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9100 et ET 3 - Nice

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Data Structures Using C (CSC2SK)

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Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

Subject Chemistry

Paper No and Title 3, INORGANIC CHEMISTRY – I (Stereochemistry, Metal-

Ligand Equilibria and Reaction Mechanism of Transition

Metal Complexes)

Module No and Title 3, dπ-pπ bonding

Module Tag CHE_P3_M

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

TABLE OF CONTENTS

1. Learning Outcomes

2. Introduction

2 Metal-metal bonding

2 Overlapping of d-orbitals

3. π-backdonation or π-backbonding

3 Types of backbonding

3 Conditions of back bonding

4. Effects of backbonding

4 Bond length

4 Hybridisation, shape and bond angle

4 Lewis acidic and basic character

4 Adduct formation

4 Back bonding in borazine (inorganic benzene)

5. Bridge bonding

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

metal bonding or/and antibonding orbitals. This type of bonding is found in metals having d 8 configuration.

2 Overlapping of d-orbitals

The overlapping of d orbitals makes different types of metal-metal covalent bonds. Fig1 shows the types of covalent M-M interaction from strongest to weakest.

Fig Overlapping of d-orbitals

Overlap of two dz2 orbitals form a sigma bond which is stronger than the π bond formed by overlap of two dyz or dxz orbitals. Overlapping of dx2-y2 orbitals is not shown as it results in M-L bond formation.

Molecular orbital diagram for the interaction of two square planar metal centers is shown in fig 2.

Fig The interaction of two square planar metal centers

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

Initially electron fills the bonding orbitals but if more than 8 electrons is added, metal-metal bonds starts canceling and antibonding orbitals are began to form. The energy of antibonding orbitals is higher than the bonding orbitals therefore, antibonding orbitals are less stable.

The table given below shows the bonding between two square-planar metals. The metals having different geometry may have different bond orders.

No. of

electrons

M-M bond

d 1 -d 1 overlap Single bond d 2 -d 2 overlap Double bond d 3 -d 3 overlap Triple bond d 5 -d 5 overlap Triple bond d 6 -d 6 overlap Double bond d 7 -d 7 overlap Single bond d 8 -d 8 overlap No bond

*d 4 -d 4 is the optimum case as it results in the formation of quadruple bond.

Some examples of multiple covalent bonds:

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

Note that 3pπ-3pπ backbonding is not possible as if metal and ligand both are from 3rd period the size of both the atoms will be very large and effective overlapping will not take place.

3 Conditions of backbonding

The following conditions should be fulfilled for effective overlapping of atomic orbitals and formation of backbonds:

  1. Metal atom should have vacant orbitals
  2. Ligand should have lone pair of electrons or filled π-orbital.
  3. At least one of the atoms should be from 2nd period. As if both are from 3rd period then effective overlapping will not take place.

4. Effects of backbonding

4 Bond length

Bond length of the molecule always decreases whether the backbonding is from central atom to side atom or from side atom to central atom. As due to backbonding partial double bond character is induced in the molecule as a result of which bond order increases and hence bond length decreases. This is the reason for short B-F bond length in BF 3 as compared to [BF4-] ion.

4 Hybridization, shape and bond angle

Taking an example of BF 3 molecule, here lone pair on fluorine atom is donated to vacant π- orbital of boron and results in side to central atom backbonding. In this the hybridsation of molecule will not change and it remains sp 2 hybridised. Also the shape of the molecule remains trigonal planar.

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

But in N(SiH 3 ) 3 as lone pair on nitrogen atom is donated to empty orbital of the silicon and central to side atom backbonding take place, the lone pair on nitrogen atom is no more in hybridization and hence the hybridization of the molecule is changes from sp 3 to sp 2 (percentage s character increases) and hence the geometry of the molecule also changes from pyramidal to trigonal planar. Now as the geometry of the molecule changes by change in hybridization. Bond angle of the molecule also changes.

4 Lewis acidic and Lewis basic character

Back bonding is nothing but intramolecular Lewis acid base interaction. For example on comparing the Lewis acidic character of BF 3 and BH 3 molecule, the Lewis acidic character of BF 3

is less than that of BH 3 because it undergoes backbonding due to which the π- orbital of the boron atom is no longer vacant and hence BF 3 cannot act as a Lewis acid.

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

Similarly NH 3 and BBr 3 forms good adducts because none of them are involved in backbonding. Therefore, we can say that the molecules which are better Lewis base can form better adducts.

