairs donated by the NH3 so must pair up 2
electrons.
 Now we can form bonds with two d orbitals, one s orbital,
2
3
and three p orbitals, giving d sp hybridization.
 Six pair of electrons form six NH3
.
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Chem 321
Prof. Saad M. Alshehri
[Co(NH3)6]
3+
hybridization
King Saud University
2
(inner orbital or low spin complex), d sp
and octahedral structure, [Co(NH3)6]
3+
3
a
diamagnetic compound
 Six pairs of electrons, one from each NH3 molecule occupy
the six hybrid orbitals. Thus, the complex has octahedral
geometry and its diamagnetic character is justified
because of the absence of unpaired electron. Since in the
formation of the complex, the inner d-orbitals (3d) are
used in hybridization, the complex, [Co(NH3)63+ is called
an inner orbital or low spin or spin paired complex.
2.
[CoF6]3 Similar
octahedral
complex,
[CoF6]3-
is,
however,
paramagnetic. In such a situation, Co+3 can be considered
to use outer d orbitals (4d) in hybridization (sp3d2) and
leaving four unpaired electrons in 3d orbitals to satisfy
its paramagnetic nature. It is thus called outer orbital or
high spin or spin free complex.
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Chem 321
Co
Co
Prof. Saad M. Alshehri
2
27
3+
2
6
2
6
2
King Saud University
7
= 1S , 2s , 2p , 3s , 3p , 4s , 3d
2
2
6
2
6
0
= 1S , 2s , 2p , 3s , 3p , 4s , 3d
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 [CoF6]
3-
6
is peramagnetic in nature In such a situation Co
+3
can be considered to use outer d orbitals (4d) in
3 2
hybridization (sp d ) and leaving four unpaired electrons to
satisfy its paramagnetic nature.
It is thus called outer
orbital or high spin or spin free complex.
 Six pair of electrons form six F.
 Now we can form bonds with two d orbitals, one s orbital,
3 2
and three p orbitals, giving sp d hybridization.
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Chem 321
Prof. Saad M. Alshehri
King Saud University
2-
3.
[NiCl4] :
 It is a tetrahedral complex and in tetrahedral complexes,
one s and three p-orbitals are hybridized to form four
equivalent orbitals.
This
[NiCl4]2-.
paramagnetic
is
illustrated
Here
below
nickel (II)
for
has
a
the
electronic configuration 3d8. The hybridization scheme is
as follows:
2
2
6
2
6
2
8
Ni 28 = 1S , 2s , 2p , 3s , 3p , 4s , 3d
2+
Ni
2
2
6
2
6
0
= 1S , 2s , 2p , 3s , 3p , 4s , 3d
26
8
 Four pair of electrons are donated by 4 Cl- ions
.
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Chem 321
4.
Prof. Saad M. Alshehri
[NiCl4]
2-
King Saud University
is paramagnetic in nature, In such a situation Ni
+2
can be considered to one s and three p -orbitals in
3
hybridization (sp ) and leaving two unpaired dorbital
electrons to satisfy its paramagnetic nature.
It is sp3 hybridization and tetrahedral complexes
5.
[Ni(CO)4]:
Ni 28 = 1S2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8
This complexes also has a tetrahedral geometry but is
diamagnetic since it contains no unpaired electron. The two
electrons of 4s orbital under the influence of CO ligand
molecules pair up with the two unpaired electrons of the 3d
orbitals leaving the 4s orbital vacant.
Ni 28 = 1S2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8
The four pairs of electrons from 4 CO molecules then
occupy the sp3 hybrid orbitals.
.
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Chem 321
Prof. Saad M. Alshehri
King Saud University
 This is a tetrahedral complexes one s and three p -orbitals
are hybridized to form four equivalent orbitals create a
tetrahedral geometry.
6.
[Ni(CN)4]2-: In this complex Ni2+ (e.g. [Ni(CN)4]2-) is
diamagnetic, the ligands in these complexes must cause
pairing up the two unpaired electrons. Resulting complexes
is diamagnetic in nature and square planer geometry.
Ni 28 = 1S2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8
Ni2+ 26 = 1S2, 2s2, 2p6, 3s2, 3p6, 4s0, 3d8
 This complexes is diamagnetic, the ligands in these
complexes must cause pairing up the two unpaired
electrons.
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Chem 321
Prof. Saad M. Alshehri
 dsp2
King Saud University
hybrid orbitals, four pairs of electrons from CN-
ligands and the geometry is squar planer
7.
[PtCl4]2-:
Consider the example of [PtCl4]2-, where the hybridization
scheme can be given as follows:
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Chem 321
Prof. Saad M. Alshehri
King Saud University
Since the 4 donated electron pairs occupy one 5d, one 6s
and two 6p orbitals, the complex [PtCl4]2- involves dsp2
hybridization. As a consequence, the geometry of the
complex is square planar with platinum at the centre and
the chloride ions at the four corners.
 The coordination entities like [PtCl4] 2- and [Ni(CN)4]2on one side and [NiCl4]2- and [Ni(CO)4] on the other side
illustrate the useful generalization called, the magnetic
criteria of the bond type, this is, the geometry of the
coordination entity can be predicted if its magnetic
behavior is known.
In the above cases, we find that diamagnetic species are
square
planar
while
the
paramagnetic
species
are
tetrahedral.
It is important to note that the hybrid orbitals are
conceived by a combination of atomic orbitals using
mathematical manipulation of the respective wave functions
involved.
8.
Limitations of Valence Bond Theory
While the VB theory, to a larger extent, explains the
formation,
structures
and
magnetic
behaviour
of
coordination compounds, it suffers from the following
shortcomings:
(i) It involves a number of assumptions.
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Chem 321
Prof. Saad M. Alshehri
King Saud University
(ii) It does not give quantitative interpretation of magnetic
data.
(iii)
It
does
not
explain
the
colour
exhibited
by
coordination compounds.
(iv) It does not give a quantitative interpretation of the
thermodynamic
or
kinetic
stabilities
of
coordination
compounds.
(v) It does not make exact predictions regarding the
tetrahedral and square planar structures of 4-coordinate
complexes.
(vi) It does not distinguish between weak and strong
ligands.
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Chem 321
Prof. Saad M. Alshehri
King Saud University
Practice problems:
1. Co in [CoCl6]3- is a high spin complex. Show the
distribution of d electrons for Co 3+ in the following
octahedral d energy level diagram.
2. Ni in [Ni(CN)4]2- is a dimagentic complex. Show the
distribution of d electrons for Ni2+ and what is the
geometry of Ni ion in this complex.
3. Co in [Co(H2O)6]2+ is a paramagnetic complex. Show the
distribution of d electrons for Cu2+ in the following
octahedral d energy level diagram.
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