Transcript Lecture 32 (Slides Microsoft 97-2003) November 1

Basic Ideas Concerning MOs
1. Number of MOs = Number of AOs.
2. Bonding (lower energy) and antibonding (higher
energy) MOs formed from AOs.
3.
e- fill the lowest energy MO first (aufbau process)
4. Maximum 2 e- per orbital (Pauli Exclusion
Principle)
5. Degenerate orbitals fill singly before they pair up
(Hund’s Rule).
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General Chemistry: Chapter 11
Slide 1 of 57
Molecular Orbitals – Learning
Objectives
• 1. Construct molecular orbital diagrams for
diatomic molecules composed of elements
from the first period elements (H and He) and
the second period elements (Li, Be, B, C, N, O
F and Ne). This includes species with +ve and
-ve charges. (Eg. O2+ and CN-).
• 2. Label MOs in the MO diagram and show
their relative energies. Indicate whether MOs
are bonding or anti-bonding.
Molecular Orbitals – Learning
Objectives
• 3. Use the molecular formula (for neutral
molecules and diatomic ions) and charge to
determine the total number of electrons that we
must accommodate using the MO picture.
• 4. Distribute all of the electrons among the
available MOs – starting with the lowest
energy MOs (sound familiar?).
Molecular Orbitals – Learning
Objectives
• 5. After counting the number of electrons in
both bonding and anti-bonding orbitals
determine the bond order.
• 6. Use the MO diagram (and the number of
electrons in the various molecular orbitals) to
determine whether a molecule is diamagnetic
or paramagnetic.
Molecular Orbitals – Learning
Objectives
• 7. Understand a surprising feature of molecular
orbital theory. We can accommodate all of the
valence electrons in various molecular orbitals
for a diatomic species and end up with a bond
order of zero!
Bond Order
• Stable species have more electrons in
bonding orbitals than antibonding.
No. e- in bonding MOs - No. e- in antibonding MOs
Bond Order =
2
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General Chemistry: Chapter 11
Slide 6 of 57
Molecular Orbitals – Nomenclature:
• For the simplest atoms (H, He, Li, Be) only 1s
and 2s orbitals are occupied in the ground
electronic state. The overlap of two 1s orbitals
can only produce a sigma (σ) bond. In the H2
molecule, for example, two 1s atomic orbitals
can combine to form a σ1s bonding molecular
orbital and a σ1s* anti-bonding molecular
orbital. When 2p orbitals come into play we
can form both σ and π molecular orbitals.
Simplest Diatomics – MO Diagrams
• MO diagrams are initially a bit confusing
because they represent the formation of
chemical bonds using both a “before picture”
(showing the relative energies of the various
atomic orbitals) and an “after picture”
(showing the relative energies of the molecular
orbitals). We’ll illustrate this with the
molecules H2, He2, H2+ and He2+.
Diatomic Molecules of the First-Period
BO = (e-bond - e-antibond )/2
BOH + = (1-0)/2 = ½
2
BOH = (2-0)/2 = 1
2
BOHe + = (2-1)/2 = ½
2
FIGURE 11-22
BOHe = (2-2)/2 = 0
2
•Molecular orbital diagrams for the diatomic molecules and ions of the firstperiod elements
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General Chemistry: Chapter 11
Slide 9 of 57
Class Examples
• Draw molecular orbital diagrams for Li2 and
Be2. Using the MO diagrams determine the
bond order for both molecules and, as well,
indicate from the MO diagrams whether the
molecules are diamagnetic or paramagnetic.
Molecular Orbitals of the Second
Period Elements
• First period use only 1s orbitals.
• Second period have 2s and 2p orbitals
available.
• p orbital overlap:
– End-on overlap is best – sigma bond (σ).
– Side-on overlap is good – pi bond (π).
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General Chemistry: Chapter 11
Slide 11 of 57
Molecules with 2nd Period Atoms
• The simplest possible molecular orbital
diagram that one could imagine for second row
elements having 2p electrons is shown on the
next slide. This slide would necessarily apply
only to homonuclear diatomics. Note the
“symmetrical disposition” of bonding and
nonbonding orbitals.
