The valence-shell electron-pair repulsion

(VSEPR) model present a simple model for calculate

the shapes of species. The model combines original

ideas of Sidgwick and Powell with extensions developed by Nyholm and Gillespie

The valence shell electron pair repulsion (VSEPR) model is based on the idea that both bonding and

nonbonding electron pairs in the valence shell of an atom ‘repel’ each other.

The basic principles of the model are as follows.

(i) Valence electron pairs round an atom (whether bonding or

nonbonding) adopt a geometry that maximizes the distance

between them..

(ii) Nonbonding electron pairs are closer to the central atom than

bonding pairs and have larger repulsions: in fact, the order of

interactions is

(iii) If double (or triple) bonds are present the four (or six)

electrons involved behave as if they were a single pair, although

they exert more repulsion than do the two electrons of a single

bond

(iv) As the terminal atoms become more electronegative relative to

the central one, bonding electron pairs are drawn away from the

central atom and so repel less.

. Each valence shell electron pair of the central atom E in a molecule EXn

containing E–X single bonds is stereochemically significant, and repulsions

between them determine the molecular shape.

. Electron–electron repulsions decrease as: lone pair–lone pair>lone pair–bonding

pair>bonding

pair–bonding pair.

. Where the central atom E is involved in multiple bond formation to atoms X,

electron–electron repulsions

decrease in the order: triple bond–single bond>double bond–single bond>single

bond–single bond.

. Repulsions between the bonding pairs in EXn depend on the difference between

the electro negativities of E and X;

The size of a bonding electron pair decreases with increasing electronegativity of the ligand or substituent.

The bond angles in nitrogen trifluoride (NF3) and oxygen difluoride F20 are less

than those in ammonia (NH3) and water (H20).

NF3 - 102.3°, NH3 - 107.2°

OF2 - 103.1°, OH2 – 104.5

In a set of halomolecules AB2E2 or AB3E, the BAB bond angles increase in the

order F < CI < Br ::::; I.

Bond angles in phosphorus trihalides are :

PF3 - 97.7°, PCl3 - 100.3°, PBr3 - 101.0°, PI3 - 102°

SF4

This molecule has 10 electrons in the valence shell of SUlphur, four bonding pairs

and one lone pair. In order to let each electron pair have as much room as possible,

the approximate geometry will be a trigonal bipyramid as in phosphorus

pentafluoride. However, the lone pair can occupy one of the two possible positions,

either equational or axial.

(a) trigonal blpyramld with equatorial lone pair.

(b) Trigonal blpyramid SF4 with axial lone pair.

(c) Experimentally determined structure of SF4

NH3, H3O+, SF4, CIF3, ICl-2 and H2O

Limitations of' the VSEPR theory:

1. VSEPR theory cannot explain the shapes of molecules which have very polar bonds e.g.

Li20 should have the same structure as water H20 but it is linear. Alkaline earth halide

molecules MX2 (M = Ca, Sr, Ba) exist only in the gas phase, the solids are ionic lattice.

Most MX2 molecules are linear, but some, such as SrF2, BaF2, are bent.

2. This theory is unable to expain the shapes of molecules having extensive delocalized

π-electron systems.

3. This theory cannot explain the shape of certain molecules which have an inert pair of

electrons.

4. This theory is not able to predict the shapes of certain transition metal complexes.

Valence Shell Electron Pair Repulsion Theory

StructuralPairs

BondedPairs (σ)

LonePairs

MolecularGeometry

Bond angle

2 2 0 Linear 180

3 3 0 Triangular planar

120

2 1 Bent < 120

4 4 0 Tetrahedral 109.5

3 1 Triangular pyramidal

< 109.5

2 2 Bent << 109.5

5 5 0 Triangular bipyramidal

120 & 90

4 1 See-saw < 120 & 90

3 2 T-shape 90

2 3 Linear 180

6 6 0 Octahedral 90

5 1 Square 90 & < 90

pyramidal4 2 Square

planar90

3 3 T-shape < 90

2 4 Linear 180