Coordination Compounds - Ligands and coordination number

Ligands are neutral molecules, anions, or cations which can donate a pair of electrons to the central metal atom or ion to form a coordinate covalent bond. Common examples include $H_2O$ (aqua), $NH_3$ (ammine), and $Cl^-$ (chlorido).

Ligands are classified by denticity: Unidentate ligands (one donor atom, e.g., $F^-$), Didentate/Bidentate ligands (two donor atoms, e.g., $C_2O_4^{2-}$ or ethane-1,2-diamine represented as $en$), and Polydentate ligands (multiple donor atoms, e.g., $EDTA^{4-}$ which is hexadentate).

Ambidentate ligands are unidentate ligands that contain more than one coordination site but bind through only one atom at a time. Examples include $NO_2^-$ (can bind via $N$ or $O$) and $SCN^-$ (can bind via $S$ or $N$).

Chelating Ligands are multidentate ligands that bind to a single metal ion through two or more donor atoms, forming a ring structure. This 'Chelate Effect' significantly increases the stability of the complex.

The Coordination Number ($C.N.$) of a metal ion in a complex is defined as the total number of ligand donor atoms to which the metal is directly bonded. It is not necessarily equal to the number of ligands if polydentate ligands are present.

The Oxidation State of the central metal is the charge it would carry if all the ligands were removed along with the electron pairs that are shared with the central atom.