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Paper 3: (Stereochemistry, Metal-Ligand Equilibria and Reaction Mechanism of Transition Metal Complexes) Module 1: Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

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

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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

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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

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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.

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4. The Valence-Shell Electron-Pair Repulsion (VSEPR) Model As discussed earlier, the VSEPR model helps in predicting the molecular structures, where:

• Atoms are bonded together by electron pairs in valence orbitals • Since electrons are negatively charged species they tend to repel other electrons

present as lone pairs • Bonding pairs of shared electrons tend to repel other bonding pairs of electrons in

the valence orbital

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The factors that affect the geometry of the molecular species are (i) the total number of electron pairs in the valence shell of the central atom, (ii) availability of the low energy orbitals and (iii) lone pairs. The LPs are assigned the positions where LP-LP repulsions are minimum and LP can expand more readily due to the larger angular volume. The table below summarizes the shapes of molecules predicted from VSEPR theory.

Table:1 Shapes of the molecules redicted from the VSEPR theory

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Here are some examples of species where d orbitals are not used: Isoelectronic CH4, NH3, H2O, HF and NH4+ in all of these the central atom uses sp3 hybrid orbitals. In CH4, all the orbitals have BPs resulting in a symmetrical structure with bond angle 109.5̊. NH3, H2O and HF have 1, 2, 3 LPs respectively. The increasing LP-LP repulsion decreases the bond angles from 109.5̊ to 107̊ in NH3 and to 104̊ in H2O as shown in figure .1

Figure 1: The structure of methane (CH4), ammonia (NH3), water (H2O), hydrogen fluoride and ammonium ion (NH4

+) from valence shell electron pair repulsion theory (VESPRT) When a proton gets attach to the LP of NH3 and forms NH4+ ion, the LP gets polarized by the positive charge of the proton and gets concentrated along the N-H axis. Due to the decreased electron density around nitrogen atoms, the three BPs open up, and as all the four electrons pairs are now BPs, a symmetrical tetrahedral structure with bond angle 109.5̊ results. The positive charge of the proton is equally shared by all the four protons. Now, let us discuss about the geometry of species which involve d orbitals. Let us consider the species PF5, SF4, ClF3, and [ICl2]- , having 10 electrons each in the valence shell of the central atom. The LPs present on P, S, Cl and I are 0, 1, 2, and 3 respectively. The 10 electrons are housed in trigonal bipyramidal geometry ie in sp3d hybrid orbitals. See figure 2

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Figure 2: Possible geometries of (a) Phosphorus Pentachloride PF5, (b) Sulphur tetrafluoride SF4, (c) Chloride trifluoride ClF3, and (d) Dichloriodate (I) ion ICl2

- ; on the basis of VSEPR theory Compounds with highly electronegative fluorine which creates partial positive charge on the central atom and reducing the size of the d orbitals. Role of outer d orbitals: In the compounds like nitronium (NO2

+) ion, nitrite (NO2 -) ion

the splitting of d orbitals should take place. This depends upon the energy of d orbitals and their diffused nature. Outer d electrons are used only in the case of

• Formation of large number of bonds (5, 6,or even 7) • Compounds with highly electronegative fluorine which creates partial positive

charge on the central atom and reducing the size of the d orbitals

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Figure 3: The VSEPR pictures of nitroniumion NO2

+ (a), nitrite ion NO2- (b), and the free radical

NO2 (c)

5. Predicting Molecular Geometries For determination of geometry of the molecule with the help of VSEPR model various steps involved are: 1. Write down the Lewis dot structure of the molecule 2. Count and arrange the total number of electron pairs around the central atom. The arrangement should be done in such a way so that valence shell electron pair repulsion can be minimized. 3. Describing the molecular geometry in terms of the angular arrangement of the bonding pairs In Lewis structures there are two types of valence electron pairs:

• bonding pairs, which are shared by atoms in bonds • nonbonding pairs, which are also called lone pairs

Let us consider the Lewis structure of ammonia,

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Which contains:

• 3 bonding pairs of electrons • 1 nonbonding pair i.e. lone pairs

The ammonia has distorted tetrahedral geometry because in this arrangement the electron pair repulsion between these four electron pairs is minimum.

