(i) The ligands are the atoms or groups of atoms bound to the central atom/ion. The atom in a ligand that is bound directly to the central metal atom is known as a donor atom.
(ii) For example, in K₄[Fe(CN)₆], the ligand is CN^- ion, but the donor atom is carbon.

(i) The central atom or ion is the one that occupies the central position in a coordination entity and binds other atoms or groups of atoms (ligands) to itself, through a coordinate covalent bond.
(ii) For example, in K₄[Fe(CN)₆], the central metal ion is Fe²⁺. In the coordination entity [Fe(CN)₆]^4-, the Fe²⁺ accepts an electron pair from each ligand, CN^- and thereby forming six coordinate covalent bonds with them.
(iii) It is referred to as a Lewis acid.

(i) Porphyrin
(ii) Cobalt
(iii) Wilkinson Catalyst [(PPh₃)₃ RhCl]

Coordination number is the number of co-ordinate bonds formed by the ligands with the central metal atom.

Coordination isomerism.

Tetraammine copper (II) sulphate.

(i) EDTA is used in the treatment of lead poisoning
(ii) Cis-platin[Pt(NH₃)₂Cl₂] is used in the treatment of cancer.

(i) Oxidation state of the central metal ion : +2 (i.e) Mn²⁺
(ii) Co-ordination number : 6
(iii) Nature of ligand: CN^- (Negative ligand)
(iv) Magnetic property: Paramagnetic nature
(v) Electronic configuration: 

(vi) Oxidation state: +2 (or) Mn²⁺ of central metal ion

Tetrahedral complexes do not exhibit geometrical isomerism. Because the relative position of donor atoms of ligand the unidentate Iigands (donor atom) attached to the central atom are same with respect to each other.

[Ma₂b₂C₂]^\(\pm\)n where a, b, c are monodentate ligands.

[Pt(py₂)(NH₃)₂Cl₂]²⁺. It exhibits both optical and geometrical isomerism. c is isomer exhibit optical isomerism also. While trans isomer exhibits geometrical isomerism only.

Medicinal uses
(i) Ca-EDTA chelate, is used in the treatment of lead and radioactive poisoning.
(ii) That is for removing lead and radioactive metal ions from the body.
(iii) Cis-platin is used as an antitumor drug in cancer treatment.
Biological Importance compounds:-
(i) Fe²⁺ - porphyrin complex: It plays an important role in carrying oxygen from lungs to tissues and tissue carbon dioxide from to lungs.
(ii) Chlorophyll - useful in photosynthesis in plants.

a) potassiumhexacyanidoferrate(II) - Potassiumhexacyanidoferrate(II) - K₄[Fe(CN)₆]
b) Pentacarbonyliron(0) - [Fe(CO)₅]
c) Pentaamminenitrito −kNcobalt(III)ion - [Co(NH₃)₅(NO₂)]²⁺
d) Hexaamminecobalt(III) Sulphate  - [CO(NH₃)₆](SO₄)₃
e) Sodiumtetrafluoridodihydroxidochromate(III)  - Na₂[CrF₄(OH)₂]

i) Na₂[Ni(EDTA)] - Sodium 2, 2', 2",2'" - (ethane-1,2 diyldinitrilo tetraacetatonickelate)(II)
ii) [Ag(CN)₂]^- - Dicyanido-kC-argentate (I) ion
iii) [CO(en)₃]₂(SO₄)₃ - Tris (ethane 1, 2 diamine)cobalt (III) sulphate
iv) [CO(ONO)(NH₃)₅]²⁺ - Pentaamminenitrito - KO cobalt (III) ion
v) [Pt(NH₃)₂Cl(NO₂)] - Diammainechloridonitro - kN - platinum (II)

(i) [Co(NH₃)₅ CI]²⁺
(ii) [CO(NO₂)₃(NH₃)₃]
(iii) [Co(NH₃)₄H₂OBr](NO₃)₂
(iv) [Co(en)₂CI₂]CI
(v) [Cu(NH₃)₄]SO₄

Metal carbonyls are classified in two different ways as described below Classification based on the number of metal atoms present.
a. Mononuclear carbonyls
These compounds contain only one metal atom. For example, [Ni(CO)₄] - nickel tetracarbonyl is tetrahedral, [Fe(CO)₅] - Iron pentacarbonyl is trigonal bipyramidal, and [Cr(CO)₆] - Chromium hexacarbonyl is octahedral.
b. Polynuclear carbonyls
Metallic carbonyls containing two or more metal atoms are called polynuclear carbonyls. Polynuclear metal carbonyls may be Homonuclear [Co₂(CO)₈], [Mn₂(CO)₁₀], [Fe₃ (CO)₁₂] or heteronuclear [MnCo(CO)₉], [MnRe(CO)₁₀] etc.

(i) It exhibits both geometrical and optical isomerism
(a) Geometrical isomers:

(b) Optical isomers:

(ii) It shows two optical isomers

(iii) Ionisation isomers
Linkage isomers
[Co(NH₃)₅(NO₂)(NO₃)₂], [CO(NH₃)₅ (ONO)](NO₃)₂
(iv) Geometrical isomers

(i) Every metal atom has two types of valencies Primary valency or ionisable valency Secondary valency or non ionisable valency
(ii) The primary valency corresponds to the oxidation state of the metal ion. It is always satisfied by negative ions.
(iii) Secondary valency corresponds to the coordination number of the metal ion or atom. It is satisfied by either negative ions or neutral molecules.
(iv) The molecules or ions that satisfy secondary valencies are called ligands.
(v) The ligands which satisfy secondary valencies must project in definite directions in space. So the secondary valencies are directional in nature whereas the primary valencies are non - directional in nature.
(vi) The ligands have unshared pair of electrons. These unshared pair of electrons are donated to central metal ion or atom in a compound. Such compounds are called coordination compounds.
Werner's representation
Eg: [Co(NH)₆]Cl₃
Cl: primary valency (dotted lines)
NH₃: secondary valency (solid lines).
Defects of Werner's theory
Werner's theory describes the structures of many co-ordination compounds successfully. However, it does not explain the magnetic and spectral properties.

