Crystal Field Theory
The valence bond theory of bonding in complex ions has many shortcomings.
In order to provide suitable answers to many such questions, crystal field theory proved far more successful and a better alternative. It was developed by H.Bethe and V.Bleck (1935).
The major difference between the two theories is that the valence bond theory considers the bonding between the metal ion and the ligands as purely covalent whereas, the crystal field theory considers the bond between the metal ion and the ligand as purely electrostatic.
The CFT is based on the assumption that the metal ion and the ligands act as point charges and the attractions between them are purely electrostatic. In case of negative ligands (anions such as Cl, Br, CN), the intersections with metal ions are ion-ion interactions. If the ligands are neutral molecules (such as NH3 H2O, CO), the interactions with the metal ion are ion-dipole interactions. The name crystal field is assigned to this theory because the electrons of the central metal ion in the environment of other ions or molecules i.e. ligands are affected by their non-spherical electric field. Such an electric field is called as crystal field and hence the name crystal field theory.
The important features of CFT are:
(i) The transition metal ion is surrounded by the ligands with lone pairs of electrons and the complex is a combination of central ion surrounded by other ions or molecules or dipoles i.e. ligands.
(ii) All types of ligands are regarded as point charges.
(iii) The interactions between the metal ion and the negative ends of anion (or ion dipoles) are purely electrostatic, i.e. the bond between the metal and ligand is considered 100% ionic.
(iv) The ligands surrounding the metal ion produce electrical field and this electrical field influences the energies of the orbitals of central metal ion, particularly d-orbitals.
(v) In the case of free metal ion, all the five d-orbitals have the same energy. Such orbitals having the same energies are called degenerate orbitals.
The five degenerated d-orbitals of the metal ion split into different sets of orbital having different energies in the presence of electrical field of ligands. This is called crystal field splitting.
(vi) The number of ligands and their arrangement (geometry) around the central metal ion will have different effect on the relative energies of the five d-orbitals. In simple words, the crystals field splitting will be different in different structures having different co-ordination numbers.
(vii) The magnetic properties, spectra and preference for particular geometry can be explain in terms of splitting of d-orbitals in different crystal fields.
Thus, to understand the crystal field theory, it is essential to have a clear picture of the disposition of the five d-orbitals in space and the geometrical arrangement of the ligands around the central metal ion. We may illustrate this concept by considering the complexes with co-ordination number 6 and 4, which are very common.