VSEPR.

The familiar VSEPR (Valence Shell Electron Pair Repulsion) approach to molecular structure was developed by Ronald Gillespie. The basic idea is that lone pairs of electrons occupy space around a central atom in much the same way as do atoms that are bonded to the central atom. The lone pairs and bonded atoms then assume that geometry that minimizes electrostatic repulsionbetween them.

Ronald Gillespie.

Electron domains and molecular geometry:

Lewis dot diagram of ammonia

each lone pair of electrons plus each atom bonded to thecentral atom constitute an electron ‘domain’

Ammonia trigonal pyramidal (derived from tetrahedral geometry)

lone pairof electrons

observed geometry isthat where the electrondomains are as far apartas possible

H

H

HN

Using VSEPR In order to use VSEPR to predict molecular structure:

1) Draw up Lewis dot diagram for the molecule or ion. The first atom (e.g. Br in BrF5) is always the central atom. Place the other atoms around the central atom.

If these are single bonds, contribute one electron per attached atom. Then add the valence electrons for the central atom = 7 for Br.

2) Work out number of electron domains = valence electron pairs (‘n’) plus attached atoms on central atom. For BrF5 n = 6.

3) Relate n to the type of structure predicted for that value of n. n = 6 = octahedral.

4) Place lone pairs in expected positions, maximizing separation of lone pairs. For BrF5, there is one lone pair, so mol. structure = square pyramidal.

red = 7 valenceelectrons for Br

place 5 Fatoms aroundcentral Br

The structure of BrF5 from VSEPR:

Lewis dot diagram

n = 6 from fiveattached atomsplus one electronpair

n = 6, parent structure= octahedral, but one siteoccupied by a lone pair

molecular or finalstructure – disregardthe lone pair

molecular structure =square pyramidal

parent structure

lone pair

Parent shapes for EXn molecules (n = 2-5)

Formula n shape shapes of structures

EX2 2 linear

EX3 3 trigonal planar

EX4 4 tetrahedral

EX5 5 trigonal

bipyramidal

Parent shapes for EXn molecules (n = 6-8)

Formula n shape shapes of structures

EX6 6 octahedral

EX7 7 pentagonal

bipyramidal

EX8 8 square

antiprismatic

Final structures for VSEPR theory.

More final structures for VSEPR.

A series of derivatives of the EX4 geometry (all with n = 4) but with increasing numbers of lone pairs:

Methane ammonia water hydrogen fluorideTetrahedral trigonal pyramid bent linear diatomic

lonepairs

Structures derived from trigonal geometry (n = 3):

boron trifluoride nitrite anion, NO2-

trigonal planar bent

trigonal planar bent

lone pair

Ozone – a bent molecule:The structure of the O3 (ozone) molecule can be predicted using VSEPR. First draw up the Lewis dot diagram:

Central atom(red valenceelectrons)

For the valence shell of thecentral oxygen atom n = 3,so parent geometry =trigonal. The final structure is thus two-coordinate bent, as seen for the ozone molecule below:

Structure of the ozonemolecule (oxygens =red atoms) ozone

Note that two pairsof e’s still count asonly one electrondomain = one attached O-atom

Structures derived from TBP (n = 5):(Note: Lone pairs go in the plane:)

Structures derived from the octahedron (n = 6):

Structures derived from the pentagonal bipyramid (n = 7) (Note: lone pairs go axial)

Example:

Note: The way the number of valence electrons (= 12) on the iodine is derived is from the seven valence electrons for iodine (group 7in the periodic table), plus one each from the F-atoms, and one from the negative charge on the complex.

Negative charge adds a valence electron to iodine.

Example: Chlorine trifluoride

NOTE: in structures derived from a TBP parent structure, the lone pairsalways lie in the plane, as seen here for the T-shaped structure of ClF3.

The structure of [IF5(C6H5)]-:

Note: an aliphatic or aromatic group is equivalent to an F.

S.Hoyer, K.Seppelt (2004) J. Fluorine Chem. ,125, 989

iodine

phenyl group

fluorine

Diphenyl(acetato)iodine(V)oxide

carbon atomsfrom phenyls

oxygen fromacetato group

two pairsof electrons= double bond

oxideoxygen

phenyl group

iodine

The structure of bis(pentafluorophenyl)xenon. VSEPR explains this type of structure, which is linear like XeF2.

(explain the latter in terms of VSEPR)

H.Bock, D.Hinz-Hubner, U.Ruschewitz, D.Naumann(2002) Angew.Chem.,Int.Ed. , 41, 448

xenon

pentafluorophenyl group

The [I(C6H5)2]+ cation:

phenyl group

iodine

Bis(trifluoroacetato)phenyl-iodine(III)

iodine

phenyl group

trifluoroacetate group

The effect of lone pairs on bond angles:

In VSEPR the lone pairs appear to occupy more space than electron pairs in bonds, with the result that bond angles are compressed away from the lone pairs. For example, in structures derived from tetrahedral parent geometry, such as water or ammonia, the H-O-H and H-N-H angles are compressed to be less than the 109.5º expected for a regular tetrahedron:

O

HH104.5o H H

H

N

106.7oH

lone pairs

water ammonia

Effects of lone pairs on bond angles in ClF3 and ClF5.

chlorine trifuoride chlorine pentafluoride

86.0o87.5o