Valence Bond Theory
Submitted to : Birendra Thapa
Department of Applied Science
and Chemical Engineering
Submitted by : Suyash Adhikari
Roll no : 079BCT090
Department of Computer
Engineering
SOME TERMINOLOGIES
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Valence Bond Theory : Valence bond theory describes a covalent bond as the overlap of half-filled
atomic orbitals (each containing a single electron) that yield a pair of electrons shared between the
two bonded atoms.
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Ligands: The ions or molecules either atom or group of atom which are attached with the central
metal atom/ion to form a complex compound are called ligands.
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Strong Ligands: The ligands which have the tendency to pair up the unpaired electrons in (n-1)d
orbital are called strong ligands. Hence, back-pairing of electrons take place in strong ligands. Eg:
CO, CN-.
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Weak Ligands: The ligands which have no tendency to pair up the unpaired electrons in (n-1)d orbital
are called weak ligands. Hence, back-pairing of electrons cannot take place. Eg: F-,Cl-
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Central Metal atom: The atom/ion to which a fixed number of atoms/group are bound in a definite
geometrical arrangement around it, is called metal atom ion. They are also referred as Lewis acids.
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Coordination Number: The total number of ligands which are directly attached to the central metal
atom/ion is known as co-ordination number of that metal atom/ion.
POSTULATES
01
02
The central metal atom/ion
makes number of empty s, p
and d atomic orbitals equal to
its coordination number for
accommodating electrons
donated by the ligands.
Vacant hybrid orbitals of
the metal atom/ion overlap
with the filled σ-orbital of
the ligand to form ligand →
metal σ-bond (L → M). This
bond is generally known as
coordinate bond
03
04
These vacant orbitals hybridize
together to form hybrid orbitals
which are the same in number
as the atomic orbitals that are
hybridizing together. These hybrid
orbitals are vacant, equivalent in
energy and have definite
geometry.
The ligands have atleast
one σ-orbital containing a
lone pair of electrons
.
05
The strong ligands (such CN- , NO3 , NH3 , CO, NO and organic ligands) have
tendency to pair up the unpaired electron in (n-1)d orbital of the metal atom
i.e. cause back pairing of electrons. But the weak ligands (such as X- , H2O)
cannot cause the back pairing of electrons in the (n-1)d orbital of the metal
ion.
06
The non-bonding electrons of the metal atom/ion are then rearranged in the (n-1) d
orbitals of metal which do not participate in-forming the hybrid orbitals. The
rearrangement of non-bonding electrons take place according to Hund’s rule of
maximum multiplicity
07
The d-orbitals involved in the hybridization may be inner (n-1) d-orbitals or
outer n d-orbitals. The complexes so formed by these two ways are
respectively referred to as low spin and high spin complexes
08
If the complex contains unpaired electrons, it is paramagnetic in nature while if it
does not contain unpaired electrons, the complex is diamagnetic.
09
The number of unpaired electrons in the complex indicates its geometry and viceversa
The shape of the complex can be explained by hybridization:
Coordination
Number
Shape
Structure
Examples
2
Linear
[CuCl2]-, [Ag(NH3)2]+, [AuCl2]-
4
Square planar
[Ni(CN)4]2-, [PdCl4]2-, [Pt(NH3)4]2+,
[Cu(NH3)4]2+
4
Tetrahedral
[Cu(CN)4]3-, [Zn(NH3)4]2+, [CdCl4]2. [MnCl4]2-,[Ni(CO)4]
6
Octahedral
[Ti(H2O)6]3+, [V(CN)6]4-,
[Cr(NH3)4Cl2]+, [Mn((H2O)6]2+,
[FeCl6]3-, [Co(en)3]3+, [Fe(NH3)6]2+
A. OCTAHEDRAL COMPLEXES:
Octahedral complexes is formed when six ligands approach to the metal so, metal in an
octahedral complex requires six hybrid orbitals for the interaction with ligands. Hybrid
orbitals. are formed by the mixing of s, p and d orbitals.
These types of complexes are formed by either d2sp3(inner orbital) or sp3d2(outer
orbital) hybridization
1.
Outer orbital complex(high spin complex.)
let us take an octahedral complex,[Ni(H2O)6]2+
Electronic configuration of Ni2+ is [Ar] 3d8. The no. of ligand(H2O) present is 6; containing 12
electrons. These ligand occupy 4s(one),4p (three)and 4d ( two) orbitals and undergoes
hybridization to give sp3d2 hybrid orbitals.
Other examples:[Cu(NH3)6]3+, [Fe(NH3)6]2+, [Fe(H2O)6]3+
2. Inner orbital complex( low spin complex)
Let us take an octahedral complex, [Cr(NH3)6]3+
Electronic configuration of Cr3+ is [Ar] 3d3. The no. of ligand(NH3) present is 6;
containing 12 electrons. These ligand occupy 3d(two),4s (one)and 4p ( three) orbitals and
undergoes hybridization to give d2sp3 hybrid orbitals.
Other examples, [CO(NH3)6]3+, [Fe(CN)6]4-, [Fe(CN)6]3-
B. Tetrahedral complexes
Tetrahedral complexes are coordination complexes that have a central metal atom surrounded by
four constituent atoms in corners of a tetrahedron. The bond angles of the bonds in this structure are
about 109.5° and involves sp3 hybridization. However, if the constituents(ligand) are different from
each other, the bond angles vary. Tetrahedral geometry is common for complexes where the
metal has d0 or d10electron configuration.
Let us take a tetrahedral complex,[MnCl4]2Electronic configuration of Mn2+ is [Ar] 3d5. The no. of ligand(Cl) present is 4; containing 8
electrons. These ligand occupy 4s(one) and 4p (three) orbitals and undergoes hybridization to give
sp3 hybrid orbitals.
Tetrahedral str.
Other examples: [FeCl4]2-, [Ni(CO)4], [Zn(NH3)4]++
3. Square planar complex:
The square planar complexes form when four ligands approach to the metal, so metal in a square
planar complex requires four hybrid orbitals for the interaction with ligands. Hybrid orbital's are formed
by the mixing of s, p and d orbital's. In a square planar complex, metal requires four orbital's of the
same energy. One d- orbital, one s and two p-orbital combine and form four hybrid orbits.
The hybridization is known as dsp2 hybridization.
The square planar geometry is prevalent for transition metal complexes with d8 configuration.
The square planar geometry is prevalent for transition metal complexes with d8 configuration
For Eg: [Ni(CN)4]2-, [Cu(NH3)4]2+ ions has square planar shape
Square planar
LIMITATIONS
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Most transition metal complexes are colored but the theory provides no explanation for their
electronic spectra.
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VBT does not explain why the magnetic properties vary with temperature.
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This theory could not explain the relative stability of the complexes.
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It does not take into account of the splitting of d-energy levels.
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The VBT does not provide any satisfactory explanation for the existence of inner orbital and
outer orbital complexes.
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It can not explain the nature of ligands.
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It cannot explain why creation complexes are more labile than others. Labile complexes are
those in which one ligand can be easily displaced by another ligand.
THANK YOU