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Crystal Field Theory
Crystal field theory is a theory that describes the electronic structure of transition metal
complexes. The theory was developed to explain the special properties of coordination
complexes like optical spectra, thermodynamic stability and magnetic properties. The theory
is based on an ionic description of the metal-ligand interaction and considers the ligand as a
point negative charge.
Octahedral Complex: In the octahedral ligand field the d-orbitals split into two sets with an
energy difference Δo where the dxy, dxz and dyz orbitals (t2g) have lower energy because they
lie between the ligand and have less repulsion by the negative charge of the ligand the dz2 and
dx2-y2 orbitals (eg), which have higher energy because they are located close to the ligand,
along the axes which results in greater repulsion by the negative charge of the ligands.
Ligand field splitting parameter Δo is the energy difference between the two sets of dorbitals eg (dz2, dx2-y2 ) and t2g (dxy, dxz and dyz ) for octahedral complexes.
High spin and low spin complexes: A strong ligand field makes the splitting energy Δ large
and thus complexes with such ligands tend to have low spin configuration because the
splitting energy Δ is larger than the spin pairing energy of electrons. Weak field ligands make
the splitting energy Δ small. Complexes with these ligands tend to be high spin because the
splitting energy Δ is smaller than the spin pairing energy of electrons.
Spectrochemical series: List of ligands ordered by the size of Δo that they produce.
weak ligands: I- <Br- < S2- < SCN- < Cl- < NO3- < N3- < F- < OH- < C2O42- < H2O < NCS-<
CH3CN- < py < NH3 < en < 2,2'-bipyridine < phen < NO2- < PPh3 < CN- < CO strong ligands
Jahn-Teller effect occurs in non-linear, mainly octahedral complex compounds. The
complex is distorted along one spatial axis (compression or extension), the degenerate dorbitals are energetically split. The reason for this is an energy gain, which is caused by the
distortion and stabilizes the complex. If a complex is stretched along an axis, the orbitals in
the direction of elongation are energetically favoured, if a complex is compressed, these
orbitals are lowered in energy. Jahn-Teller effect for octahedral complexes is observed for
compounds of ions with d4- high spin- (Cr2+, Mn3+) d9- (Cu2+, Ag2+) and d7- low-spinconfiguration (Co2+, Ni3+).
Colour of transition metal complexes: The energy difference Δ in many complexes is about
equal to the energy of a photon of visible light. When a complex is illuminated by a photons
with the correct energy, it is absorbed and the electron jumps from the lower energy d orbitals
to the higher one. Since the complex absorbs light of that frequency, we see the complementary colour of light.
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