• This arrangement is for the valence electron pairs. What about the atoms in a compound?

• The molecular geometry is the location of the atoms of a compound in space • We can predict the molecular geometry from the electron pair geometry

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6. Four or Fewer Valence-Shell Electron Pairs Here we’ll try to understand some molecules or ions, which obey the octet rule: [Note-In the prediction of geometry of the molecule a double or triple bond is counted as one bonding pair] Example: Using the VSEPR model we can predict the geometries of a) SnCl3 - and b) O3

7. Effect of Nonbonding Electrons and Multiple Bonds on Bonds Angles

VSEPR model can be used to explain certain exceptions from ideal bond geometries observed in some structures, for example in the case of water, methane and ammonia, it is assumed that all have tetrahedral electron-pair geometries but NH3 and H2O are slightly distorted from an ideal tetrahedron.

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When the central atom has‘d’ orbitals available, then it may have more than 4 electron pairs around it. Such central atoms exhibit a variety of molecular geometries:

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The trigonal bipyramidal arrangement for atoms with 5 pairs of valence electrons contains two geometrically distinct types of electron pairs, axial and equitorial:

If there is a non-bonding pair of electrons (a "larger" electron cloud), it will go in the axial position to minimize electron repulsion. The octahedral structure contains 6 pairs of valence electrons. All positions are equivalent and at 90°from other electron pairs. If there is one nonbonding pair of electrons, it makes no difference where we place them. However, if there are two nonbonding pairs of electrons, the second pair will be 180° from the first to minimize steric interactions

8. Molecules with no central atom For this, let’s consider acetic acid:

• The first carbon has four pairs of valence electrons and will be tetrahedral

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• The second carbon has three (multiple bonds count as one in VSEPR) and will be trigonal Planar

• The oxygen on the right has four and will be tetrahedral (only has two bonds pairs and thus it will appear as a "bent" conformation)

9. Limitations of VSEPR theory Though satisfactory for many species, the VSEPR theory fails for most of the 14 electron systems. For IF7 and ReF7 (where no LP exists) the expected structure is pentagonal bipyramidal with sp3d3 hybridization. It fails to explain the species with LPs. Thus it doesn’t give a correct structure for XeF6 or SbF6

3- (distorted octahedron in which the LP is trying to emerge out of the triangular phase; IF6

- (lower symmetry octahedron); Pb(II), As(III) and Sb(III) complexes with hexadentate ligands (pentagonal bipyramid, where the LP seems to occupy the axial orbital so that it appears that in these complexes, the LP occupies less space than the BPs). Further, though the alkaline earth halides are ionic in a solid state, they are covalent in the vapour phase, where some of them have a bent V-shape. This cannot be explained as the alkaline earth ions, after the formation of the dihalides, do not have any electron pair on them. Generally it fails to predict the shapes of isoelectronic species and transition metal compounds. This model does not take relative sizes of substituents and stereochemically inactive lone pairs into account. See figure. 3

Figure 3: Structure of XeF6 and SbF3 in which the lone pair of electrons is emerging out of the triangular face

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10. Summary The module discusses about the followings

• The valence-shell electron-pair repulsion (VSEPR) theory, that helps in the prediction of the geometries/ shapes of the molecule from their Lewis dot structures

• The basic assumptions of VSEPR theory which state that, the electron pairs are negatively charged and repel each other

• The VSEPR Model, that helps in the prediction of molecular geometry .The best spatial arrangement of the bonding pairs of electrons in the valence shell is the one in which the repulsions between electron pairs are minimized

• Predicting of molecular geometries from Lewis dot structures containing bonding & nonbonding pairs of electrons

• The effect of Nonbonding Electron pairs and Multiple Bonds on Bond Angles of the molecule

• Application of VSEPR Model for determining the structure of molecules with no-central atom.

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