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Subject Chemistry
Paper No and Title Paper 3: (Stereochemistry, Metal-Ligand Equilibria and
Reaction Mechanism of Transition Metal Complexes)
Module No and Title Module 1: Valence-Shell Electron-Pair Repulsion (VSEPR)
Theory
Module Tag CHE_P3_M1
CHEMISTRY Paper 3: (Stereochemistry, Metal-Ligand Equilibria and
Reaction Mechanism of Transition Metal Complexes)
Module 1: Valence-Shell Electron-Pair Repulsion (VSEPR)
Theory
2
TABLE OF CONTENTS
1. Learning Outcomes
2. Introduction
3. Basic Assumptions of VSEPR Theory
4. The Valence Shell Electron-pair Repulsion (VSEPR) Model
5. Predicting Molecular Geometries
6. Four or Fewer Valence-Shell Electron Pairs
7. The Effect of Nonbonding Electrons and Multiple Bonds on Bonds Angles
8. Molecules with no central atom
9. Summary
CHEMISTRY Paper 3: (Stereochemistry, Metal-Ligand Equilibria and
Reaction Mechanism of Transition Metal Complexes)
Module 1: Valence-Shell Electron-Pair Repulsion (VSEPR)
Theory
3
1. Learning Outcomes
After studying this module, you shall
• Learn the basic assumptions of VSEPR theory
• Understand the VSEPR Model
• Be able to predict molecular geometries that deal with Lewis structures containing
bonding & nonbonding pairs of electrons
• Analyze the effect of Nonbonding Electrons and Multiple Bonds on Bond Angles
• Know the application of VSEPR model for predicting the geometrical shapes of
molecules through their Lewis structures.
• Explore the application of VSEPR Model for determining the structure of
molecules with no-central atom.
2. Introduction
Molecules of different substances have diverse shapes. Atoms attach to each other in
various geometric arrangements. The overall molecular shape is determined by its bond
angles in space. The shapes of the molecules can be predicted from their Lewis structures
as prescribed in the model presented in the Valence-Shell Electron-Pair Repulsion
(VSEPR) theory. The base of VSEPR theory was laid down by N.V. Sidgwick and H.M.
Powell in the 1930’s but modern formulation of VSEPR theory was proposed by R.
Nyholm and R.J. Gillespie. VSEPR theory is used for predicting the shapes of individual
molecules based on the extent of interactions of electron pairs in the valence shell of the
atoms. It explains the shape of the molecules having localized electron pairs, bonded or
nonbonded. The shape of a molecule is very important for studying its physical and
chemical properties.
The VESPs are regarded as occupying the localized orbitals with proper orientations in
space so as to minimize the coulombic repulsion between the electron pairs leading to
stable spatial arrangement. The stable spatial arrangements of 2,3,4,5 and 6 electron pairs
with minimum inter-electron repulsion are linear, trigonal planar, tetrahedral, trigonal
bipyramidal and octahedral respectively.
3. Basic Assumptions of the VSEPR Theory
The bond angles in a species depends upon (i) interatomic van der waal repulsion
amongst the nonbonded atoms, (ii) coulombic repulsion due to partial charges on the
atoms due to electronegativity differences, or (iii) repulsion between the electron pairs on
CHEMISTRY Paper 3: (Stereochemistry, Metal-Ligand Equilibria and
Reaction Mechanism of Transition Metal Complexes)
Module 1: Valence-Shell Electron-Pair Repulsion (VSEPR)
Theory
4
the bonded atoms. Of all the three terms, (iii) i.e. repulsion between the electron pairs on
the bonded atoms, is the most important affecting the geometry of the species. There are
three types of repulsion that take place between the electron pairs of a molecule:
• The lone pair-lone pair repulsion
• The lone pair-bonding pair repulsion
• The bonding pair-bonding pair repulsion.
The repulsion between the lone pair (LP) electrons are greater than those between the
bonded pairs (BP). For a stable molecule these repulsions must be minimised. When
repulsion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest
deviation from the ideal shape) is preferred. The order of the repulsion between various
types of LP is given as:
LP-LP>LP-BP>BP-BP
This is due to the absence of second nucleus at the distal end of the LP. The nucleus at
the either ends of the electron pair tends to polarize the electron cloud in the internuclear
regions. The LP is attracted only its own nucleus tends to occupy larger angular volume
then BP. Further, double bonds occupy more angular space than single bonds. Also
bonding to more electronegative substituent occupies less space than bonding to a less
electronegative substituent. If the central atom belongs to a third or higher period, the
above rules apply for bonding to halogens and oxygen atoms only. For other atoms, the
LP occupies nonbonding s orbitals and bonding is through p orbitals, e.g. in phosphine
(bond angle 94̊) or in arsine (bond angle 92̊).
The valence bond theory and VSEPR theory is usually compared with each other. The
valence bond theory deals with the molecular shape through orbitals that are energetically
accessible for bonding and mainly concerns with the formation of sigma and pi bonds.
Another model named molecular orbital theory also describes that how atoms and
electrons are assembled into molecules and polyatomic ions.
VSEPR theory was structurally accurate and molecular geometries of covalent molecules
have been predicted successfully. VSEPR theory has been criticized for not being
quantitative. The shape of a molecule can be related to following five basic
arrangements.
CHEMISTRY Paper 3: (Stereochemistry, Metal-Ligand Equilibria and
Reaction Mechanism of Transition Metal Complexes)
Module 1: Valence-Shell Electron-Pair Repulsion (VSEPR)
Theory
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