Crystal Field Theory
Introduction:
Many theories were formulated for explaining the geometries of complexes. Crystal field theory is one of them. This theory determines the effect of different electrostatic fields caused by the different geometries of the complexes and different basic strengths of the ligands, on the energies of d- orbitals of the central metal ion.
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Crystal Field Theory:
Salient features of crystal-field theory are:
a. The central metal cation is surrounded by ligands which contain one or more lone pairs of electrons.
b. The ionic ligands, like F-, Cl-, CN-, etc., are regarded as negative point charges, or point charges and the neutral ligands, like H2O, NH3, etc., are regarded as point dipoles or just dipoles. (CFT regards neutral ligands as dipolar) If the ligand is neutral, the negative end of this ligand dipole is oriented towards the metal atom.
c. The interaction between the metal cation and the ligands is regarded as purely electrostatic, i.e, the metal-ligand bond is considered to be 100%ionic.
Crystal field splitting of d orbitals:
In an isolated gaseous atom, all the five d orbital are degenerate. (They have same energy).
Crystal-field theory is basically concerned with the effect of different arrangement of surrounding ligands on the energy of th d orbitals, and the sub-division of the orbitals into two groups, t2g and eg.
On the approach of the ligands in a complex, the electrons in the d orbital of the central ion are repelled by the lone pairs of the ligands. This repulsion will raise the energy level of the d orbitals. All the ligands approaching the energy of each orbital will increase by the same amount. In other words, they will remain degenerate.
Since d orbitals differ in their orientation, those orbitals lying in the direction of the ligands is raised to a larger extent than the others. So, five degenerate d orbitals will split into two sets, having different amount of energies. This splitting of five degenerate d-orbitals of the metal ion under the influence of approaching ligands, into two sets of orbitals having different energies is called as Crystal- field splitting.
The two sets are named as:
d- Orbitals facing in the direction of ligands - eg
d- Orbitals away from the path of the ligands – t2g
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Crystal Field Stabilization Energy:
One set of d- orbital will have higher energy than the other. The set eg will have higher energy than the set t2g, because, eg lies directly in the path of the ligands.
The energy difference or the energy gap between these two sets is denoted by ∆ or 10Dq. This energy difference arises because of the difference in the electrostatic field exerted by the ligands on t2g and eg sets of the orbitals. This energy is called as CRYSTAL FIELD SPLITTING ENERGY.
Crystal Field Theory Color:
Most of the transition metal complexes are colored. According to CFT, the color of the metal complexes is due to the d-d transitions between the eg and t2g electrons. The energy difference between the eg and t2g electrons is so small that even absorption of low radiation in the visible light may cause excitation from a lower to a higher d level. As a result of absorption of some selected wave lengths of light, the complexes appear to be colored.
Many theories were formulated for explaining the geometries of complexes. Crystal field theory is one of them. This theory determines the effect of different electrostatic fields caused by the different geometries of the complexes and different basic strengths of the ligands, on the energies of d- orbitals of the central metal ion.
Please express your views of this topic Thomas Atomic Theory by commenting on blog.
Crystal Field Theory:
Salient features of crystal-field theory are:
a. The central metal cation is surrounded by ligands which contain one or more lone pairs of electrons.
b. The ionic ligands, like F-, Cl-, CN-, etc., are regarded as negative point charges, or point charges and the neutral ligands, like H2O, NH3, etc., are regarded as point dipoles or just dipoles. (CFT regards neutral ligands as dipolar) If the ligand is neutral, the negative end of this ligand dipole is oriented towards the metal atom.
c. The interaction between the metal cation and the ligands is regarded as purely electrostatic, i.e, the metal-ligand bond is considered to be 100%ionic.
Crystal field splitting of d orbitals:
In an isolated gaseous atom, all the five d orbital are degenerate. (They have same energy).
Crystal-field theory is basically concerned with the effect of different arrangement of surrounding ligands on the energy of th d orbitals, and the sub-division of the orbitals into two groups, t2g and eg.
On the approach of the ligands in a complex, the electrons in the d orbital of the central ion are repelled by the lone pairs of the ligands. This repulsion will raise the energy level of the d orbitals. All the ligands approaching the energy of each orbital will increase by the same amount. In other words, they will remain degenerate.
Since d orbitals differ in their orientation, those orbitals lying in the direction of the ligands is raised to a larger extent than the others. So, five degenerate d orbitals will split into two sets, having different amount of energies. This splitting of five degenerate d-orbitals of the metal ion under the influence of approaching ligands, into two sets of orbitals having different energies is called as Crystal- field splitting.
The two sets are named as:
d- Orbitals facing in the direction of ligands - eg
d- Orbitals away from the path of the ligands – t2g
Having problem with Primary Carbocation keep reading my upcoming posts, i will try to help you.
Crystal Field Stabilization Energy:
One set of d- orbital will have higher energy than the other. The set eg will have higher energy than the set t2g, because, eg lies directly in the path of the ligands.
The energy difference or the energy gap between these two sets is denoted by ∆ or 10Dq. This energy difference arises because of the difference in the electrostatic field exerted by the ligands on t2g and eg sets of the orbitals. This energy is called as CRYSTAL FIELD SPLITTING ENERGY.
Crystal Field Theory Color:
Most of the transition metal complexes are colored. According to CFT, the color of the metal complexes is due to the d-d transitions between the eg and t2g electrons. The energy difference between the eg and t2g electrons is so small that even absorption of low radiation in the visible light may cause excitation from a lower to a higher d level. As a result of absorption of some selected wave lengths of light, the complexes appear to be colored.