Crystal Field Theory

Question 1

What assumptions do crystal field theory make?

Answer 1

Assumes that the bonding between M and L is purely ionic
Metal complex is a point positive charge (central metal) surrounded by a set of negative charges (ligands)
Bonding energy is from electrostatic forces

Question 2

What is the additional secondary effect of CF theory?

Answer 2

Electrons in different d orbitals interacts with the ligands to different extents
This results in the splitting of the d orbitals into groups of different energy

Question 3

What is crystal field splitting energy?

Answer 3

This is the energy gap between the two groups of orbitals. It has the symbol triangle

Question 4

Describe the d orbital energy in a normal metal ion

Answer 4

The 5 d orbitals are degenerate as they have the same energy

Question 5

Describe d orbital energy of an ion placed in a spherical field

Answer 5

Causes electrostatic repulsion between field and d electrons
All d orbitals would increase in energy

Question 6

What is an octahedral field?

Answer 6

This is where the negative charge of the spherical shell is concentrated at 6 points arranged along the xyz axis at the vertices of an octahedron

Question 7

Describe the d orbital energy in an octahedral field

Answer 7

dx2-y2 and dz2 point directly at ligands which leads to large repulsions so these are higher in energy
dxy, dxz and dyz point between the ligands so they experience less repulsion and are lower in energy

Question 8

What labels are the groups of orbitals given in an octahedral field?

Answer 8

dx2-y2 and dz2 are called eg

dxy, dxz and dyz are called t2g

Question 9

To what extend are the eg orbitals destabilised and the t2g stabilised

Answer 9

t2g is stabilised by -0.4^o(-2/5^o)

eg is destabilised by +0.6^o(3/5^o)

Question 10

How does the overall energy of the octahedral field, compared with that of the spherical field?

Answer 10

The average energy of the 5d orbitals is unchanged

The overall energy is the same

Question 11

What is crystal field stabilisation energy?

Answer 11

This is the stabilisation associated with forming an octahedral complex
This is because most of the electrons are in the t2g which are lower in energy that a spherical field ion d orbitals
The more negative the CFSE, the greater the stabilisation on forming an octahedral complex

Question 12

How are the d orbitals filled?

Answer 12

D orbitals are filled using the aufbau principle, pauli exclusion principle and hunds rule
t2g are filled first as they are lower in energy

Question 13

What are the options for filled after the 3 t2g orbitals have been singly filled?

Answer 13

1) pair up with an electron in t2g which will lead to electron repulsions
2) go into an eg orbital which involves a higher energy orbital

Question 14

What is a low spin complex?

Answer 14

This is where the t2g orbitals are doubly filled before the eg orbitals are filled

Question 15

What is a high spin complex?

Answer 15

This is where t2g and eg orbitals are all singly filled before the electrons start pairing up

Question 16

What determines whether a complex will be high spin or low spin?

Answer 16

This depends on the magnitude of ^o and pairing energy
If pairing energy is greater, the complex will be high spin
If ^o is greater, the complex will be low spin

Question 17

How do you calculate CFSE?

CFSE is crystal field splitting energy (energy in the gap)

Answer 17

CFSE= n(-0.4^o) + m(0.6^o)

n is the number of electrons in t2g
m is the number of electrons in eg

Question 18

What is a weak field case?

Answer 18

A lower energy is achieved in the upper orbital is occupied (high spin)

There is a small energy gap

Question 19

What is a strong field case?

Answer 19

A lower energy is achieve by occupying only the lower orbitals despite the cost of the pairing energy (low spin)

There is a large energy gap

Question 20

How is CFSE equation modified for low spin complexes?

Answer 20

You need to account for pairing energy

CFSE= n(-0.4^o) + m(0.6^o) + piP
pi is the number of additional electron pairs compared to the high spin state

Question 21

What factors affect ^? (The energy gap)

Answer 21

-The charge of the metal- higher charge lead to large ^
- the nature of the metal- ^ is larger for second and third row block d due to larger d orbitals (greater ML overlap)
- nature of the ligand
(Weak field give small ^, strong field give large ^)
- geometry of complex (more ligands means more repulsion so larger gap)

Question 22

Explain why these factors affect ^?

Answer 22

Factors that increase the strength of interaction between the M and L and pull ligand in closer lead to a larger energy gap
This is because there are more repulsions

Question 23

Which factors affect pairing energy?

Answer 23

Larger d orbitals lead to smaller pairing energy as repulsions are decreased

Question 24

What is a strong field ligand?

Answer 24

These are ligands that give rise to large ^

Likely to be low spin

Question 25

What is a weak field ligand?

Answer 25

These are ligands that give rise to small values of ^

Likely to be high spin