Crystal Field Theory

Question 1

Who developed CFT and when

Answer 1

1. Hans Bethe

2. 1929

Question 2

Why did Bethe develop CFT

Answer 2

1. In order to interpret colour spectra and magnetism in crystals
2. For the solid state

Question 3

What did Van Vleck do

Answer 3

1. 1932

2. Used CFT to interpret some properties of TM complexes- solutions

Question 4

What are 3 assumptions used in CFT

Answer 4

1. Interactions between ligands and metals are electrostatic (ionic)
2. The metal and ligands are point charges; lone-pairs of ligand electrons are treated as negative charges; the metal centre as a point positive charge
3. No metal-ligand covalent interactions are considered

Question 5

What are 3 shortcomings of CFT

Answer 5

1. Metal-ligand interactions are not purely electrostatic (ionic)
2. The metal and ligands are not point charges (have a steric demand)
3. There is some covalency in metal-ligand interactions

Question 6

Describe the spherical case for a 3d metal

Answer 6

1. The metal Mn+ is placed at the origin of the cartesian axes (X,Y,Z all perpendicular)
2. The ligands (L) are considered to be at a long way from the metal (non-bonding distance)
3. The metal feels the effect of the ligands as though it is spread out on the surface of a sphere
4. This sets up a spherical electrical field
5. The energy of the d-orbitals remains equivalent (degenerate)
6. The overall energy of the orbitals and the electrons within them, are uniformly raised (relative to the free metal ion) because of the electron-electron electrostatic repulsions
7. As the ligand-sphere is brought into bonding distance with the metal, some orbitals are stabilised relative to the spherical field and some are de-stabilised
8. The stabilised orbitals will feel less e-e repulsion to the ligands relative to the spherical field and reverse of true for the de-stabilised orbitals

Question 7

What happens to the energy of d-orbitals in a spherical field

Answer 7

1. D-orbitals energies are raised relative to the spherical field
2. Due to e-e repulsions

Question 8

Does the arrangement of 6 ligands around a metal ion have inversion symmetry

Answer 8

1. Yes

2. Gerade ‘g’

Question 9

In an octahedral case which orbitals will raise in energy and why

Answer 9

1. The ligands will point directly at the lobes of the metal dz^2 and dx^2-y^2 orbitals
2. The electron-electron repulsions will raise the energy of the electrons in theses two 3d orbitals with respect to the spherical field

Question 10

What is the name for the average energy of d-orbtials

Answer 10

1. Barycentre

Question 11

In an octahedral case which orbitals will lower in energy and why

Answer 11

1. The energy of the electrons in the metal d-orbitals whose lobes point between the ligands, dxy and dxz and dyz, is lowered with respect to the spherical field

Question 12

What happens to the total energy of the orbitals

Answer 12

1. Remains the same

Question 13

What label is given to stabilised orbitals in an octahedral complex

Answer 13

1. t2g

Question 14

What label is given to destabilised orbitals in an octahedral complex

Answer 14

1. eg

Question 15

What is the name and given to the energy gap between the two sets of orbitals

Answer 15

1. Delta(o) or delta(oct) or 10 Dqo

2. Crystal field splitting energy

Question 16

What is the relationship between the total stabilisation of the t2g and destabilisation of the eg orbitals

Answer 16

1. They are equal

Question 17

How much is each t2g orbital stabilised by

Answer 17

1. -0.4Deltao relative to the free ion in a spherical field

Question 18

How much is each eg orbital destabilised by

Answer 18

1. +0.6deltao relative to the free ion in a spherical field

Question 19

Describe bonding in [Ti(OH2)6]3+ including electron configuration of Ti ion

Answer 19

1. 6 H2O ligands- octahedral geometry

2. Ti3+ - [Ar]3d1

Question 20

How can CFT be used to explain the optical absorption spectrum of a d1 complex such as [Ti(OH2)6]3+

Answer 20

1. One peak at highest absorption- wavelength at this point is lambda max
2. Lambda max is equivalent to deltao
3. The band arises as a result promotion of the single electron in the t2g subset to the higher energy eg subset
4. This is the only possible d-d transition for this complex

Question 21

At lambda max what is the energy of the absorbed photon of light equal to

Answer 21

1. Energy=hv=deltao

Question 22

What are the two possible geometries for ML4 complexes containing d block metals

Answer 22

1. Tetrahedral

2. square planar

Question 23

Describe the tetrahedral case for a 3d metal

Answer 23

1. The metal ion Mn+ is placed at the origin of the d-orbital cartesian axes
2. 4 ligands- L
3. These 4 ligand point charges are now placed at alternate corners of the cube (not in centre of faces) and a tetrahedral crystal field is created

Question 24

Does the tetrahedral arrangement of ligands have inversion symmetry

Answer 24

1. No

2. Labels don’t contain g

Question 25

In the tetrahedral case which orbitals are destabilised and why

Answer 25

1. dxy, dxz, dyz are pointed more towards the charges | 2. They are destabilised and rise in energy with respect to that in the spherical field (barycentre)

Question 26

In the tetrahedral case which orbitals are stabilised and why

Answer 26

1. dx2-y2 and dz2 are directed halfway between the point charges and so are less influenced 2. They are stabilised with respect to the barycentre

Question 27

What is the label for the stabilised orbitals in the tetrahedral complex

Answer 27

1. e

Question 28

What is the label for the destabilised orbitals in the tetrahedral complex

Answer 28

2. t2

Question 29

How much are the e orbitals raised by

Answer 29

1. +0.4dt with respect to the spherical field

Question 30

How much are the t2 orbitals destabilised by

Answer 30

1. -0.6dt with respect to the spherical field

Question 31

Is dt or do larger and why

Answer 31

1. dt

Question 32

What is the general relation between Dt and Do

Answer 32

1. Dt = around 4/9Do

Question 33

Describe the square planar case

Answer 33

1. Start with octahedral field and remove z-axis ligands | 2. The metal-ligand e-e repulsions along the z-axis are removed and orbitals with a 'z' component are stabilised

Question 34

What is the order of stabilisation in the square planar case

Answer 34

1. Most stable= dyz, dxz 2. dz2 3. dxy 4. least stable= dx2-y2

Question 35

How does the stabilisation of square planar compare with octahedral

Answer 35

1. Dxy and dz2 change over in energetic ordering 2. 4 different energies 3. dz2 has the biggest stabilisation as it was pointing directly at ligands along z axis

Question 36

What is the energy difference between orbitals called in square planar case

Answer 36

1. Dsp

Question 37

What is the relationship between Dsp and Do

Answer 37

Dsp= around 1.3Do

Question 38

What are d8 configurations observed for

Answer 38

1. Ni2+, Pd2+, Pt2+

Question 39

When can square planar geometries be favoured over tetrahedral

Answer 39

1. d8 configurations

Question 40

What is a tetragonal distortion

Answer 40

1. In between 2 extremes of octahedral and square planar geometries- complex that exhibits tetragonal distortion 2. M-L extension along the z-axis and M-L compression along the x and y-axes

Question 41

What is the CF splitting pattern like for a tetragonal distortion

Answer 41

1. Like that for square-planar 2. dxy + dz2 do not cross over energetically 3. E(dz2)>E(dxy) as there are still ligands along z

Question 42

What is the Crystal Field Stabilisation Energy

Answer 42

1. CFSE | 2. Is the stability that results from placing a transition metal ion in the crystal field generated by a set of ligands

Question 43

How is CFSE for a transition metal ion in a complex calculated

Answer 43

1. On the basis of the number of electrons occupying the d orbitals in a CFSP