Co-ordination Compounds

Ligands, Co-ordination number, Denticity and Chelation

Co ordination number:
Total number of electron pairs donated by the central metal atom through coordinated bond. Depending on the valencies of atoms coordination no. is defined.

 ion CN
Cr+1 6
Sn+4 6
Pb+4 6
Pt+4 6

Sidgwick Rule of Effective Atomic Number:
E.A.N = Atomic number − oxidation number + 2 coordination number
E.A.N = No. of electrons on central metal + electrons donated by ligands / metal / bridge ligand
Z = Atomic number, o.s → oxidation state
C.N = Coordination number of central metal.

E.A.N (Mn) = 25 − 0 + 2 × 5 + 1 = 36

Part1: View the Topic in this Video from 19:27 to 59:16

Part2: View the Topic in this Video from 0:40 to 12:25

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1. In case of monodentate ligands,
Coordination number = number of ligands

2. In polydentate ligands,
Coordination number = number of ligands × denticity

3. Effective Atomic Number (EAN)
This concept was proposed by Sidgwick. In a complex, the EAN of metal atom is equal to the total number of electrons present in it.
EAN = Z − ON of metal + 2 × CN
(where, Z = atomic number of metal atom
ON = oxidation number of metal
and CN = coordination number of complex)