Crystal field theory Flashcards
What does the crystal field theory assume?
Ligands are point negative charges and the binding within a complex is purely ionic
What phase is it assumed the metal is in?
Gas phase
Where are the negatively charged ligands assumed to be from?
infinity
When drawing an energy diagram for octahedral complexes, how many orbitals are there before any splitting occurs? How do they compare to each other?
5
They have the same energy - degenerate
For an octahedral complex, how many orbitals are high energy? Which ones are they?
2
dz²
dx²-y²
What is the high energy level called for octahedral complexes?
eg
How many orbitals are low energy for octahedral? Which ones are they?
3
dxy
dyz
dxz
What is the low energy level called for octahedral complexes?
t2g
What is the gap between eg and t2g called?
ΔO (oct)
For the octahedral complex, what is the value from the middle to the eg level?
+3/5 ΔO
For the octahedral complex, what is the value from the middle to the t2g level?
-2/5 ΔO
What happens when the ligands approach from infinity along the x,y and z axes?
The electrons in the d-orbitals start to sense them
Why do dz² and dx²-y² rise in energy?
The negative charges are being brought closer together which means they become destabilised
Why are dxy, dxz and dyz not as affected and are lower in energy?
They point in between the axes and are lower in energy
In a tetrahedral complex, where do the ligands point?
In between the axes
In a tetrahedral complex, which orbitals are better aligned with the ligands?
dxy
dyz
dxz
In an octahedral complex, where do the ligands point?
dz² and dx²-y² point along the xyz axes
dxz, dyz and dxy point in between the axes
In a tetrahedral complex, which orbitals are high energy?
dxy
dyz
dxz
In a tetrahedral complex, which orbitals are low energy?
dz² and dx²-y²
In a tetrahedral complex, what is the high energy level called?
t2
In a tetrahedral complex, what is the low energy level called?
e
Why are the labels for octahedral and tetrahedral complexes different?
There is no symmetry in tetrahedral molecules
the g stands for gerade so it is lost when describing tetrahedral energy levels
What is the difference in energy between t2 and e?
Δtet
What is the energy gap between the middle and the t2 level?
+2/5Δt
What is the energy gap between the middle and the e level?
-3/5Δt
How does the splitting term for Δtet compare to Δoct?
Smaller
What is a tetragonal distortion?
The two ligands of an octahedral complex along the z-axis are pulled away from the metal to form a square planar complex
What does tetragonal distortion remove?
the degeneracy of eg
What happens when the two ligands on the z axes of an octahedron complex are removed?
The d-orbitals with a z component lower in energy
Why do the d orbitals with an x,y component rise in energy during the formation of a square planar complex?
The ligands in the x-y plane are pulled in more
In a square planar complex, when does the dz² orbital fall in energy to below that of the dxy?
Eventually the two ligands are removed to infinity
For octahedral complexes, where do you fill the orbitals up from first?
From the bottom
t2g
What is pairing energy (P)?
The energy needed to pair an electron in the same orbital
When you get to d4, what do you use to determine where you put the 4th electron?
depends on the size of ΔO and the pairing energy (P)
What are weak field ligands?
They have a small splitting energy ΔO
They are high spin
Where is the more energetically favourable place to put the 4th electron for a weak field ligand?
In the higher energy level
to make it t2g3 eg1
How do you increase the spin of an orbital?
Have all the electrons pointing up
What are strong field ligands?
They have a large splitting energy ΔO
They are low spin
How do you make an orbital low spin?
Pair up the electrons
Where is the most favourable place to put the 4th electron for a strong field ligand?
in the lowest energy orbital paired up
What does paramagnetic mean?
Unpaired electrons
What does diamagnetic mean?
Paired electrons
Where is tetrahedral filling of electrons likely to start?
At the bottom as ΔG will be small
