Define the following terms:
$1$. Ligands
$2$. Coordination number

(N/A) Ligands: The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These may be simple ions such as $Cl^{-}$,small molecules such as $H_{2}O$ or $NH_{3}$,larger molecules such as $H_{2}NCH_{2}CH_{2}NH_{2}$ or $N(CH_{2}CH_{2}NH_{2})_{3}$,or even macromolecules such as proteins. Ligands are electron pair donors and are therefore referred to as Lewis bases.
Coordination number $(CN)$: The coordination number of a metal ion in a complex is defined as the number of ligand donor atoms to which the metal is directly bonded. The coordination number depends on the denticity of the ligands. For example,in the complexes $[PtCl_{6}]^{2-}$ and $[Ni(NH_{3})_{4}]^{2+}$,the coordination numbers of $Pt$ and $Ni$ are $6$ and $4$,respectively. Similarly,in the complex ions $[Fe(C_{2}O_{4})_{3}]^{3-}$ and $[Co(en)_{3}]^{3+}$,the coordination number of both $Fe$ and $Co$ is $6$ because $C_{2}O_{4}^{2-}$ and $en$ (ethane-$1,2$-diamine) are bidentate ligands. The coordination number is determined only by the number of sigma bonds formed between the ligand donor atoms and the central metal ion.