Question

What are the salient features of crystal field theory? What are its limitations?

Answer 1

Crystal field theory (CFT) is a bonding model that explains many important properties of transition-metal complexes, including their color, magnetism, structures, stability and reactivity. The central assumption of CFT is that metal-ligand interactions are purely electrostatic in nature.

Complete answer:
The salient features of crystal field theory are:
$\bullet$ In a complex central metal atom or ion is surrounded by various ligands.
$\bullet$ Ligands are negatively charged ions or neutral molecules, having lone pair of electrons example-${H_2}O,N{H_3}$
$\bullet$ Metal ions and ligands are considered point charge, their interaction is purely electrostatic.
$\bullet$ When ligands approach central metal, degeneracy of $d -$ orbitals in metal is removed due to repulsion between metal and ligands and split into different energy levels. The repulsion is more if ligands approach along the axis of $d -$ orbitals and acquire higher energy. While repulsion is less if ligands approach between the axis of $d -$ orbitals and acquire less energy. Thus the difference between these splitted energies is called crystal field stabilization energy (CFSE) and denoted by small delta.
$\bullet$ Electrons of metal are filled in split $d -$ orbital. According to Hund’s rule i.e. from lower energy levels.
$\bullet$ Since there is no orbital overlapping hence no covalent character in bonding.
$\bullet$ The magnitude of CFSE depends on nature and number of ligands and also the geometry of the complex.
If CFSE$> 0$ then stable complex,
If CFSE$< 0$then unstable complex.
Limitation of Crystal field theory are:
$\bullet$ The assumption that the interaction between ligand and metals is purely electrostatic cannot be said to be very realistic.
$\bullet$ The theory fails to explain the behavior of certain metals which cause large splitting while others show small splitting.
$\bullet$ The theory rules out the possibility of having p bonding this is a serious drawback because it is found in many complexes.
The theory gives no significance to the orbits of the ligands. Therefore, it cannot explain any properties related to ligand orbitals and their interaction with metal orbitals.
$\bullet$ This theory always took only $d -$ orbitals into consideration. Contribution of $s$ and $p$ block was never taken into this theory.

Note:
A consequence of CFT is that the distribution of electrons in the d-orbitals may lead to net stabilization of some complexes depending on the specific ligand field geometry and metal d-electron configurations. It is a simple matter to calculate the stabilization since all that is needed is the electron configuration and knowledge of the splitting patterns.