(i) Coordination entity : A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, $\mathrm{[CoCl_3(NH_3)_3]}$ is a coordination entity in which the cobalt ion is surrounded by three ammoniamoleculesand three chloride ions. Other examples are $\mathrm{[Ni(CO)_4]}, \mathrm{[PtCl_2(NH_3)_2]},$ $\mathrm{[Fe(CN)_6]^{4-}}, \mathrm{[Co(NH_3)_6]^{3+}}$ . (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 $\mathrm{H_2O} \text{ or } \mathrm{NH_3}$, larger molecules such as $\mathrm{H_2NCH_2CH_2NH_2}$ or $\mathrm{N(CH_2CH_2NH_2)_3}$ 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,$\mathrm{[PtCl_6]^{2-}}$ and $\mathrm{[Ni(NH_3)_4]}$2+, the coordination number of Pt and Ni are 6 and4 respectively.Similarly, in the complex ions, $\mathrm{[Fe(C_2O_4)_3]^{3-}}$ and $\mathrm{[Co(en)_3]^{3+}},$the coordination number of both, Fe and Co, is 6 because $\mathrm{C_2O_4^{2-}}$ 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, $\mathrm{[Co(NH_3)_6]^{3+}}$is octahedral, $\mathrm{[Ni(CO)_4]}$ istetrahedral and $\mathrm{[PtCl_4]^{2-}}$ is square planar. (v) Homoleptic and heteroleptic complexe : Complexes in which a metalis bound to only one kind of donor groups, e.g., $\mathrm{[Co(NH_3)_6]^{3+}}$, are knownas homoleptic. Complexesin which a metal is bound to more than onekind of donor groups, e.g., $\mathrm{[Co(NH_3)_4Cl_2]^{+}}$, are known as heteroleptic.