Crystal field theory explains interactions between ligands and transition metal atoms. When ligands approach the free metals atoms, it breaks the degeneracy of their d-orbitals. This causes the splitting of degenerated orbitals into different sets of energy levels.
All five d-orbitals degenerate in a free metal atom. Orbitals that are close to approaching ligands go to a higher energy state due to greater repulsion. Whereas, the orbitals that are away from approaching ligands move toward a lower energy state. The creation of energy gaps between the orbitals is called crystal field splitting.
There are the following factors that affect the crystal field splitting. These are the nature of ligands, coordination number, arrangement of ligand, size of a metal atom, charge on the metal atom, size of ligands, electronegativity, and interatomic distance.
Factors affecting crystal field splitting
The following factors affect the splitting of crystal fields in a coordination metal atom.
1. Nature of ligands
The extent of splitting depends upon the nature of ligands. Ligands are classified based on their donating and accepting capabilities. There are two main types of ligands.
Strong ligands
Strong ligands are good electron donors. They share their electrons easily. When strong ligands approach, they cause a greater extent of repulsion. This creates a large energy gap between two sets of orbitals. So, electrons cannot jump to higher energy levels easily. Therefore, they prefer to remain in lower energy orbitals.
Strong ligands force the d electrons to be paired up. This pairing of electrons starts after 3rd electron. These complexes are known as low spin complexes. They show colors of smaller wavelengths. In the case of strong ligands, splitting energy is greater than pairing energy.
Examples of Strong Ligands
- NO2−
- PPh3
- CN−
- CO, etc
Weak ligands
Weak field ligands are not good electron donors. They do not share their electrons easily. When weak ligands approach, they cause a lesser repulsion as compared to the strong ones. There is a small energy gap between two sets of orbitals. So, electrons can enter higher energy orbitals easily. These complexes show paramagnetic behavior.
The pairing of electrons starts after the 6th electron. These are known as high spin complexes. They show colors of larger wavelengths. In the case of weak ligands, splitting energy is less than pairing energy.
Examples of Weak Ligands
- I−
- Br−
- S2−
- Cl−
- N3−
- F−, etc
Spectrochemical series
Spectrochemical series are arrangements of ligands based on their strength or increasing order of ligand field splitting energy.
2. Coordination number
Crystal field splitting also depends upon the coordination number. Coordination number refers to the number of ligands approaching the central metal atom. Based on coordination number, there are several types of splitting that take place in coordination complexes.
Octahedral splitting
When six ligands approach the central metal atom, it is octahedral splitting. In octahedral complexes, ligands are approaching through the axis. Octahedral splitting will give either high spin complexes or low spin complexes. This high/low spin of complexes depends upon the strength of ligands.
Square planner / tetragonal splitting
Square planar and tetragonal also called distorted octahedral complexes are obtained by removing two ligands from the z-axis. When two ligands (up and down) are completely removed from octahedral complexes, we have a square planner splitting whereas, during removing (if only distance increases) we have tetragonal splitting.
Tetrahedral splitting
When four ligands approach the central metal atom, tetrahedral splitting gets formed. Tetrahedral splitting generally gives high spin complexes ligands approach the axis. No one of the orbitals is in the direction of approaching ligands. So there is less repulsion and hence, less splitting.
3. Arrangement of ligands
Arrangement of ligand around central metal atom also affects the crystal field splitting. In octahedral complexes, ligands are approached through the x, y, and z-axis. These ligands are arranged at the vertex of the octahedron. This will break the degeneracy of d orbitals and split them into two sets of orbitals.
In case of tetrahedral complexes, four ligands approach the central metal atom between the axis. Ligands are arranged at the corner of a tetrahedron. None of the d orbitals is directly in direction of approaching ligands. So there is less splitting in tetrahedral splitting as compared to the splitting of octahedral complexes.
4. Size of a metal atom
The size of the metal atom also affects crystal field splitting energy. When we move from top to bottom in a group, the crystal field splitting energy increases. This is due to the increase in the size of metal atoms. As the size of the metal atom increases, there would be greater interaction between the metal atom and approaching ligands.
