Coordination Compounds

Notes

Coordination Compounds

Coordination compounds are compounds in which a central metal atom or ion is surrounded by a number of ions or molecules called ligands. The ligands are attached to the central metal atom or ion through coordinate bonds.

1. Addition Compounds

When aqueous solutions of two salts are mixed together and crystallisation is carried out, an addition compound may be formed.

Addition compounds are mainly of two types:

• Double salts
• Complex salts

2. Double Salts

Double salts are addition compounds which completely dissociate into their constituent ions in aqueous solution.

Examples:

• Potash alum – K₂SO₄·Al₂(SO₄)₃·24H₂O
• Mohr's salt – FeSO₄·(NH₄)₂SO₄·6H₂O

Double salts lose their identity in aqueous solution because they completely dissociate into constituent ions.

3. Complex Salts

Complex salts contain a complex ion which retains its identity in aqueous solution and does not dissociate completely into simple ions.

Example: K₄[Fe(CN)₆]

In this compound, [Fe(CN)₆]⁴⁻ exists as a complex ion.

4. Important Terms Used in Coordination Compounds

Complex Ion

A complex ion is formed by the combination of a central metal atom or ion with ligands.

Central Metal Atom or Ion

The metal atom or ion present at the centre of a coordination entity is called the central metal atom or ion.

Oxidation Number

The oxidation number represents the oxidation state or charge assigned to the central metal atom or ion in a coordination compound.

Coordination Number

The coordination number of a central metal atom or ion is the number of ligand donor atoms directly bonded to it.

Ionisation Sphere

The ions present outside the square bracket constitute the ionisation sphere. These ions can ionise in aqueous solution.

Coordination Sphere

The central metal atom or ion and the ligands enclosed within square brackets constitute the coordination sphere.

Example: [Co(NH₃)₆]Cl₃

• [Co(NH₃)₆]³⁺ is the coordination sphere.
• Cl⁻ ions are present in the ionisation sphere.

Ligand

An ion or molecule which can donate one or more lone pairs of electrons to the central metal atom or ion is called a ligand.