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A CH20147 > Crystal Field Theory > Flashcards

Crystal Field Theory Flashcards

1
Q

Who developed CFT and when

A
  1. Hans Bethe

2. 1929

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2
Q

Why did Bethe develop CFT

A
  1. In order to interpret colour spectra and magnetism in crystals
  2. For the solid state
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3
Q

What did Van Vleck do

A
  1. 1932

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

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4
Q

What are 3 assumptions used in CFT

A
  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
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5
Q

What are 3 shortcomings of CFT

A
  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
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6
Q

Describe the spherical case for a 3d metal

A
  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
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7
Q

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

A
  1. D-orbitals energies are raised relative to the spherical field
  2. Due to e-e repulsions
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8
Q

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

A
  1. Yes

2. Gerade ‘g’

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9
Q

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

A
  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
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10
Q

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

A
  1. Barycentre
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11
Q

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

A
  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
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12
Q

What happens to the total energy of the orbitals

A
  1. Remains the same
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13
Q

What label is given to stabilised orbitals in an octahedral complex

A
  1. t2g
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14
Q

What label is given to destabilised orbitals in an octahedral complex

A
  1. eg
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15
Q

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

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

2. Crystal field splitting energy

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16
Q

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

A
  1. They are equal
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17
Q

How much is each t2g orbital stabilised by

A
  1. -0.4Deltao relative to the free ion in a spherical field
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18
Q

How much is each eg orbital destabilised by

A
  1. +0.6deltao relative to the free ion in a spherical field
19
Q

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

A
  1. 6 H2O ligands- octahedral geometry

2. Ti3+ - [Ar]3d1

20
Q

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

A
  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
21
Q

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

A
  1. Energy=hv=deltao
22
Q

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

A
  1. Tetrahedral

2. square planar

23
Q

Describe the tetrahedral case for a 3d metal

A
  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
24
Q

Does the tetrahedral arrangement of ligands have inversion symmetry

A
  1. No

2. Labels don’t contain g

25
Q

In the tetrahedral case which orbitals are destabilised and why

A
  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)

26
Q

In the tetrahedral case which orbitals are stabilised and why

A
  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
27
Q

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

A
  1. e
28
Q

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

A
  1. t2
29
Q

How much are the e orbitals raised by

A
  1. +0.4dt with respect to the spherical field
30
Q

How much are the t2 orbitals destabilised by

A
  1. -0.6dt with respect to the spherical field
31
Q

Is dt or do larger and why

A
  1. dt
32
Q

What is the general relation between Dt and Do

A
  1. Dt = around 4/9Do
33
Q

Describe the square planar case

A
  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

34
Q

What is the order of stabilisation in the square planar case

A
  1. Most stable= dyz, dxz
  2. dz2
  3. dxy
  4. least stable= dx2-y2
35
Q

How does the stabilisation of square planar compare with octahedral

A
  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
36
Q

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

A
  1. Dsp
37
Q

What is the relationship between Dsp and Do

A

Dsp= around 1.3Do

38
Q

What are d8 configurations observed for

A
  1. Ni2+, Pd2+, Pt2+
39
Q

When can square planar geometries be favoured over tetrahedral

A
  1. d8 configurations
40
Q

What is a tetragonal distortion

A
  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
41
Q

What is the CF splitting pattern like for a tetragonal distortion

A
  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
42
Q

What is the Crystal Field Stabilisation Energy

A
  1. CFSE

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

43
Q

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

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

A CH20147 (10 decks)

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