Crystal Field Theory Flashcards
What assumptions do crystal field theory make?
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
What is the additional secondary effect of CF theory?
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
What is crystal field splitting energy?
This is the energy gap between the two groups of orbitals. It has the symbol triangle
Describe the d orbital energy in a normal metal ion
The 5 d orbitals are degenerate as they have the same energy
Describe d orbital energy of an ion placed in a spherical field
Causes electrostatic repulsion between field and d electrons
All d orbitals would increase in energy
What is an octahedral field?
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
Describe the d orbital energy in an octahedral field
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
What labels are the groups of orbitals given in an octahedral field?
dx2-y2 and dz2 are called eg
dxy, dxz and dyz are called t2g
To what extend are the eg orbitals destabilised and the t2g stabilised
t2g is stabilised by -0.4^o(-2/5^o)
eg is destabilised by +0.6^o(3/5^o)
How does the overall energy of the octahedral field, compared with that of the spherical field?
The average energy of the 5d orbitals is unchanged
The overall energy is the same
What is crystal field stabilisation energy?
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
How are the d orbitals filled?
D orbitals are filled using the aufbau principle, pauli exclusion principle and hunds rule
t2g are filled first as they are lower in energy
What are the options for filled after the 3 t2g orbitals have been singly filled?
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
What is a low spin complex?
This is where the t2g orbitals are doubly filled before the eg orbitals are filled
What is a high spin complex?
This is where t2g and eg orbitals are all singly filled before the electrons start pairing up
What determines whether a complex will be high spin or low spin?
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
How do you calculate CFSE?
CFSE is crystal field splitting energy (energy in the gap)
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
What is a weak field case?
A lower energy is achieved in the upper orbital is occupied (high spin)
There is a small energy gap
What is a strong field case?
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
How is CFSE equation modified for low spin complexes?
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
What factors affect ^? (The energy gap)
-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)
Explain why these factors affect ^?
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
Which factors affect pairing energy?
Larger d orbitals lead to smaller pairing energy as repulsions are decreased
What is a strong field ligand?
These are ligands that give rise to large ^
Likely to be low spin
What is a weak field ligand?
These are ligands that give rise to small values of ^
Likely to be high spin
