Valence Shell Electron Pair Repulsion Theory (VSEPR Theory)

Assumption:

VSEPR Rules

1. Electron pairs tend to _____________ repulsions. For a given no. of electron pairs around the central atom (the occupancy) certain geometries will minimize electron pair repulsions.

Occupancy	Ideal Geometry of e- prs.	Examples
2	linear	CO2, BeCl2
3	trigonal planar	BF3 (CN = 3, trig. planar) SO2 (CN = 2, bent)
4	tetrahedral	CH4 (CN = 4, tetrahed.) NH3 (CN = 3, trig. pyr.) H2O (CN = 2, bent)
4	square planar	trans. metal cmplxs, d8 PtCl42-
5	trig. bipyramidal	PF5
6	octahedral	SF6

 Note that when the occupancy (the total no. of electron pairs around the central atom) equals the coordination number (CN), then the name for the geometry of the electron pairs will be the same as the name of the geometry of the molecule. When CN is less than the occupancy, then the name of the geometry will be different. For example, water, H₂O, has an approximately tetrahedral arrangement of electrons around the oxygen (occupancy = 4). However, the geometry is bent, not tetrahedral since only two atoms are bonded to oxygen (so CN = 2).

2. Repulsions increase in the order BP-BP BP-LP LP-LP.

a. When lone pairs are present, the bond angles are ___________ than predicted by rule 1.

CH₄ vs NH₃

A closer look at lone pairs:

Stick and ball model of ammonia.

Electrostatic potential isosurface for NH₃. This shows where a proton would protonate NH₃.

To a first approximation we can think of the NH₃ HOMO as being equivalent to a lone pair on nitrogen:

NH₃ HOMO. (Highest occupied molecular orbital)

Another view:

Stick and ball model of NH₃.

Electrostatic potential contour plot for NH₃. Magenta means negative potential energy. Again this shows where the proton would want to go and hence, where the lone pair is.

b. Lone pairs chose the ___________ site, e.g._____________________________

BrF₃

c. If all sites are equal, lone pairs will be ___________________

XeF₄

3. Double bonds ____________________________________

SOF₄

Stick and ball model of SOF₄.

Bonding electron density isosurface.

Which bond is "thicker"?

Electron density contour plot.

S=O bond is __________ than S-F bonds therefore F-S-F angle (equatorial) is < 120^o

4. Bonding pairs to electronegative substituents occupy __________ space than those to more electropositive substituents.

X-P-X bond angles:

PF₃ = 97.8^o < PCl₃ = 100.3^o < PBr₃ = 101.5^o < PI₃ = 102^o

Why?

Y-N-Y bond angles in NY₃ :

NF₃ = ____ NH₃ = ____

Why?

Examples:

SO₃

SO₃^2-

SO₂

SF₄

Electron density contour plot of SF₄.

Stick & ball model of SF₄.

Electrostatic potential map for SF₄.

Note the axial F atoms are more electron rich than the equatorial ones.