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

Coordination entity : A coordination entity constitutes a central atom/ion, usually of a metal, to which are attached a fixed number of other atoms or groups each of which is called a ligand. It may be neutral or charged.
Examples: [Co(NH₃)₆]³⁺, [PtCl₄]^2–, [Fe(CN)₆]^3–, [NiCl₂(OH₂)₄]

Ligand: The groups attached to the central metal ion (or atom) in a complex are called ligands. The ligands may be anions like CN^–, C^–, C₂O₄^2– ion neutral molecules like H₂O, NH₃, CO. Irrespective of their nature all types of ligands have lone pair of electrons.

Coordination number: Total number of ligand atoms which are bound to a given metal ion is called its coordination number. Coordination number of a metal ion is also equal to the total number of coordinate bonds present in a complex.
Coordinations polyhedron: The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and tetrahedral. For example, [Co(NH₃)₆]³⁺ is octahedral, [Ni(Co)₄] is tetrahedral and [PtCl₄]^2– is square planar.


Homoleptic: Complexes in which a metal is bound to only one kind of donor groups, e.g., [Co(NH₃)₆]³⁺, are known as homoleptic.

Heteroleptic: Complexes in which a metal is bound to more than one kind of donor groups, e.g., [Co(NH₃)₄Cl₂]⁺, are known as heteroleptic.