The trend of the splitting energy is, therefore;
5. Charge on the metal atom
Charge on the metal atom also influences crystal field splitting. The higher the oxidation state, the greater would be crystal field splitting and vice versa. Just like the ligand series, there is spectrochemical series of metal atoms. These are also arranged in increasing order.
Spectrochemical series of metal atoms:
As the oxidation state increases the size of the metal atom decreases. This leads to an increase in the charge density of metal atoms. This allows ligands to come closer to the central metal atom.
6. Size of ligands
The size of ligands also affects crystal field splitting. There is an inverse relationship between the size of ligands and the splitting energy. The smaller the size of ligands, the greater would be splitting energy. The reason behind this is that when a ligand is of a smaller size it comes very close to the metal atom. This will cause greater repulsion which leads to a higher energy gap between two sets of orbitals.
7. Electronegativity of ligands
Electronegativity also affects crystal field splitting energy. There is an inverse relationship between the electronegativity of ligands and the splitting energy. The greater the electronegativity of ligands, the smaller would be the splitting energy. As the ligands are more electronegative, they do not share their electrons easily. Hence, there presents weak interaction between metal atoms and ligands.
8. Interatomic distance
Interatomic distance also affects crystal field splitting energy. The greater the distance between the metal atom and surrounding ligands smaller would be the splitting energy. This is due to fewer repulsions.
Distance between ligand and central metal atom depends upon;
- Electronegativity
- Size ligands
- Size of metal atom
Related topics
Concepts Berg
What are the factors affecting crystal field theory?
There are the following factors that affect crystal field splitting energy
- Nature of ligands
- Coordination number
- Arrangement of ligand
- Size of a metal atom
- Charge on the metal atom
- Size of ligands
- Electronegativity
- Interatomic distance
Differentiate between high spin and low spin complexes?
Low spin Complexes (strong field)
- These complexes are given by strong ligands.
- There is a large energy gap between two sets of orbitals.
- In low spin complexes, the splitting energy is greater than the pairing energy.
- Low spin complexes show diamagnetic behavior.
- They have less number of unpaired electrons.
- These are also called inner orbital complexes.
- For example, [Fe(CN)6]-3, [Co(NH3)6]+3
High spin Complexes (weak field)
- These complexes are given by weak ligands.
- There is a small energy gap between two sets of orbitals.
- In high spin complexes, the splitting energy is smaller than the pairing energy.
- High spin complexes show paramagnetic behavior.
- They have a larger number of unpaired electrons.
- These are also called outer orbital complexes.
- For example, [Fe(H2O)6]+3, [Co(F)6]-3
Crystal field splitting in octahedral complexes?
In octahedral complexes, six ligands approach the central metal atom. This breaks degenerate orbitals into two sets (t2g and eg) of orbitals. In these complexes, ligands are approaching through the axis. Octahedral splitting will give either high spin complexes or low spin complexes. This depends upon the strength of ligands. For example , [Fe(CN)6]-3, [Co(NH3)6]+3 , [Fe(H2O)6]+3, [Co(F)6]-3, etc.
Crystal field splitting in tetrahedral complexes?
In tetrahedral complexes, four ligands approach the central metal atom. This breaks degenerate orbitals into two sets (b) of orbitals. As ligands are approaching between the axis, tetrahedral complexes are generally high spin complexes No one of the orbitals is in the direction of approaching ligands. So there is less repulsion and less splitting. For example, [Co(Cl)4]2-, [Cu(Cl)4]2-, [Ni(Co)4]0, etc.
How does the size of the ligand effect crystal field splitting?
There is an inverse relationship between the size of the ligand and splitting energy. The greater the size of the ligand, the smaller would be the crystal field splitting energy. This is due to the larger size ligands can’t come closer to the central metal atom and there would be fewer repulsions.
References
- Basic Inorganic chemistry (3rd edition) by F.Albert Cotton (Texas A&M University), Geoffrey Wilkinson (University of California, Berkeley), Paul L. Gaus (The College of Wooster)
