Download Chapter 10 Homework Problem Molecular Orbital Diagram for Cr

CHEM 432
Chapter 10 Homework Problem
Molecular Orbital Diagram for Cr(CO)6
In this problem you are asked to draw the molecular orbital diagram for the
organometallic complex Cr(CO)6.
Answer all questions, which are sequentially numbered, on separate sheets of paper.
Label them with your name and staple them together before handing them in.
You may discuss ideas about this assignment with your classmates, but the answers
you hand in should be your own work. You are also encouraged to discuss any of this
material with me.
Preliminary Considerations
1. (5 pts) What is the oxidation state of Cr in this complex? How many valence
electrons does Cr have? Does the complex obey the 18-electron rule? Show work.
2. (2 pts) What is the point group of Cr(CO)6?
3. (5 pts) Assign symmetry types (labels) to the basis set orbitals of Cr. Sketch the
orbitals next to each symmetry type designation.
The Carbonyl Ligand
Let’s now consider the CO ligand and how it would bond to the metal.
Energies of the basis set atomic orbitals are given below in eV.
C 2s
-20.03811
C 2p
-10.82129
O 2s
-34.33875
O 2p -16.09289
What follows is a list of energies (in eV) for all CO molecular orbitals.
1
2
3
4
5
6
7
8
-39.1585
-22.1435
-18.9168
-18.9168
-14.4329
-4.5810
-4.5810
63.9790
1
CHEM 432
4. (2 pts) Draw a Lewis structure for CO.
5. (10 pts) Draw a molecular orbital diagram for CO using the information given
above.
6. (5 pts) Compare the structure you drew in question 4 to the molecular orbital
diagram in question 5. Can you assign the electron pairs in the Lewis structure to
specific occupied molecular orbitals in the MO diagram? Draw as many one-to-one
relationships as possible.
7. (3 pts) Which end of the CO ligand bonds to the metal? Looking at your answers to
questions 4 and 5, is there an orbital on that atom that can serve as a σ donor to the
metal? Draw a picture of this orbital. Remember: ligands use frontier molecular
orbitals to bond with the metal.
8. (3 pts) Does CO also use π orbitals to bond with the metal? If so, is this ligand a π
donor or a π acceptor? Base your answers on the CO molecular orbital diagram you
drew in question 5.
9. (2 pts) Sketch the π orbitals on the ligand involved in bonding with the metal.
10. (5 pts) Draw the symmetry-adapted linear combinations (SALCs) of the CO σ and π
molecular orbitals in the point group of the complex.
Chromium Hexacarbonyl
Energies of the metal atomic orbitals are given below in eV.
Cr 3d -8.38305
Cr 4s -5.54879
Cr 4p -1.34885
11. (10 pts) Build a molecular orbital diagram for Cr(CO)6.
12. (10 pts) Sketch the valence molecular orbitals of the complex.
13. (8 pts) Based on the MO diagram, which of the following can be used to describe
CO’s mode of bonding to the metal: a σ donor, a π donor, a π acceptor? Explain.
2