What is Valence Bond Theory in the context of coordination compounds?

Valence Bond Theory (VBT) in coordination compounds explains the formation of coordinate covalent bonds between the central metal atom and ligands. It describes how atomic orbitals of the metal overlap with those of the ligands to form molecular orbitals, resulting in the complex's structure and properties.

What is hybridization in the context of Valence Bond Theory?

Hybridization in VBT refers to the mixing of atomic orbitals of the central metal atom to form new hybrid orbitals. These hybrid orbitals then overlap with ligand orbitals to form coordinate covalent bonds. The type of hybridization depends on the geometry of the complex and the number of ligands.

How does Valence Bond Theory explain the formation of inner orbital complexes?

Inner orbital complexes, according to VBT, form when the metal uses its inner d-orbitals for hybridization and bonding with ligands. This typically occurs with strong-field ligands and results in low-spin complexes. The d-orbitals participate directly in bond formation, leading to structures like octahedral or square planar.

How does Valence Bond Theory explain the magnetic properties of coordination compounds?

VBT explains magnetic properties based on the number of unpaired electrons in the metal's d-orbitals. In outer orbital complexes, more unpaired electrons lead to higher paramagnetism. Inner orbital complexes often have fewer unpaired electrons, resulting in lower paramagnetism or even diamagnetism.

What are outer orbital complexes in Valence Bond Theory?

Outer orbital complexes in VBT form when the metal uses its outer d-orbitals (along with s and p) for hybridization and bonding with ligands. This usually happens with weak-field ligands and results in high-spin complexes. The inner d-orbitals remain unhybridized and can accommodate unpaired electrons.

What are the limitations of Valence Bond Theory in explaining coordination compounds?

Limitations of VBT include: inability to explain colors of complexes, difficulty in explaining the spectrochemical series, inability to predict the strength of ligand fields, and challenges in explaining the magnetic moments of some complexes. It also becomes more complicated for complexes with more than six ligands.

How does Valence Bond Theory differ from Crystal Field Theory?

Valence Bond Theory focuses on the overlap of atomic orbitals between the metal and ligands, while Crystal Field Theory considers the electrostatic interactions between the metal and ligands. VBT explains bonding and hybridization, whereas CFT primarily explains the splitting of d-orbitals and resulting colors and magnetic properties.

How does Valence Bond Theory explain the color of coordination compounds?

VBT doesn't directly explain color, which is a limitation of the theory. It focuses more on bonding and structure. Color is better explained by Crystal Field Theory, which considers d-orbital splitting and electronic transitions. This is one reason why multiple theories are needed to fully describe coordination compounds.

How does Valence Bond Theory account for the trans effect in square planar complexes?

VBT explains the trans effect through the influence of certain ligands on the metal's hybrid orbitals. Ligands that form strong π bonds or have high trans influence can weaken the metal-ligand bond trans to them by altering the electron distribution in the hybrid orbitals, making the trans position more labile.

How does Valence Bond Theory explain the spectrochemical series?

VBT struggles to fully explain the spectrochemical series, which is a limitation of the theory. It can partially account for it by considering the extent of orbital overlap between metal and ligand orbitals. Stronger overlap generally corresponds to higher positions in the spectrochemical series, but this explanation is not comprehensive.

What is the significance of dsp2 hybridization in coordination compounds?

dsp2 hybridization is significant in forming square planar complexes. It involves one d-orbital, one s-orbital, and two p-orbitals of the metal, resulting in four hybrid orbitals arranged in a square planar geometry. This hybridization is common in d8 metal complexes like Pt(II) and Pd(II).

What is the role of σ and π bonding in Valence Bond Theory of coordination compounds?

In VBT, σ bonds form through head-on overlap between metal hybrid orbitals and ligand orbitals. π bonds can form through side-on overlap of metal d-orbitals with appropriate ligand orbitals. The presence and strength of π bonding can affect the overall stability and properties of the complex.

How does Valence Bond Theory explain the stability of octahedral complexes?

VBT explains the stability of octahedral complexes through the formation of six equivalent sp3d2 or d2sp3 hybrid orbitals on the metal. These hybrid orbitals overlap with ligand orbitals to form strong σ bonds in all six directions, resulting in a stable octahedral structure.

What is the difference between high-spin and low-spin complexes in Valence Bond Theory?

In VBT, high-spin complexes typically form outer orbital complexes with weak-field ligands, where electrons occupy unhybridized d-orbitals following Hund's rule. Low-spin complexes form inner orbital complexes with strong-field ligands, where electrons pair up in hybrid orbitals before occupying higher energy orbitals.

How does Valence Bond Theory explain the formation of tetrahedral complexes?

VBT explains tetrahedral complexes through sp3 hybridization of the metal's orbitals. Four equivalent hybrid orbitals are formed, which overlap with ligand orbitals to form four coordinate covalent bonds arranged tetrahedrally. This is common in complexes with d10, d5, or d0 electron configurations.

