CBSE Class 12 Chemistry Coordination Compounds Chapter Notes

2. Coordination entity: A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. Example: in K₄[Fe(CN)₆], [Fe(CN)₆]^4- represents coordination entity.

3. Central atom or ion: In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion. E.g.: in K₄[Fe(CN)₆], Fe²+ is the central metal ion.

4. Ligands: A molecule, ion or group that is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is called ligand. It may be neutral, positively or negatively charged. Example: H₂O, CN^-, NO⁺ etc.

5. Donor atom: An atom of the ligand attached directly to the metal is called the donor atom. For example: in the complex K₄[Fe(CN)₆], carbon is a donor atom. 

6. Coordination number: The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. For example: in the complex K₄[Fe(CN)₆], the coordination number of Fe is 6.

7. Coordination sphere: The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the coordination sphere. For example: in the complex K₄[Fe(CN)₆], [Fe(CN)₆]^4- is the coordination sphere.

8. Counter ions: The ions present outside the coordination sphere are called counter ions. For example: in the complex K₄[Fe(CN)₆], K⁺ is the counter ion.

9. Coordination polyhedron: The spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and tetrahedral. For example: [PtCl₄]^2- is square planar, Ni(CO)₄ is tetrahedral while [Cu(NH₃)₆]³⁺ is octahedral.

10. Charge on the complex ion: The charge on the complex ion is equal to the algebraic sum of the charges on all the ligands coordinated to the central metal ion.

11. Denticity: The number of ligating (linking) atoms present in ligand is called denticity.

12. Unidentate ligands: The ligands whose only one donor atom is bonded to metal atom are called unidentate ligands. Example: H₂O, NH₃, CO, CN-

13. Didentate ligands: The ligands which contain two donor atoms or ions through which they are bonded to the metal ion. For example: ethylene diamine (H₂NCH₂CH₂NH₂) has two nitrogen atoms, oxalate ion has two oxygen atoms which can bind with the metal atom. | COO^-  - COO^-

14. Polydentate ligand: When several donor atoms are present in a single ligand, the ligand is called polydentate ligand. For example: in N(CH₂CH₂NH₂)₃, the ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA^4–) is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.

15. Chelate: An inorganic metal complex in which there is a close ring of atoms caused by attachment of a ligand to a metal atom at two points. An example is the complex ion formed between ethylene diamine and cupric ion, [Cu(NH₂CH₂NH₂)₂]²⁺.

16. Ambidentate ligand: Ligands which can ligate (link) through two different atoms present in it are called ambidentate ligand. Example: NO₂^-,and SCN^-. NO₂^- can link through N as well as O while SCN^- can link through S as well as N atom.

17. Werner’s coordination theory: Werner was able to explain the nature of bonding in complexes. The postulates of Werner’s theory are:

a. Metal shows two different kinds of valencies: primary valence and secondary valence.

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