CBSE Class 12 Chemistry Coordination Compounds Chapter Notes

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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 (EDTA4–) 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.

 CBSE Class 12 Chemistry - Coordination Compounds Chapter Notes 1

b. The ions/ groups bound by secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. 
c. The most common geometrical shapes in coordination compounds are octahedral, square planar and tetrahedral.
 
18. Oxidation number of central atom: The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. 
 
19. Homoleptic complexes: Those complexes in which metal or ion is coordinate bonded to only one kind of donor atoms. For example:[Co(NH3)6]3+ 
 
20. Heteroleptic complexes: Those complexes in which metal or ion is coordinate bonded to more than one kind of donor atoms. For example: [CoCl2(NH3)4]+, [Co(NH3)5Br]2+ 
 
21. Isomers. Two or more compounds which have same chemical formula but different arrangement of atoms are called isomers.
 
22. Types of isomerism: 
a. Structural isomerism 
i. Linkage isomerism 
ii. Solvate isomerism or hydrate isomerism 
iii. Ionisation isomerism 
iv. Coordination isomerism 
b. Stereoisomerism 
i. Geometrical isomerism 
ii. Optical isomerism 
 
23. Structural isomerism: This type of isomerism arises due to the difference in structures of coordination compounds. Structural isomerism, or constitutional isomerism, is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders. 
 
a. Ionisation isomerism: This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. Example: [Co(NH3)5Br] SO4 and [Co(NH3)5 SO4] Br 
 
b. Solvate isomerism: It is isomerism in which solvent is involved as ligand. If solvent is water it is called hydrate isomerism, e.g.,[Cr(H2O)6]Cl3 and [CrCl2(H2O)4] Cl2. 2H2O 
 
c. Linkage isomerism: Linkage isomerism arises in a coordination compound containing ambidentate ligand. In the isomerism, a ligand can form linkage with metal through different atoms. 
Example: [Co(NH3)5ONO]Cl2 and [Co(NH3)5NO2]Cl2 
 
d. Coordination isomerism: This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex.
Example:[Co(NH3)6][Cr(C2O4)3] and [Cr(NH3)6][Co(C2O4)3] 
 
24. Stereoisomerism: This type of isomerism arises because of different spatial arrangement. 
 
a. Geometrical isomerism: It arises in heteroleptic complexes due to different possible geometrical arrangements of ligands. 
 
b. Optical isomerism: Optical isomers are those isomers which are non superimposable mirror images.
 

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