(i) Coordination entity : A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl3(NH3)3] is a coordination entity in which the cobalt ion is surrounded by three ammoniamoleculesand three chloride ions. Other examples are [Ni(CO)4],[PtCl2(NH3)2], [Fe(CN)6]4–,[Co(NH3)6]3+ .
(ii) Ligand : The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These maybe simple ionssuch as Cl– , small molecules such as H2O ‌or‌ ‌N‌H3, larger molecules such as H2NCH2CH2NH2 or N(CH2CH2NH2)3 or even macromolecules, such as proteins.
(iii)Coordination number : The coordination number [C.N.]of a metalion in a complex can be defined as the number of ligand or donor atomsto which the metal is directly bonded. For example, in the complex ions,[PtCl6]2– and [Ni(NH3)4]2+, the coordination number of Pt and Ni are 6 and4 respectively.Similarly, in the complex ions, [Fe(C2O4)3]3– and [Co(en)3]3+,the coordination number of both, Fe and Co, is 6 because C2O42– and en(ethane-1,2-diamine) are bidentate ligands.
(iv) Coordination polyhedron : The spatialarrangement of theligand atoms which are directly attached to the central atom/iondefines a coordination polyhedron about the central atom. The mostcommon coordination polyhedra are octahedral, square planar andtetrahedral. For example, [Co(NH3)6]3+is octahedral, [Ni(CO)4] istetrahedral and [PtCl4]2– is square planar.
(v) Homoleptic and heteroleptic complexe : Complexes in which a metalis bound to only one kind of donor groups, e.g., [Co(NH3)6]3+, are knownas homoleptic. Complexesin which a metal is bound to more than onekind of donor groups, e.g., [Co(NH3)4Cl2]+, are known as heteroleptic.