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Crystal Field Theory (CFT)
What is Crystal Field Theory?
This theory describes the interaction between transition metals and ligands. There is an attraction between the metal ion (having positive charge) and non-bonding electrons of ligands (having negative charge). When ligands approach the central metal ion, degeneracy of d or f orbital is lifted due to the production of the static electric field, which is present due to surrounding charge distribution. This theory helps to explain certain characteristics like magnetic properties, colour, and hydration energies. There occurs a repulsion between the electrons in the d orbital of the central atom and those present in the ligand; as a result, electrons of d orbitals which are closer to the ligand will be higher in energy than those further away, which results in splitting in the energy of d orbitals. There are four factors on which splitting generally depends:
- Nature of metal ion
- The oxidation state of the metal ion
- Arrangement of the ligands around the central atom
- Nature of ligands surrounding the metal ion
High Spin and Low Spin Concept:
The соmрlexiоn with the greаter number оf unраired eleсtrоns is knоwn аs the high sрin соmрlex, the lоw sрin соmрlex соntаins the lesser number оf unраired eleсtrоns. High sрin соmрlexes аre exрeсted with weаk field ligаnds whereаs the сrystаl field sрlitting energy is smаll Δ. The орроsite аррlies tо the lоw sрin соmрlexes in whiсh strоng field ligаnds саuse mаximum раiring оf eleсtrоns in the set оf three t2 аtоmiс оrbitаls due tо lаrge Δо.
High sрin – Mаximum number оf unраired eleсtrоns.
Lоw sрin – Minimum number оf unраired eleсtrоns.
Exаmрle: [Со(СN)6]3- & [СоF6]3-
- [Co(CN)6]3- – Low spin complex
- [CoF6]3- – High spin complex
Splitting in Octahedral Complexes:
In an octahedral, six ligands are present around the central atom. The splitting of d orbital takes place into two levels:
Bottom three energy levels(t2g) - dxy, dyz, dxz
Upper two energy levels (eg) - dx2-y2, dz2
In an octahedral complex, the attraction of electrons takes place towards the axis. An orbital having a lobe on the axis will move to a higher energy level. It means energy of eg levels are higher as compared to t2g levels.
In tetrahedral complexes, four ligands are present around the central atom. The splitting of d orbital takes place into two levels:
Top three energy levels (t2g) - dxy, dyz, dxz
Bottom two energy levels (eg) - dx2-y2, dz2
The reason behind this splitting is due to poor overlapping of the orbital between metal and ligand. In this, orbitals are directed on the axes, but ligands are not.
Square Planar Complexes:
In square planar complexes, four ligands are present around the central atom. In this, electrons of ligands get attracted towards the XY plane only. The four different energy levels for this complex are:
dx2-y2, dz2, dxy and both dyz, dxz.
Crystal Field Stabilization Energy:
It is the difference between the ligand field energy and energy in an isotropic field. There are mainly four factors on which CFSE depends:
- Number of d electrons
- Spin pairing energy
- Nature of the ligand
The complete detail of the splitting of d orbital along with nature of ligand in both octahedral and tetrahedral environment is given below:
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