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–, C2O42– 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.
What will be the correct order for the wavelength of absorption in the visible region for the following:
[Ni (NO2)6]4–, [Ni(NH3)6]2+, [Ni(H2O)6]2+?
Out of the following two coordination entities which is chiral (optically active)?
(a) cis-[CrCl2(ox)2]3+
(b) [trans-[CrCl2 (ox)2]3–
A solution of [Ni (H2O)6]2+ is green but a solution of [Ni(CN)4]2– is colourless. Explain.