4 Backbonding in borazine (Inorganic benzene)

Borazine is also known as inorganic benzene because its structure is similar to that of benzene. In borazine nitrogen and boron are bonded as a 6 membered ring or has 6π electron system similar to that of benzene. But as compared to benzene, borazine is kinetically less stable and thermodynamically more stable. This is due to backbonding between boron and nitrogen atoms which induced polarity within the molecule and the bond between nitrogen and boron become ionic.

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

Now in borazine although nitrogen is more electronegative than boron but due to backbonding there will be a partial negative charge on boron and partial positive charge on nitrogen (nitrogen has lone pair of electron which are backbonded with vacant pi orbital of boron). For example if borazine is going to react with HCl then following reaction will take place:

5. Bridge bonding

Bridge bonding is another type of backbonding. It causes polarization in the molecule having vacant orbitals as they participates in hybridization. Due to which hybridization always increases to 1 step (i. sp-sp 2 and sp 2 -sp 3 ) and percentage s character decreases.

5 Types of bridge bonding

Bridge bonding is of two type i. 3 centered-2 electron and 3 centered-4 electron bridge bonding. Bridge bonding is only possible if there is no backbonding and no steric crowding with in the molecule. BH 3 involves in bridge bonding and one extra σ bond is formed with another BH 3 molecule. Here, the shared pair electrons of boron and hydrogen atom are donated to the vacant π-orbital of the next boron atom. In this way 2 electrons are shared between 3 atoms i. 3centered- 2electron bridge bonding will take place. The resultant B 2 H 6 molecule is shown below:

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

  1. The formation of bridge bonds results in formation of a 4 membered ring and in order to stable the ring structure, the bridge bonds are longer than that of terminal bonds
  2. Also, the bridge bonds are stronger because there are two electrons shared between three atoms and hence force by which electrons are hold with in the bond is more.

This can be explain by considering methylation of B 2 H 6. As it takes place on terminal bonds instead of bridge bonds and following product is form.

5 Cleavage of bridge bonding

There are two types of cleavage possible in molecules i. homolytic cleavage and heterolytic cleavage. In homolytic cleavage two electrons in a cleaved bond are divided equally between the products. And in heterolytic cleavage the bond break in such a way that the shared pair of electrons remain with one of the fragments. Taking an example of B 2 H 6 , bulkier bases such as N(CH 3 ) 3 results in homolytic cleavage and form two molecules of BH 3 molecule because the bulky groups (i. CH 3 ) cause steric crowding therefore N(CH 3 ) 3 cannot attack on boron having more number of terminal hydrogen.

On the other hand lighter base such as NH 3 causes heterolytic cleavage and results in the formation of [BH2+] and [BH4-] as shown below:

Chemistry Paper 3: INORGANIC CHEMISTRY – I (Stereochemistry,

Metal-Ligand Equilibria and Reaction Mechanism of

Transition Metal Complexes)

6. Summary

  • The overlapping of d orbitals makes different types of metal-metal covalent bonds.
  • Overlap of two dz2 orbitals form a sigma bond which is stronger than the п bond formed by overlap of two dyz or dxz orbitals.
  • The energy of antibonding orbitals is higher than the bonding orbitals in a molecular orbital therefore, antibonding orbitals are less stable.
  • Backbonding involves a synergic process in which the electrons from the filled π-orbitals or lone pair of the ligand are donated into an empty orbital of the metal and at the same time back donation of electrons take place from the nd orbital of the metal to the empty π* antibonding orbital of the ligand.
  • Backbonding is basically of two type i. central atom to side atom and side atom to central atom.
  • Back bonding can affect the following properties of a molecule:
  1. Bond length
  2. Hybridization, shape and bond angle
  3. Lewis acidic and basic characters
  4. Adduct formation
  • Bridge bonding is another type of backbonding. It causes polarization in the molecule having vacant orbitals as they participates in hybridization.
  • It is of two types i. 3 centered-2 electron and 3 centered-4 electron bridge bonding.
  • Bridge bonding is only possible if there is no backbonding and no steric crowding with in the molecule.
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9100 et ET 3 - Nice

Course: Data Structures Using C (CSC2SK)

31 Documents
Students shared 31 documents in this course

University: Acharya Nagarjuna University

Was this document helpful?
____________________________________________________________________________________________________
Chemistry
Paper 3: INORGANIC CHEMISTRY Ð I (Stereochemistry,
Metal-Ligand Equilibria and Reaction Mechanism of
Transition Metal Complexes)
Module 3: dπ-pπ bonding
Subject
Chemistry
Paper No and Title
3, INORGANIC CHEMISTRY Ð I (Stereochemistry, Metal-
Ligand Equilibria and Reaction Mechanism of Transition
Metal Complexes)
Module No and Title
3, dπ-pπ bonding
Module Tag
CHE_P3_M3

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