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Activity 5B: Ligand Field Model (continued): Applications for Metal Complexes
Background
The properties of metal complexes can be understood from the energy splitting of the metal ion’s dorbitals in the lower symmetry that the ligands impose. This feature combined with the number of d
electrons of the metal determines which orbitals are occupied and the possible spectroscopic transitions
and the number of unpaired electrons. The energy splitting of the d-orbitals can be understood in terms of
several models, from the simple electrostatic charge model of crystal field to the sophisticated molecular
orbital model of bonding. An understanding of the order of ligands according to their abilities to split the
d-orbitals (the spectrochemical series) requires a basic understanding of both models. This activitiy
guides you to explore and understand the applications of ligand field splitting in metal complexes.
Learning Objectives
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Understand and apply the ligand field model (crystal field theory) in several geometries
(octahedral, tetrahedral, tetragonal, square planar) (this was accomplished in Activity 5A)
Explain the color and magnetic behaviors of metal complexes
Understand the spectrochemical series (both ligands and metals) in terms of bonding interactions
Success Criteria
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Draw detailed and correct d-orbital splitting diagrams for octahedral, tetrahedral, tetragonal and
square planar geometries (this was done in Activity 5A)
Predict the low-spin, high-spin behavior and the number of unpaired electrons and the magnetic
moment of a metal complex
Relate the magnitude of the ligand field splitting to the color of the compound, the nature of the
ligands and the geometry of the complex
Pre-Activity assignment: Problem 4.35
Post-activity assignment: Problems 4.31, 4.36, 4.37, 4.51 and 4.56
Plan
Work in groups of two or three to answer the following questions and problems.
Model 1: Table 4.1 Electronic Configurations and Crystal Field Stabilization Energies for Metal
Ions in Octahedral Fields (page 71 of Rodgers) (Glen E. Rodgers, “Descriptive Inorganic,
coordination and Solid-State Chemistry,” Brooks/Cole, Thompson Learning, USA, 2002, ISBN 012-592060-1).
Key Questions
Activity for Fundamental Inorganic Chemistry contributed by Susan C. Jackels, Seattle University.
1. Table 4.1 lists the electronic configurations for three cases: Unsplit case, High-spin case and Lowspin case. On the figure below, you will find the labels for case 1, case 2 and case 3. Assign the
three cases to the corresponding case in Table 4.1.
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Energy Splitting of d-orbitals in various strengths of octahedral ligand field
Case 3
dz2, dx2-y2 (eg)
Δo
Energy
Case 2
dxy,, dyz, dxz (t2g)
Case 1
I- < Br- < Cl- < SCN- < NO3- < F- < OH- < C2O42- < H2O < NCS- < py
<NH3 < en < NO2- < Pph3 < CN- < CO
Increasing ligand field strength
Above Figure
Case 1
Case 2
Case 3
Table 4.1
_______ (note: Case 1 is at the origin in the Figure)
_______
_______
2. In the Figure above, what happens to the d-orbital splitting as the ligand field strength increases?
3. For the numbers of d electrons between 4 and 7 in Table 4.1, what is the difference between the
High-spin and the Low-spin cases?
4. Why is there only one case (listed in the middle between High-spin and Low-spin) for 1 – 3 d
electrons and 8 – 10 electrons?
5. What are the factors that determine whether a given complex, [ML6]n+, will have an electronic
configuration in the High-spin or the Low-spin case?
Activity for Fundamental Inorganic Chemistry contributed by Susan C. Jackels, Seattle University.
6. Write the definition of “pairing energy” here
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7. The definition of Crystal Field Stabilization Energy (CFSE) is “the decrease in energy, relative to
the unsplit case, of a coordination compound caused by the splitting of the metal d orbitals by a
field of ligands. Use the diagram and equation below to calculate the CFSE for a complex having
6 d-electrons in both the high-spin and the low-spin cases. Note that high CFSE means that the
complex is more stabilized with respect to the unsplit case.
Unsplit State
Octahedral Ligand Field, ML6
eg +3/5 Δo
d-orbitals
Barycenter, 0
t2g
-2/5 Δo
CFSE = nt2g (2/5 Δo) + neg (-3/5 Δo) + qP
What is nt2g?
What is neg?
What is q?
What is P?
8. In class we noted that the complexes with 3 d-electrons and 6 d-electrons (low spin) were most
likely to have octahedral coordination. For which numbers of d-electrons is the CFSE greatest
(assuming that P is about 40% of Δo). For what other numbers of d-electrons would you expect
octahedral coordination to be common? Why?
Model 2: Table 4.2 and the Spectrochemical Series on page 73 of Rodgers.
1. What are the units used for Δo? This is a common spectroscopic unit.
Activity for Fundamental Inorganic Chemistry contributed by Susan C. Jackels, Seattle University.
2. Using the conversion factor 1000 cm-1 = 12.0 kJ/mole, calculate the CFSE of [Co(CN)6]3-.
3. Using the text on pages 72 – 76, list the factors affecting the magnitude of the crystal field
splitting, Δ:
4. The spectrochemical series for common ligands is given below:
I- < Br- < Cl- < SCN- < NO3- < F- < OH- < C2O42- < H2O < NCS- < py <NH3 < en < NO2- < Pph3 < CN- <
CO
Which ligand has the lowest ability to split the d-orbitals? The highest ability?
5. One series of ligands, namely, the halide anions, does seem to fit the crystal field model we
explored in activity 5A. Explain how this series fits the model.
6. Clearly, more than the crystal field model is needed to explain the entire series. The ligands in the
middle of the series include some neutral molecules that function as Lewis bases in bonding to the
metal ion. A) Using ammonia as an example, explain how ammonia functions as a Lewis base in
bonding to a metal ion. B) What kind of bond is formed?
7. The ligands at the high end of the series benefit from a covalent bonding effect called πbackbonding. Ligands that exhibit this effect have a lone pair that is donated to the metal
accompanied by low-energy empty p (π) orbitals that can overlap with a metal d-orbital to create a
“back bond” to the ligand from the metal. Draw a picture of a d-orbital (four-lobed type)
overlapping with a ligand p orbital. See Figure 4.11 for guidance.
Activity for Fundamental Inorganic Chemistry contributed by Susan C. Jackels, Seattle University.