Download crystal field theory, spectrochemical series, high spin

CRYSTAL FIELD THEORY, SPECTROCHEMICAL SERIES, HIGH SPINLOW SPIN COMPLEXES AND JAHN-TELLER EFFECT
Crystal Field Theory:
Ligand is considered to be a negative charge and as it approaches the central metal
ion, the ‘d’ electrons of metal are repelled to different extent. This gives rise to loss
degeneracy of d orbitals.
Octahedral complex:
dx2-y2 and dz2 orbitals lie along the x, y, z axis while dxy,dxz and dyz orbitals lie in
between the axis. Six ligands are considered as approaching along the three axis. So
they interact more with dx2-y2 and dz2 (eg) than with dxy, dxz and dyz orbitals (t2g).
So energy of eg becomes more and that of t2g becomes less (degeneracy is lost)
Energy difference in t2g and eg is given by ∆o (crystal field splitting parameter). The eg
orbitals are raised by +3/5∆o while t2g orbitals are lowered by -2/5∆o than barycentre.
Spectrochemical Series:
For a particular metal ion, if ligands are changed, then ∆o also changes. It means that
different ligands interact with different strengths.
A series of ligands with increasing order of strengths is called spectrochemical series.
I− < Br− < S−2 < SCN− < Cl− < NO2− < F− < OH− < H2O < NCS− < py < bipy < NO2− < CN− < CO
The ligands interact weakly : - weak field ligands e.g. I−, Br−, S−2, SCN−, Cl−
The ligands interact strongly: - strong field ligands e.g. NO2− ,CN− ,CO
Similarly, if metal ions are different with same ligand, ∆o are different. Metals with
more positive charge and from 2nd and 3rd transition series interact more (higher
splitting).
High spin and Low Spin Complexes:
if ∆o > pairing energy of electrons, electrons pair and low spin complexes sre formed
if ∆o < pairing energy high spin complexes are formed
Tetrahedral complexes:
Four ligands approach from alternate corners of a cube while metal ion is at the center
of the cube. Here t2 will interact more than e orbitals because t2 (in between axes) are
closer to the ligands than e orbitals (along the axes)
So energy of t2 is raised by + 2/5∆T and energy of e is lowered by -3/5∆ T
It is observed that always ∆T < ∆o , so only high spin tetrahedral complexes are
formed.
Jahn-Teller Effect:
If electronic configuration is asymmetric then degeneracy of orbitals is lost further
and some distortions take place. e.g for octahedral d9, high spin d4 and low spin d7
complexes a tetragonal distorted geometry is observed.
Distortion is elongation of two axial bonds and contraction of four equatorial bonds or
elongation of four equatorial bonds and contraction of two axial bonds.
Generally elongation of two axial bonds and contraction of four equatorial bonds is
preferred thermodynamically.
Square Planar Complex:
It is the extreme case of Jahn-Teller Effect where two axial bonds are so much
elongated that they are removed. As there is no ligand on one axis the d orbitals split
further.
Question:
1) Explain the extreme case of Jahn Teller effect in detail.
2) Explain the crystal field splitting in octahedral complexes and
show for which electronic configurations high- and low-spin
complexes are possible.