Coordination compounds are compounds or complexes that do not follow the conventional ionic or covalent bonding model. These complexes contain metals in neutral and various oxidation states, coordinated with cationic, anionic, and neutrally charged ligands. The bonding pattern between the ligands and the central metal ion is defined as a coordinate covalent bond in which the ligand acts as a source of electrons donated into the vacant d-orbitals of the transition metal complexes. 

Transition metals are mainly involved in the preparation of coordination complexes because of their unique chemical properties, like their ability to show variable oxidation states and the availability of d-orbitals. For example, Hemoglobin is a coordination complex (containing Fe metal, coordinated with the porphyrin ring has nitrogen as the coordinating site, the lone pair on nitrogen is donated to the iron metal), Cisplatin (a drug to cure cancer).

Coordination compounds can be categorised as double salts and coordination complexes. 

Properties of Double salts

1. Double salts are found in solid-state only.
2. Double salts completely ionise into ions and exhibit properties of their constituents like carnallite (KCl.MgCl2.6H2O) will exhibit properties of KCl and MgCl2.
3. Double salts do not show isomerism.
4. Double salts generally contain water of hydration.
5. Mostly double salts are readily soluble in water.
6. Double salts are also used in medicinal chemistry. For example, potash alum, KAl(SO4)3, has antiseptic properties.
7. Double salts do not exhibit intense colours generally.
8. Early transition-metal complexes are also used in the treatment of AIDS patients.

Properties of Coordination Complexes

1. Coordination complexes can exhibit optical activity, i.e. they can rotate the plane of polarised light.
2. Coordination complexes are found in solid, liquid, aqueous as well as in gaseous states.
3. Coordination complexes also contain water of hydration.
4. Coordination complexes exhibit various magnetic properties that use in the analysis methods.
5. Coordination complexes exhibit intense colours used in industrial processes.
6. Coordination complex also has enormous medicinal value like complexes of gold used in rheumatoid arthritis, complexes of rhenium and technetium used in, Platinum and ruthenium are used in anticancer drugs.

Application of the double salts and Coordination Complexes

1. Coordination complexes use in the fabric industries for dyeing the cloths.
2. Coordination complexes are used extensively in the catalysis industry; they are used as homogenous as well as heterogeneous catalysts. 
3. (NH4)Al(SO4)2·12H2O ammonium alum is a double salt used in water purification.
4. KCr(SO4)2·12(H2O), chrome alum is a double salt of chromium and potassium. It is used in the leather tanning industry and also used in the photography industry.
5. Calcium ammonium nitrate, also called CAN, calcium ammonium nitrate, is mostly used as fertiliser for plants, and it’s also used for instant freeze packs as a substitute for ammonium nitrate.
6. Vitamin B12 contains cobalt metal in a +3 oxidation state. It is the first organometallic coordination complex found in nature.
7. Vitamin B12 consists of a tetrapyrrole porphyrin ring complex with a central Co+3 ion, and its coordination number is 6.
8. Cisplatin is used in cancer treatment.
9. Coordination complex Ni(CO)4 is used in the extraction of Ni in high purity in Mond’s Process.

Conclusion 

Double salts are simply two more ionic compounds that are crystallised together as one crystal but dissociate completely into separate ions on dissolution in water. Coordination complexes involve coordinate covalent bonds between the central metal ion and ligands (electron-pair donors) and involve a coordinate sphere that does not dissociate completely into ions like conventional ionic compounds. Coordination complexes find a variety of applications in medicine and industry.