To explain bonding in complexes valence bond theory was proposed by Linus Pauling. The main postulates of VB theory are as follows:

(i) The central atom loses a requisite number of electrons to form the cation. The number of electrons lost is equal to the valence of the resulting cation.

(ii) The central cation makes available a number of vacant orbitals equal to its coordination number for the formation of dative bonds with the ligands.

(iii) The cation orbitals hybridize to form new set of equivalent hybrid orbitals with definite directional characteristics.

(iv) The non-bonding metal electrons occupy the inner d-orbitals and do not participate in the hybridization.

(v) In the presence of strong ligands such as CN^-, NO, CO, the d-electrons are rearranged vacating some d-orbitals (when the number of d-electrons are more than 3 only) which can participate in hybridization.

(vi) In the presence of weak ligands such as F^-, Cl^-, H₂O, etc., the d-electrons are not rearranged.

(vii) The d-orbitals involved in the hybridization may be either (n - 1)d orbitals or outer d-orbitals.

The complexes formed by the involvement of (n - 1)d orbitals in hybridization are called inner orbital complexes or low spin complexes. The complexes formed by the involvement of d-orbitals of outer orbit are called outer orbital complexes or high spin complexes.

(viii) Each ligand contains a lone pair of electrons. A dative bond is formed by the overlap of a vacant hybrid orbital of metal ion and a filled orbital of ligand.

(ix) The complex will be paramagnetic, if any unpaired electrons present, otherwise diamagnetic.

(x) The number of unpaired electrons in a complex gives out the geometry of the complexes or vice versa.

Which of the following statements is wrong?

Halide complexes are generally low spin complexes

The correct answer is option 1. Halide complexes are generally low spin complexes.

Low Spin Complexes

In a low spin complex, the electrons in the metal's d orbitals are paired up. This is because the ligand field is strong enough to cause the electrons to pair up, even though this means that some of the d orbitals will be half-filled.

The factors that favor the formation of low-spin complexes include:

A strong field ligand: A strong field ligand is a ligand that can split the d orbitals into two energy levels, with the lower energy level being much lower than the higher energy level. This makes it energetically favorable for the electrons to pair up in the lower energy level.

A small metal ion: A small metal ion has a high charge density, which makes it more attractive to the ligand's electrons. This also makes it more likely that the electrons will pair up in the lower energy level.

High Spin Complexes

In a high spin complex, the electrons in the metal's d orbitals are unpaired. This is because the ligand field is not strong enough to cause the electrons to pair up, and it is more energetically favorable for the electrons to remain unpaired.

The factors that favor the formation of high-spin complexes include:

A weak field ligand: A weak field ligand is a ligand that cannot split the d orbitals into two energy levels. This means that the electrons in the d orbitals will not be paired up, even if this means that some of the d orbitals will be half-filled.

A large metal ion: A large metal ion has a low charge density, which makes it less attractive to the ligand's electrons. This makes it less likely that the electrons will pair up in the lower energy level.

In the case of halide complexes, the halide ligands are weak field ligands. This means that they do not tend to cause the electrons in the metal's d orbitals to pair up. Therefore, halide complexes can be either low spin or high spin, depending on the other factors involved.

For example, [FeCl₆]^3– is a low spin complex because the iron ion has a high charge density, which makes it more attractive to the ligand's electrons. This causes the electrons in the d orbitals to pair up in the lower energy level.

On the other hand, [CoCl₄]^2– is a high spin complex because the cobalt ion has a lower charge density. This makes it less likely that the electrons in the d orbitals will pair up, and they will remain unpaired.