What is the significance of d2sp3 hybridization in coordination compounds?

d2sp3 hybridization is significant in forming octahedral complexes. It involves two d-orbitals, one s-orbital, and three p-orbitals of the metal, resulting in six hybrid orbitals arranged octahedrally. This hybridization is common in inner orbital complexes with strong-field ligands.

How does Valence Bond Theory explain the formation of square pyramidal complexes?

VBT explains square pyramidal complexes through sp3d hybridization. Five hybrid orbitals are formed: four arranged in a square plane and one perpendicular to this plane. This hybridization scheme accounts for the five coordinate bonds in square pyramidal geometry.

What is the role of electronegativity in Valence Bond Theory of coordination compounds?

Electronegativity plays a crucial role in VBT by influencing the nature of the metal-ligand bond. More electronegative ligands tend to form stronger covalent bonds with the metal, affecting the degree of orbital overlap and the resulting complex stability. It also influences the distribution of electron density in the complex.

What is the significance of sp3d2 hybridization in coordination compounds?

sp3d2 hybridization is significant in forming octahedral complexes, particularly outer orbital complexes. It involves one s-orbital, three p-orbitals, and two d-orbitals of the metal, resulting in six hybrid orbitals arranged octahedrally. This hybridization is common with weak-field ligands and results in high-spin complexes.

How does Valence Bond Theory explain the formation of linear complexes?

VBT explains linear complexes through sp hybridization of the metal's orbitals. Two hybrid orbitals are formed 180° apart, which overlap with ligand orbitals to form two coordinate covalent bonds in a linear arrangement. This is common in complexes with d10 electron configurations, like [Ag(NH3)2]+.

What is the concept of "back bonding" in Valence Bond Theory?

Back bonding in VBT refers to the donation of electron density from filled metal d-orbitals to empty ligand orbitals (usually π* antibonding orbitals). This strengthens the metal-ligand bond and can stabilize the complex. It's particularly important in complexes with ligands like CO, CN-, and NO+.

How does Valence Bond Theory explain the stability of 18-electron complexes?

VBT explains the stability of 18-electron complexes through the concept of achieving a noble gas configuration. When the sum of the metal's electrons and those donated by ligands equals 18, all bonding orbitals are filled, resulting in a stable electronic configuration similar to the next noble gas.

What is the role of ligand field strength in Valence Bond Theory?

Ligand field strength in VBT influences whether a complex forms an inner or outer orbital complex. Strong-field ligands promote inner orbital complex formation with low spin, while weak-field ligands lead to outer orbital complexes with high spin. This affects the hybridization scheme and magnetic properties.

How does Valence Bond Theory account for the Jahn-Teller effect?

VBT doesn't directly account for the Jahn-Teller effect, which is another limitation of the theory. The Jahn-Teller effect involves geometric distortions in certain complexes to remove orbital degeneracy. While VBT can describe the resulting bonding after distortion, it doesn't predict or explain the cause of the distortion itself.

What is the significance of d4sp2 hybridization in coordination compounds?

d4sp2 hybridization is significant in forming trigonal prismatic complexes. It involves four d-orbitals, one s-orbital, and two p-orbitals of the metal, resulting in six hybrid orbitals arranged in a trigonal prismatic geometry. This hybridization is less common but can occur in certain transition metal complexes.

How does Valence Bond Theory explain the concept of isomerism in coordination compounds?

VBT explains isomerism by considering different possible arrangements of ligands around the central metal atom. Geometric isomers result from different spatial arrangements of the same ligands, while linkage isomers involve different bonding modes of ambidentate ligands. VBT can describe the bonding in each isomer but doesn't predict which isomer will form.

What is the role of chelation in Valence Bond Theory of coordination compounds?

Chelation in VBT involves the formation of multiple coordinate bonds between a single polydentate ligand and the metal. VBT explains this through the overlap of multiple ligand orbitals with appropriate metal hybrid orbitals. Chelation often increases complex stability due to the entropy effect and the formation of ring structures.

How does Valence Bond Theory explain the formation of bridging ligands in polynuclear complexes?

VBT explains bridging ligands in polynuclear complexes through the ability of certain ligands to form coordinate bonds with more than one metal center. This involves the overlap of ligand orbitals with hybrid orbitals from multiple metal atoms. The resulting structures can be described using appropriate hybridization schemes for each metal center.

How does Valence Bond Theory explain the concept of hard and soft acids and bases in coordination compounds?

VBT contributes to understanding hard and soft acids and bases by considering orbital overlap. Hard acids and bases typically involve ionic bonding or highly polar covalent bonds, explained by overlap of orbitals with similar energy. Soft acids and bases involve more covalent bonding, explained by efficient overlap of orbitals with similar size and energy.

What is the role of crystal field stabilization energy in Valence Bond Theory?

