Valence Bond Theory of Coordination Compounds

The Valence Bond Theory is the basis upon which a very delicate interaction between metal ions and ligands is represented. The central metal atom or ion is surrounded by molecules or ions called ligands. One of the most important concepts of VBT arises from the ability of the ligand to donate electron pairs to the metal center, thereby forming coordinate bonds.

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This Story also Contains

1. Interpretation of Coordination Compounds According to VBT
2. Weaknesses of VBT
3. Some Solved Examples
4. Summary

Interpretation of Coordination Compounds According to VBT

According to this theory, the metal atom or ion under the influence of ligands can use its (n-1)d or nd orbitals along with its ns and np for hybridization to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridized orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding. The different types of hybridization and their respective shapes are given below

Let us consider the case of [Ni(CN)₄]²⁺ and try to predict the hybridization of this complex

Here nickel is in a +2 oxidation state and the ion has a valence electronic configuration 3d⁸4s

In the presence of Cyanide ions, the electrons will be paired up and the hybridization of Ni in the complex will bed sp² as shown below

Each of the hybridized orbitals receives a pair of electrons from a cyanide ion. The compound is diamagnetic as evident from the absence of unpaired electrons.

Weaknesses of VBT

While the Valence Bond theory, to a larger extent, explains the formation, structures, and magnetic behavior of coordination compounds, it has some shortcomings which are listed below:
(i) It involves several assumptions.
(ii) It does not give a quantitative interpretation of magnetic data.
(iii) It does not explain the color exhibited by coordination compounds.
(iv) It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.
(v) It does not make exact predictions regarding the tetrahedral and square planar structures of 4-coordinate complexes.
(vi) It does not distinguish between weak and strong ligands.

Recommended topic video on(Valence Bond Theory of Coordination Compounds)

Some Solved Examples

Example 1
Question:

The molecule in which hybrid molecular orbitals involve only one d-orbital of the central atom is:

1)[Ni(CN)₄]²⁻
2) BrF₅
3)XeF₄
4)[CrF₆]³⁻

Solution

Hybridization of the given molecules-

(1)[Ni(CN)₄]²⁻→dsp²
(2)BrF₅−Sp³ d²
(3)XeF₄−Sp³ d²
(4)(CrF₆)³⁻−d²Sp³

So, in [Ni(CN)₄]²⁻ molecule hybrid MOs involve only one d-orbital of the central atom.

Therefore, the correct option is (1).

Example 2
Question:

According to the valence bond theory, the hybridization of a central metal atom is dsp² for which one of the following compounds?

1)NiCl₂⋅6H₂ONiCl₂⋅6H₂O

2)K₂[Ni(CN)₄] K₂[Ni(CN)₄]

3)[Ni(CO)₄][Ni(CO)₄]

4)Na₂[NiCl₄]Na₂[NiCl₄]

Solution

Configuration of Ni=[Ar]4s²3d⁸
Configuartion of Ni2+=[Ar]3d⁸

Thus, Ni²⁺ has 2 unpaired electrons which can be paired up in the presence of a strong field ligand like CN^-as depicted below

Thus, [Ni(CN)₄]²⁻ has a dsp² hybridisation.
Hence, the correct answer is option (2).

Example 3
Question:

In Wilkinson’s catalyst, the hybridization of the central metal ion and its shape are respectively :

1) sp³d, trigonal bipyramidal
2) sp³ tetrahedral
3) dsp²,square planar
4) d²sp³octahedral

Solution:

As we learned in

Hybridization -

sp³d² - square bipyramidal or octahedral

d²sp³ - octahedral

sp³ - tetrahedral

dsp² - square planar

wherein

sp³d² - outer complex

d²sp³ - inner complex

sp³−[Ni(Cl)_4]²⁻ dsp²−[Pt(CN)₄]²⁻

sp³−[Ni(Cl)₄]²⁻ dsp²−[Pt(CN)₄]²⁻

The Wilkinson catalyst is [RhCl(pph3)₃] and the hybridization and its shape are dsp²and square planar respectively.

Hence, the answer is an option (3).

Example 4
Question:

Identify the pair in which the geometry of the species is T-shape and square-pyramidal, respectively :

1)ClF₃ and IO₄^-
2)ICl₂^-and ICl₅
3) XeOF₂and XeOF₄
4)IO₃^-and IO₂F₂^-

Solution:

As we learned in

Structure of XeOF₂ -

Sp³d hybridized and T-shaped structure

- wherein

Structure of Xenon XeOF₄ (oxytetrafluoride) -

Sp³d² hybridized and square pyramidal structure

- wherein

Summary

In a nutshell, the Valence Bond Theory is maybe one of the most important tools in knowing the bonding and properties of coordination compounds, overhead in scientific and industrial use. In short, VBT describes how the metal ion interacts with ligands to form stable complexes while taking their geometrical arrangement on the principles of hybridization. This theory focuses on the level of strength of a ligand and the hybridization types that give rise to a diversity of structures, instituting tetrahedral and square planar geometries.