|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
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.
According to the theory the postulates are:
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
In CoCl3.(NH3)5 the ionizable chlorides are only 2
In COCl3.(NH3)4 the ionizable ions are only one chloride ion
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
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.
For any coordination compound: To find the shape using valence bond theory following steps to be followed
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
Example [Fe(Co)5]: (inner orbital complex and diamagnetic )
EXAMPLE: in [CoF6]3-…… (outer orbital complex and paramagnetic )
Drawbacks of valence bond theory:
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
Ligands –have negative charge
Axial set non-axial set
dxy,dyz, dzx dx2-y2,dz2
This is crystal field splitting.
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)
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 )
square planar complex:
In the different order is seen i.e
Please note for all the complexes:
for strong ligands : the CFSE is more therefore pairing will occur
for weak ligands : the CFSE Is less