Define the following terms:
$1$. Coordination entity
$2$. Central metal ion

Coordination entity: $A$ coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example,$[CoCl_{3}(NH_{3})_{3}]$ is a coordination entity in which the cobalt is surrounded by three ammonia and three chloride ions. Other examples are $[Ni(CO)_{4}]$,$[PtCl_{2}(NH_{3})_{2}]$,$[Fe(CN)_{6}]^{4-}$,and $[Co(NH_{3})_{6}]^{3+}$.
Central metal ion/atom: In a coordination entity,the atom or ion to which a fixed number of ions or groups are bound in a definite geometrical arrangement is called the central metal ion or atom. For example,in the coordination entities $[NiCl_{2}(H_{2}O)_{4}]$,$[CoCl(NH_{3})_{5}]^{2+}$,and $[Fe(CN)_{6}]^{3-}$,the central ions are $Ni^{2+}$,$Co^{3+}$,and $Fe^{3+}$ respectively. These central ions or atoms act as electron pair acceptors and are also referred to as Lewis acids.