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
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.
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) + 2SO42-(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
In coordination compound, metal ion is surrounded by a number of oppositely charged ions or neutral molecules more than its normal valency.