FIGURE 11-25 (PART A)
Possible molecular orbital energy-level scheme for diatomic
molecules of the second-period elements
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Slide 13 of 57
MO Diagrams - Surprises
• The MO diagram presented on the previous
slide does not adequately explain all properties
of diatomic molecules formed from second
period elements. Overlap of 2p atomic orbitals
produces six MOs whose order energy order
can vary with atomic number of the bonded
atoms.
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General Chemistry: Chapter 11
Slide 14 of 57
MO Diagrams – Surprises – C2:
• Two possible MO diagrams are illustrated for
the C2 molecule on the next slide. The
presentation of MOs here is similar to that
used in drawing orbital diagrams for atoms. By
experiment we know that the C2 molecule (4
valence electrons contributed by each C atom
for a total of 8) is diamagnetic. Which of the
MO diagrams accounts for this diamagnetism?
•Experiment shows C2 to be diamagnetic,
supporting a modified energy-level diagram
Expected MO Diagram for C2
Modified MO Diagram for C2
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General Chemistry: Chapter 11
Slide 16 of 57
Rationalization of the C2 “Problem”:
• With homonuclear molecules constructed from
atoms having only ns electrons the
visualization/construction of MOs is simple.
For diatomic molecules containing both 2s and
2p electrons MOs can be “constructed” using
four atomic orbitals as opposed to two. This
accounts for variations in the order of MO
energies and we speak of s and p mixing. (Any
swimming pool analogies?)
FIGURE 11-25 (PART B)
Modified molecular orbital energy-level schemes for diatomic
molecules of the second-period elements
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General Chemistry: Chapter 11
Slide 18 of 57
FIGURE 11-26 (PART 1)
Molecular orbital occupancy diagrams for the homonuclear
diatomic molecules of the second-period elements
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General Chemistry: Chapter 11
Slide 19 of 57
Homonuclear Molecules Containing
2nd Period Atoms (MO Diagrams)
• The next few slides represent the MO
diagrams (molecular energy levels) for a series
of homonuclear diatomics. The MO diagrams
for O2, F2 and Ne2 are what we would expect
to see in the absence of 2s and 2p orbital
mixing. Why? These diagrams account for the
bond order of each molecule and also magnetic
properties. We can construct similar diagrams
for O2+ and O2- (and other molecules).
Molecular orbital occupancy diagrams for the homonuclear
diatomic molecules of the second-period elements
(Symmetrical disposition of Mos)
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General Chemistry: Chapter 11
Slide 21 of 57
Molecular Oxygen is Paramagnetic
• We discussed earlier the fact that Lewis
structures do not account satisfactorily for the
paramagnetism of the O2 molecule. The MO
picture of chemical bonding does account fro
this paramagnetism. A really “cool”
experiment to demonstrate paramagnetism in
oxygen involves trapping liquid oxygen
between the poles of a permanent magnet.
A Special Look at O2
FIGURE 10-3
Paramagnetism of
Oxygen
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General Chemistry: Chapter 11
Slide 23 of 57
Heteronuclear Diatomics – cont’d:
• In heteronuclear diatomics the different
energies of the 2s and 2p orbitals on the two
bonded atoms make “s and p mixing” more
favourable. Why? Here we use new
designations for the MOs since the s and p
subscripts used for homonuclear diatomics (eg.
σ2s) now have little meaning.
A Look at Heteronuclear Diatomic
Molecules
FIGURE 11-27
•The molecular orbital diagram of CO
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General Chemistry: Chapter 11
Slide 25 of 57
Tests 2 and 3
• We’ll examine solutions for a few of the
questions on yesterday’s test. The final term
test is on November 16th. The material for this
test will have been largely covered by the end
of today’s class. In the first instance I’ll ask
you to look at sp3d and sp3d2 hybridization
schemes at home.