Lecture 22 PDF

Preview

Summary

Lecture 22...

Description

Lecture 22

Tuesday, November 28, 2017

Exam 3 Subject - Crystal (Ligand) Field Theory - In crystal field theory, the interaction between the ligands and the metal is assumed primarily electrostatic.

• The primary effect of the ligand electron pairs is to break the degeneracy of the d-orbitals.

- Splitting of metal d-orbitals in a octahedral complex • Repulsion between the ligand electron pairs and electrons in the d orbitals raises the energies of all of the d orbitals, but not to the same extent.

• The closer they are, the more repulsed they are • eg: high energy, sigma bonds - Higher energy because there is more repulsion

the ligand electron pairs lie along the x, y and z axes of a Cartesian coordinate system "

• t2g: low energy, pi bonds

t2g: π bonds

eg: σ bonds

𝑑𝑧2 𝑎𝑛𝑑 𝑑𝑥2− 𝑦2 lobes point directly at the ligand electron pairs and thus " are higher in energy "

dxy, dxz and dyz are not raised as much because the ligand electron pairs point between the lobes of the orbitals

1

Lecture 22

Tuesday, November 28, 2017

- Graphically

- Electron Configurations of Octahedral Complexes • Because the d orbitals have different energies, there are two arrangements of the electrons in these orbitals with certain numbers of d electrons.

• High spin - fill both t2g and eg orbitals with one electron before pairing any electrons • Low spin - fill the t2g set of orbitals completely before putting any electrons in the eg set of orbitals - These configurations may have different energies because of the difference in energies of the t2g and eg orbitals and the different numbers of paired electrons. The difference in energy of between that of the octahedral field and of the hypothetical spherical field is called the Crystal Field Stabilization Energy (CFSE).

• Hypothetical spherical field: hypothetical spherically symmetrical environment (in which the negative charge due to the ligands is evenly distributed over a sphere instead of being localized at six points) defines the barycentre of the array of levels, with the two eg orbitals lying at 35 ∆O above the barycentre and the three t2g orbitals lying at 25 ∆O below it

• High spin: takes less energy to put an electron in eg than to pair it with t2g - High spin will never have P • Low spin: takes less energy to pair an electron in t2g than to put it in eg - 7 electrons and higher require P • CFSE = -0.4Δo(# t2g e-) +0.6Δo(# eg-) + P(# additional e- pairs) - P = pairing energy • This problem will be worth ~12 points on the test (probably one that is 7 electrons or higher, because it is easy to make a mistake in measuring the distance in energy)

- Whether a low or high spin configuration is observed depends on the relative magnitudes of Δo and P • Example: d4 configuration

2

Lecture 22

Tuesday, November 28, 2017

High Spin #

Configuration

Low Spin

Unpaired electrons

CFSE (∆O)

Configuration

Unpaired electrons

CFSE (∆O)

1

t2g1

1

-0.4

2

t2g2

2

-0.8

3

t2g3

3

-1.2

4

t2g3eg1

4

-0.6

t2g4

2 -1.6∆O + P

5

t2g3eg2

5

0.0

t2g5

1 -2.0∆O + 2P

6

t2g4eg2

4

-0.4

t2g6

0 -2.4∆O + 2P

7

t2g5eg2

3

-0.8

t2g6eg1

1 -1.8∆O + 2P

8

t2g6eg2

2

-1.2

9

t2g6eg3

1

-0.6

10

t2g6eg4

0

0.0

• A complex will be high spin when P > Δo. Such complexes are also referred to as weak field complexes

• A complex will be low spin when P < Δo. Such complexes are also referred to as strong field complexes

- Factors Affecting Pairing Energy and Δo • Pairing energy: two factors effect pairing energy - 1. Inherent - repulsion which must be overcome to force two electrons into the same orbital. This remains essentially constant for all transition metals in a particular row.

P > Δo

P < Δo

- 2. Loss of exchange energy - there are more possible arrangements for configurations in which the electrons are unpaired than for those in which the electrons are paired.

• This means that configurations with paired electrons are of higher energy.

• The difference in energy between the configurations with paired and unpaired electrons is proportional to the number of pairs of electrons that can be arranged to form n parallel spins and thus is greatest for a d5 configuration.

• This is the explanation of the special stability of the half-filled d shell. • The loss of exchange energy increases in the order

3

Lecture 22

Tuesday, November 28, 2017

• Δo - 1. Position of the metal in the Periodic Table. • Δo increases as you move down a column in the Periodic Table because the size of the d orbitals increase and the electrons in these orbitals are closer to the ligand electron pairs.

• The result of this is that nearly all second and third row transition metal complexes are low spin. - 2. Oxidation state of the metal • Δo increases as the oxidation state of the metal increases because the ligands move closer to the metal, and thus their electron pairs are closer to the metal d electrons

- 3. Nature of the ligands • Spectrochemical series (Δo increases from left to right) - I-...