a) [FeF₆]^4-: Fe atom - outer electronic configuration 3d⁶ 4s²

F^- is weak field ligand
In [FeF₆]^4-the hybridisation takes place is sp³d²
The number of unpaired electrons = 4.
\(\therefore \mu =\sqrt { 4(4+2) } =\sqrt { 24 } \)
The molecule is paramagnetic due to the presence of unpaired electrons.
The geometry of the molecule is octahedral.
b) [Fe(CN)₆]^4-

In [Fe(CN)₆]^4- complex, the CN^- ligand is a powerful ligand, it forces the unpaired electrons in the 3d level to pair up inside.
Hence the species has no unpaired electron after hybridisation So the molecule is diamagnetic.

The geometry of the molecule is octahedral.

Most of the elements exhibit, two types of valence namely primary valence and secondary valence and each element tend to satisfy both the valences.
The primary valence is referred the oxidation state of the metal atom.
The secondary valence as the coordination number. For example, according to Werner, the primary and secondary valences of cobalt are 3 and 6 respectively.
The primary valence of a metal ions ae always satisfied by negative ions.
For example in the complex CoCI₃.6NH₃. The primary valence of Co is +3 and is satisfied by 3CI^- ions.
The secondary valence is satisfied by negative ions, neutral molecules, positive ions or the combination of these.
For example, in CoCl₃.6NH₃ complex primary valence of cobalt +3 and it is satisfied by 3 CI^-.
The secondary valence of cobalt is 6 and is satisfied by six neutral ammonia molecules. where as in CoCI₆.NH₃. 
Secondary valence of Co = 5{It is satisfied five neutral molecules and a Cl^- ion}
According to Werner, there are two spheres of attraction around a metal atom/ion in a complex.

The inner /coordination sphere:
The groups present in this sphere are firmly attached to the metal.
The outer sphere / ionisation sphere:
The groups present in this sphere are loosely bound to the central metal ion and hence can be separated into ions upon dissolving the complex in a suitable solvent.
The primary valencies are non-directional. while the secondary valencies are directional.
The geometry of the complex is determined by the special arrangement of the groups which satisfy the secondary valence.

(i) A red blood corpuscle (RBC) is composed of heme group, which is Fe²⁺- Porphyrin complex. it plays an important role in carrying oxygen from lungs to tissues and carbon dioxide from tissues to lungs.
(ii) Chlorophyll, a green pigment present in green plants and algae, is a coordination complex containing Mg²⁺ as central metal ion surrounded by a modified Porphyrin ligand called corrin ring. It plays an important role in photosynthesis, by which plants converts CO₂ and water into carbohydrates and oxygen.
(iii) Vitamin B₁₂ (cyanocobalamin) is the only vitamin consist of metal ion. it is a coordination complex in which the central, metal ion is Co⁺ surrounded by Porphyrin like ligand.
(iv) Many enzymes are known to be metal complexes, they regulate biological processes. For example, Carboxypeptidase is a protease enzyme that hydrolytic enzyme important in digestion contains a zinc ion coordinated to the protein.

Even though VBT explains many of the observed properties of complexes, it still has following limitations.
(i) It does not explain the colour of the complex.
(ii) It considers only the spin only magnetic moments and does not consider the other components of magnetic moments.
(iii) It does not provide a quantitative explanation as to why certain complexes are inner orbital complexes and the: others are outer orbital complexes for the same metal. For example, [Fe(CN)₆]^4- is diamagnetic (low spin) whereas [FeF₆]^4-is paramagnetic (high spin).

(i) Greater the strength of the ligands greater is the value of \({ \triangle }_{ 0 }\)
(ii) Higher the ionic charge on the central metal ion. Greater will be value of \({ \triangle }_{ 0 }\)

(i) Coordination compounds which possess chirality exhibit optical isomerism similar to organic compounds.
(ii) The pair of two optically active isomers which are mirror images of each other are called enantiomers.
(iii) Their solutions rotate the plane of the plane polarised light either clockwise or anticlockwise and the corresponding isomers are called 'd' (dextrorotatory) and 'I' (levorotatory)  forms respectively.
(iv) The octahedral complexes of type \(\left[\mathrm{M}(\mathrm{xx})_{3}\right]^{\mathrm{n} \pm}, \left[\mathrm{M}(\mathrm{xx})_{2} \mathrm{AB}\right]^{\mathrm{n\pm}}\) and \(\left[\mathrm{M}(\mathrm{xx})_{2} \mathrm{~B}_{2}\right]^{\mathrm{n\pm}}\) exhibit optical isomerism.
Examples:
(i) The optical isomers of \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) 
(ii) The coordination complex \(\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right]^{+}\) has three isomers, two optically active cis forms and one optically inactive trans form.

In all the complexes, the central metal ion is Co³⁺, among the given ligands CN^- is the strongest ligand, which causes large crystal
field splitting i.e maximum Δ₀

Molecular formula: MSO₄Cl.6H₂O
Formation of white precipitate with Barium chloride indicates that SO^2-₄ ions are outside the coordination sphere, and no precipitate with AgNO₃ solution indicates that the Cl^- ions are inside the coordination sphere. Since the coordination number of M is 6.Cl^- and 5 H₂O are ligands, remaining 1 H₂O molecular and SO^2-₄ are in the outer coordination sphere.