Crystal Field Theory I
Crystal field theory
：
In Crystal field theory, a ligand lone pair is moldeled as a point
charge that repels electrons of the d orbitals of the central metal ion. This approach
concentrates on the resulting splitting of the d orbitals into groups of different
energies.
Octahedral complexes: in an octahedral ligand field the d-orbitals split in two energy
levels. Two of the orbitals, namely dz2 and dx2-y2 (eg-orbitals) point directly towards
the ligands, and three orbitals, namely dxy dxz and dyz (t2g-orbitals) point between the
ligands. The first ones have a greater electrostatic repulsion by the negative charges of
the ligands, the latter three d- orbitals have a lower electrostatic repulsion.
Ligand field splitting parameters: The difference in energy between the eg orbitals
and the t2g orbitals is called Δo (o for octrahedral).
For octahedral complexes with the electronic configurations d4, d5, d6 and d7 exist two
possibilities to fill the orbitals with electrons, depending on the value of Δo in
comparison to the value of the spin pairing energy.
In complexes with strong ligands, which cause a large Δo, first the lower energy
orbitals are completely filled. The result for the mentioned electronic configurations
are so called “low spin complex”. In complexes with weak ligands, the eg and t
2g
orbitals are filled according Hund’s rule. The result for the mentioned electronic
configurations are so called “high spin complex”
(Hund’s rule: when more than one orbital has the same energy, electrons occupy first
separate orbitals with parallel spins.↑↑)
The Spectrochemical Series:
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
central atom: Δo increases down a group and increases with increasing oxidation
number
Color of transition metal complex: The transition metal complex can absorb the
energy of the visible light, this energy of the visible light is used to excite from the
lower energy orbital to the higher energy orbital. Because only certain wavelengths (λ)
of light are absorbed - those matching exactly the energy difference - the compounds
appears colored (complementary color!!).
Literature
Shriver, D. F., Atkins, P. W., Lanford, C. H., Inorganic Chemistry (2006), fourth
edition