Crystal field stabilization energy is not directly addressed by VBT, which is a limitation of the theory. VBT focuses on bonding and hybridization rather than energy differences between orbitals. This concept is better explained by Crystal Field Theory, highlighting the complementary nature of different theories in coordination chemistry.

How does Valence Bond Theory explain the concept of redox stability in coordination compounds?

VBT explains redox stability by considering the electronic configuration and bonding in complexes. Stable oxidation states often correspond to filled or half-filled subshells, which VBT describes through appropriate hybridization schemes. The strength of metal-ligand bonds, as explained by orbital overlap, also contributes to redox stability.

What is the role of electrochemical series in Valence Bond Theory of coordination compounds?

The electrochemical series in VBT helps predict the stability of different oxidation states of metals in complexes. VBT uses this information to determine likely hybridization schemes and bonding arrangements. It aids in understanding which complexes are more likely to form and their relative stabilities.

Valence Bond Theory of Coordination Compounds

Chemistry
Coordination compounds
Valence Bond Theory of Coordination Compounds

Valence Bond Theory of Coordination Compounds

Shivani PooniaUpdated on 02 Jul 2025, 07:42 PM IST

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.

This Story also Contains

Interpretation of Coordination Compounds According to VBT
Weaknesses of VBT
Some Solved Examples
Summary

Valence Bond Theory of Coordination Compounds

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

Coordination Number	Type of Hybridisation	Shape
4	sp³	Tetrahedral
4	dsp²	Square Planar
5	sp³d	Trigonal Bipyramidal
6	sp³d²	Octahedral
6	d²sp³	Octahedral

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 sp²as 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.

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 dsp²hybridisation.
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.

Frequently Asked Questions (FAQs)

Q: What is the role of molecular orbital theory in complementing Valence Bond Theory for coordination compounds?

Molecular Orbital Theory complements VBT by providing a more comprehensive description of bonding in coordination compounds. While VBT focuses on localized bonds, MO theory considers the formation of molecular orbitals across the entire complex. This helps explain phenomena that VBT struggles with, such

Q: How does Valence Bond Theory explain the concept of ligand substitution reactions?

VBT explains ligand substitution reactions by considering changes in hybridization and orbital overlap. The process involves breaking existing metal-ligand bonds and forming new ones, which VBT describes through changes in the hybridization scheme of the metal and new orbital overlaps with incoming ligands.

Q: What is the significance of sd hybridization in coordination compounds?

sd hybridization, while less common, is significant in explaining some linear complexes, particularly those involving transition metals in low oxidation states. It involves one s-orbital and one d-orbital of the metal, resulting in two hybrid orbitals arranged linearly. This can occur in certain d9 complexes.

Q: How does Valence Bond Theory account for the Irving-Williams series?

VBT doesn't directly explain the Irving-Williams series, which is a limitation. The series describes the relative stability of complexes formed by divalent first-row transition metals. While VBT can describe the bonding in each complex, it doesn't inherently predict the trend in stability observed in the series.

Q: How does Valence Bond Theory explain the concept of lability and inertness in coordination compounds?

VBT explains lability and inertness by considering the strength and nature of metal-ligand bonds. Labile complexes have weaker metal-ligand bonds, often due to poor orbital overlap or competing interactions. Inert complexes have stronger bonds, typically due to effective orbital overlap and stable electronic configurations.

Q: How does Valence Bond Theory explain the formation of bent complexes?

VBT explains bent complexes through sp2 hybridization of the metal's orbitals. Three hybrid orbitals are formed, with two used for bonding to ligands and one containing a lone pair. This results in a bent geometry, as seen in complexes like [Cu(NH3)2]+, where the bond angle is less than 180°.

Q: What is the significance of dsp3 hybridization in coordination compounds?

dsp3 hybridization is significant in forming trigonal bipyramidal complexes. It involves one d-orbital, one s-orbital, and three p-orbitals of the metal, resulting in five hybrid orbitals arranged in a trigonal bipyramidal geometry. This hybridization is seen in five-coordinate complexes like PCl5.

Q: How does Valence Bond Theory account for the nephelauxetic effect?

VBT doesn't directly account for the nephelauxetic effect, which is the expansion of d-orbitals in complexes compared to the free ion. However, VBT's consideration of covalent bonding between metal and ligands indirectly relates to this effect, as increased covalency leads to greater orbital expansion.

Q: What is the role of steric factors in Valence Bond Theory of coordination compounds?

Steric factors in VBT influence the preferred geometry and stability of complexes. Bulky ligands can affect the hybridization scheme by favoring geometries that minimize steric repulsion. VBT considers these factors when explaining why certain complexes adopt specific structures or exhibit particular properties.

Q: What is the significance of f-orbital involvement in Valence Bond Theory of coordination compounds?

f-orbital involvement in VBT is significant for explaining the bonding in lanthanide and actinide complexes. While less common, f-orbitals can participate in hybridization schemes, leading to unique geometries and properties. This expands VBT's applicability to a wider range of elements beyond transition metals.

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