Question

Explain with two examples each of the following: coordination entity, ligand, coordination number, coordination polyhedron, homoleptic and heteroleptic complexes;

Answer 1

(i) Coordination entity:

A coordination entity is an electrically charged radical or species carrying a positive or negative charge. In a coordination entity, the central atom or ion is surrounded by a suitable number of neutral molecules or negative ion (called, ligands).

For examples:

[Ni(NH₃)₆]²⁺, [Fe(CN)₆]⁴⁺ = cationic complex
[PtCl₄]^2-, [Ag(CN)₂]^- – anionic complex
[Ni (CO)₄], [CO (NH₃)₄Cl₂] = neutral complex

(ii) ligand:

The neutral molecules or negatively charged ions that surround the metal atom in a coordination entity or a coordinal complex are known as ligands.
For example

Cl^-, OH^-. Ligands are usually polar in nature and possess at least one unshared pair of valence electrons.

(iii) Coordination number:

The total number of ligands (either neutral molecules or negative ions) that get attached to the central metal atom in the coordination sphere is called the coordination number of the central metal atom. It is also referred to as its ligaricy.
For example:

(a) In the complex K₂[PtCl₆], there are six chloride ions attached to Pt in the coordinate sphere. Therefore, the coordination number of Pt is 6.

(b) similarly, in the complex [Ni (NH₃)₄Cl₂], the cooordination number of the central atom (Ni) is 6.

(iv) Coordination polyhedron:

Coordination polyhedrons, about the central atom can be defined as the spatial arrangement of the ligands that are directly attached to the central metal ion in the coordination sphere.
For example:
(a) Square planar

(b) Square planar

(v) Homoleptic complexes:

These are those complexes in which the metal ion is bound to only one kind of a donor group For e.g.:
[CO (NH₃)₆]³⁺
[pt Cl₄]²

(vi) Heteroleptic complexes:

Heteroleptic complexes are those complexes where the central metal is bound to more than one type of a donor group.
For e.g.:
[CO(NH₃)₄C₂]⁺
[CO(NH₃)₅ Cl]²⁺