CHM 501 Lecture

Molecular Structure and Simple Covalent Bonding Models

Covalent bonding can be thought of as the situation when groups of atoms stay associated with each other independent of the phase

Valence Bond Theory: bonding occurs when pairs of electrons accumulate between nuclei

Why?


Paired electrons screen the nuclear charge, so nuclear-electron attraction overcomes electron-electron repulsion giving a net attraction of nuclei.

Lewis Dot structures

Electrons are distributed about atoms pictorially.
Only use valence electrons.
Lines represent an electron pair, either as a bond or a lone pair.
The formal charge about each atom should be minimized (neutral is best).
The octet "rule" works well for C, N, O, F but otherwise is merely advisory.


Resonance is a superposition of degenerate (equivalent) VB structures - this helps minimize charge distribution.

The net structure is a weighted average of all possible dot structures; the weighting is based on the relative energy of each resonance structure: low-energy structures contribute a lot and high-energy structures contribute little.


Neutral structure perhaps contributes less than ionic structures (Why?)

Valence Shell Electron Pair Repulsion model (VSEPR)

A simple application of Coulomb's Law to Lewis Dot structures - electron pairs distribute themselves to minimize electron-electron repulsion

Rules:

1) Draw the best Lewis Dot structure with minimum formal charges

2) Count the number of lone pairs and bonds (sb=db=tb) about the center of interest

3) Distribute the electron density as follows:
Count
Distribution
Geometry
2
linear
180o angles
3
trigonal
planar
120o angles
4
tetrahedral
109o angles
5
trigonal bipyramid
120o and
90o angles
6
octahedral
90o angles
7
pentagonal bipyramid
72o and
90o angles

4) Distribute the lone pairs and bonds to minimize repulsions using the following:

lp-lp > lp-bp > bp-bp (tb > db > sb)

5) Fine geometry is dictated by lp-bp repulsions




Hybridization : one way to "generate" molecular orbitals to be used in VB theory

Wave interference can occur on-site (same atom) as well as between atoms: this costs energy but the energy is recovered upon bond formation.

Hybridization probably occurs because of atom-atom repulsions so is more prevalent among first row atoms than anywhere else on the Periodic Table.

Understanding the relative phases of orbitals is important because constructive interference (orbitals with the same phase) increases orbital size while destructive interference (orbitals of opposite phase) decreases the orbital size.

Some common types of hybrids:
 (convention : the unique axis is defined as the z axis)
hybrid
geometry
orbitals used
sp
linear
s + pz
sp2
trigonal
s + px + py
sp3
tetrahedral
s + px + py + pz
sd3
tetrahedral
s + dxy + dyz + dxz
dsp2
square planar
s + px + py + dx2-y2
dsp3
trigonal bipyramid
s + px + py + pz + dz2
d2sp3
octahedral
s + px + py + pz + dz2 + dx2-y2