Coordination Compounds

Contents

1 Coordination Compounds
2 Double Salts
3 Differences between Coordination Compounds and Double Salts

Coordination compounds are a special class of compounds in which the central metal atom is surrounded by ions or molecules beyond their normal valency. These are also referred to as complex compounds. These compounds are called Coordination compounds.

Many biologically important compounds are coordination compounds in which complicated organic species are bound to metal ions. The common examples are : haemoglobin which is a coordination compound of iron, chlorophyll which is a coordination compound of magnesium, vitamin B12 which is a coordination compound of cobalt etc.

The coordination compounds contain a central metal atom or ion surrounded by a number of oppositely charged ions or neutral molecules more than its normal valency.

For example: When aqueous ammonia is added to green solution of nickel chloride, NiCl₂, the colour changes to purple. The Ni²⁺ ions almost disappear from the solution. The solution on evaporation, yields purple crystals corresponding to the formula [Ni(NH3)₆]Cl₂. Such a compound is called coordination (or complex) compound. The properties of the complex compound are completely different from those of Ni²⁺ ions or ammonia molecules.

NiCl₂ +6NH₃ → [Ni(NH3)₆]Cl₂
Coordination compound

When the compound is dissolved in water, it ionises to give a new species (Ni(NH₃)₆]²⁺. Such an ion is called complex ion.

[Ni(NH3)₆]Cl₂ → [Ni(NH3)₆]²⁺ + 2Cl^–

At this stage, it may be noted that the species in the square brackets does not ionise. It remains as single entity. It is known as complex entity.

Double Salts

Double salts are addition or molecular compounds which are formed by two apparently saturated compounds but they lose their identity when dissolved in water.

The common double salts are:

Mohr’s salt : FeSO₄. (NH₄)2SO₄.6H₂O

Potash alum : K₂SO₄.Al₂(SO₄)₃.24H₂O

Carnallite : KCl.MgCl₂.6H₂O

For example: Mohr’s salt dissolves in water and gives the characteristic properties of Fe2+, NH4+, and SO42- ions. Thus, double salts are stable in solid state but break up into constituents when dissolved in water.

FeSO₄.(NH₄)₂SO₄ → Fe²⁺ + (aq) + 2NH4⁺(aq) + 2SO₄^2-(aq)

On the other hand, the coordination compounds retain their identities in the solid state as well as when dissolved in water or any other solvent. Their properties are completely different from the constituents (metal and ions or molecules).

For example, [Ni(NH₃)₆]Cl₂ does not show the properties of NiCl₂ or ammonia. Similarly, complex ion such as [Fe(CN)₆]^4- of K₄[Fe(CN)₆] does not dissociate into Fe²⁺ and CN¯ ions.

Differences between Coordination Compounds and Double Salts

The main points of differences between coordination compound and double salt are :

Double Salt	Coordination Compound
Double salts exist only in solid state and dissociate into ions in aqueous solution or in any other solvent.	Coordination compounds exist in the solid state as well as when dissolved in water or any other solvent.
They lose their identity in solution.	They do not completely lose their identity in solution.
The properties of the double salt are essentially the same as those of constituent compounds.	The properties of coordination compound are different from the constituents.
In a double salt, metal ions exhibit their normal valency.	In coordination compound, metal ion is surrounded by a number of oppositely charged ions or neutral molecules more than its normal valency.