CHEM2210 Chapter Notes - Chapter 10: Crystal Field Theory, Octahedral Symmetry, Spectrochemical Series
Tanabe-Sugano Diagram
- Tanabe-Sugano diagrams are used in coordination chemistry to predict
electromagnetic absorptions of metal coordination compounds of
tetrahedral and octahedral complexes.
- The analysis derived from the diagrams can also be compared to
experimental spectroscopic data.
- Armed with spectroscopic data, an approximation to the crystal field
splitting energy (10Dq), generated by ligands attached to a metal
centre, can be found.
Crystal Field Splitting Energy
- Within Crystal Field Theory, the interaction of the metal and ligand
arise from the positive charge of the metal and negative charge on the
ligands.
- The theory is developed by looking at the five degenerate d-orbitals
and how the energies are changed on being surrounded by the
negative point charges of the ligands.
- As the ligands are moved closer to the metal the repulsion between the
electrons of the metal and ligands break the degeneracy of the d-
orbitals. In the case of an octahedral complex 6 ligands surround a
metal center with a single pair on each axis.
- This raises the energies of dx^2-y^2, dz^2 relative to those of dxy dxz dyz.
This energy split is called Δoct. The tetrahedral energy split is about
4/9Δoct.1
Racah Parameters
- Racah parameters were generated as a means to describe the effects
of electron-electron repulsion within the metal complexes.
- The Racah parameters are A, B and C. In the case of Tanabe-Sugano
diagrams each electron configuration split has an energy that can be
related by the B value. A is ignored because it is roughly the same for
any metal center and C is generally approximated as being 1/4B.
- What B represents is an approximation of the bond strength between
the ligand and metal.
- Comparisons between tabulated free ion B and B of a coordination
complex is called the nephelauxetic ratio (the effect of reducing
electron-electron repulsion via ligands).
- This effect is what gives rise to the spectrochemical series of ligands
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Document Summary
Tanabe-sugano diagrams are used in coordination chemistry to predict electromagnetic absorptions of metal coordination compounds of tetrahedral and octahedral complexes. The analysis derived from the diagrams can also be compared to experimental spectroscopic data. Armed with spectroscopic data, an approximation to the crystal field splitting energy (10dq), generated by ligands attached to a metal centre, can be found. Within crystal field theory, the interaction of the metal and ligand arise from the positive charge of the metal and negative charge on the ligands. The theory is developed by looking at the five degenerate d-orbitals and how the energies are changed on being surrounded by the negative point charges of the ligands. As the ligands are moved closer to the metal the repulsion between the electrons of the metal and ligands break the degeneracy of the d- orbitals. In the case of an octahedral complex 6 ligands surround a metal center with a single pair on each axis.
