Question

Why are low spin tetrahedral complexes rarely observed?

Answer 1

Hint- Here, we will proceed by giving important features of crystal field theory. Then, we will discuss the hybridization which exists in a tetrahedral compound and about the crystal field splitting energy.

Complete answer:
The crystal field theory (CFT) is a model for the bonding relationship between the ligands and transition metals. It describes the effect of the attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of the ligand. When the ligands approach the central metal ion, the degeneration of electronic orbital states , usually d or f orbitals, is broken because of the static electric field produced by a distribution of the surrounding charge. CFT effectively accounts for some of the transition metal complexe's magnetic properties, colors, and hydration energies but it does not attempt to describe bonding.
A tetrahedral compound has ${\text{s}}{{\text{p}}^3}$ hybridization, where four ligands form a tetrahedron structure around the metal ion. During crystal field splitting, the d-orbitals split into two groups-one with a higher energy and the other with a lower energy. In the case of low spin complexes, the energy required to pair electrons in a lower energy d-orbital is less than the energy required to place the additional electron into a higher energy d-orbital. If the situation is just the opposite, then there is high spin splitting.
Since the ligands in a tetrahedral complex do not approach the d-orbitals directly, instead they approach between the axes, the splitting energy is much lower than that of the pairing energy, and thus the electrons jump to the higher energy level d-orbitals rather than pairing, resulting in high-spin complexes being formed.
In case of high spin complex, Splitting energy < Pairing energy
In case of low spin complex, Splitting energy > Pairing energy
According to crystal field theory,
If CFSE > Pairing energy, then electron pairing against the Hund’s rule is possible.
Also, the relation between CFSE values for tetrahedral and octahedral complexes is given by
${\text{CFS}}{{\text{E}}_{\text{T}}} = \dfrac{4}{9}{\text{CFS}}{{\text{E}}_{\text{O}}}$ where ${\text{CFS}}{{\text{E}}_{\text{T}}}$ denotes the crystal field splitting energy for tetrahedral compounds and ${\text{CFS}}{{\text{E}}_{\text{O}}}$ denotes the crystal field splitting energy for octahedral compounds.
Clearly, we can see that the crystal field splitting energy for tetrahedral compounds (i.e., ${\text{CFS}}{{\text{E}}_{\text{T}}}$) is less than the crystal field splitting energy for octahedral compounds (i.e., ${\text{CFS}}{{\text{E}}_{\text{O}}}$).
Here, ${\text{CFS}}{{\text{E}}_{\text{T}}}$ < Pairing energy which means pairing against the Hund’s rule doesn’t occurs and hence, low spin complexes are not formed (or rarely formed). Here, outer orbital complexes are formed. Irrespective of the strength of the ligand (strong ligand or weak ligand), ${\text{s}}{{\text{p}}^3}$ hybridization exists.

Note- The crystal field stabilization energy or crystal field splitting energy (CFSE) is the stability that results from placing a transition metal ion in the crystal field generated by a set of ligands. This happens because when the d orbitals are split in a ligand field, some of them are lower in energy than they used to be.