Class 12 Chemistry Coordination Compounds Bonding in coordination compounds

Bonding in coordination compounds

The first theory in order to explain it was given by Alfred Werner in 1892.

He performed various experiments to show that the surrounding atoms exist around central atom. He actually conducted ppt. studies

CoCl2+NH3àCoCl3.6NH3

When CoCl3.6NH3 was precipitated with AgNO3 it gave 3 moles of AgCl this shows that 3 Chloride ions are not directly bonded with cobalt that is why it was precipitated with silver nitrate which gave him the idea about primary and secondary valances’ and accordingly he postulated his theory.

Werner’s theory

According to the theory the postulates are:

  1. Metal exhibit 2 types of vacancies: primary valency and secondary valency
  2. Primary valency gives the information about oxidation state and secondary valency gives the information about coordination number
  3. Primary valency is variable whereas secondary valency is always fixed
  4. Secondary valency that is coordination number determines the geometry of molecule or we can say polyhedral of the molecule.
  5. Metal stabilize all its vacancies

Depending upon this theory various structures of coordination compound was explained :

In CoCl3. (NH3)6 à In this NH3 is secondary valency and Cl is primary valency

 Class_12_Coordination_Compounds_Different_Structures

In CoCl3.(NH3)5 the ionizable chlorides are only 2

Class_12_Coordination_Compounds_Different_Structures_1

In COCl3.(NH3)4 the ionizable ions are only one chloride ion

Class_12_Coordination_Compounds_Different_Structures_2

Please note the dark lines shows ionizable part and light lines show non ionizable part in all the figures.

Limitations of Werner’s theory:

He was able to explain many facts about coordination compounds but failed to give any information about why only certain elements participate in coordinate bond, why the coordination entity has special geometry …

Due to these reasons other theory was proposed that is valence bond theory

Valence bond theory

It was given by Pauling in 1931

  1. He proposed the idea of donating lone pair to central metal atom.
  2. Bonding in coordination compound occur due to overlap of orbital of ligand with vacant orbital of central metal atom
  3. All the vacant d orbitals have same energy. but the degeneracy of d orbital breaks when ligand approaches
  4. Hybridization is considered while drawing polyhedral
  5. Metal ions in presence of ligands can use their (n-1)d ns np or ns np Nd.

If the inner d orbital is used than the complex is regarded as inner orbital complex and if outer d orbital is used than the complex is outer orbital complex.

  1. The ligands decide which orbitals out of these to be used and accordingly the geometry is decided.

If hybridization

  • sp3- tetrahedral
  • dsp2-square planar
  • Sp3d-trigonal bipyramidal
  • Sp3d2- octahedral
  • d2sp3-octahedral

For example:

For any coordination compound: To find the shape using valence bond theory following steps to be followed

  1. Remove the electrons from the metal and form it the ion
  2. Rearrange metal electrons if necessary
  3. Hybridization
  4. Overlapping of hybrid orbitals of metal with ligand

Let us take one example: of example [Co(NH3)6]3+. In this central metal atom Co atomic no. is 27. The electronic configuration of Co = (Ar)183d74s2

Co3+=(Ar)183d6

 Class_12_Coordination_Compounds_Structure_1

Example [Fe(Co)5]: (inner orbital complex and diamagnetic )

 Class_12_Coordination_Compounds_Structure_3

EXAMPLE: in [CoF6]3-…… (outer orbital complex and paramagnetic )

 Class_12_Coordination_Compounds_Structure_2

 

Drawbacks of valence bond theory:

  1. This theory couldn’t have valid reason behind that why some complexes of metal oxidation state is inner orbital while in some other complexes the same metal atom ion in same state form outer orbital complex.
  2. The magnetic behavior explained wasn’t satisfactory
  3. This theory couldn’t give the information about color of compounds
  4. This theory failed to distinguish between strong and weak ligand

TO OVERCOME THE SHORT COMINGS A NEW THEORY WAS PROPOSED: CRYSTAL FIELD THEORY

Crystal field splitting theory

It was given by Hans Bethe Ans John van vleck

Postulates

  1. It assumes the central metal atom and ligands as point charges
  2. When a complex is formed: central metal atom positive charge

Ligands –have negative charge

  1. This theory considers the interaction between central metal atom and ligand is purely electrostatic
  2. When a complex is formed the central metal atom is surrounded by oppositely charged ligands
  3. No hybridization takes place
  4. To form a bond the ligand molecule must approach towards central metal atom
  5. In absence of external magnetic field, the d orbital of central metal atom is degenerate but this degeneracy breaks when ligand approaches.
  6. The d orbital splits into two sets:

Axial set                                        non-axial set

dxy,dyz, dzx                                              dx2-y2,dz2

 Class_12_Coordination_Compounds_d_Orbitals

 

This is crystal field splitting.

  1. Repulsive forces occur between electrons of metal and with lone pair ligands due to which energy of electron fluctuate or changes.

For octahedral complexes

To form octahedral complex the ligands, have to approach central metal atom along the coordination axis. During the approach the d orbitals whose lobes lie along the axis will experience more repulsion due to this their energy will increase and the other non axial set will suffer less repulsion. as a result, the non-axial will have less energy as compare to axial set (eg greater than  t2g)

Class_12_Coordination_Octahedral_Complex

Tetrahedral complex:

The ligands have to approach central metal atom in between the coordinationaxis. during the approach the d orbital’s whose lobes lie along the axis will experience less repulsion due to this their energy will increase and the other non axial set will suffer more repulsion. as a result, the non-axial will have more energy as compared to axial set (t2g greater than eg )

 Class_12_Coordination_Tetrahedral_Complex

 

square planar complex:

In the different order is seen i.e

dx2-y2,dxy,dz2,dyz,dzx

Class_12_Coordination_Square_Planar_Complex

Please note for all the complexes:

for strong ligands : the CFSE is more therefore pairing will  occur

for weak ligands : the CFSE Is less

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