Ligands and Their Classification by Denticity

A ligand is any ion or neutral molecule that possesses at least one lone pair of electrons on a donor atom and is capable of donating that lone pair to a central metal atom or ion to form a coordinate bond. Common donor atoms include nitrogen (as in $\text{NH}_3$), oxygen (as in $\text{H}_2\text{O}$), and the halogens (as in $\text{Cl}^-$). Ligands are classified according to their denticity — the number of donor atoms through which a single ligand molecule or ion binds simultaneously to one metal centre.

A monodentate ligand uses only one donor atom to bind the metal. Simple examples include $\text{NH}_3$ (one N donor), $\text{H}_2\text{O}$ (one O donor), $\text{Cl}^-$, $\text{Br}^-$, $\text{CN}^-$ (one C donor), and $\text{CO}$ (one C donor). Even though $\text{NH}_3$ has only one lone pair available for donation, a single $\text{Co}^{3+}$ ion can still be surrounded by six separate $\text{NH}_3$ molecules, as in $[\text{Co}(\text{NH}_3)_6]^{3+}$ — six different monodentate ligand molecules, each donating once, are what fills the six coordination positions.

A bidentate ligand has two donor atoms, both of which coordinate to the same metal ion at once, forming a ring called a chelate ring. Ethylenediamine, $\text{H}_2\text{N}\text{-}\text{CH}_2\text{-}\text{CH}_2\text{-}\text{NH}_2$ (abbreviated en), is the classic example: both nitrogen atoms bind the same metal, closing a five-membered ring (metal + N + C + C + N). The oxalate ion, $\text{C}_2\text{O}_4^{2-}$, is another common bidentate ligand, binding through two of its oxygen atoms.

A polydentate (or multidentate) ligand has three or more donor atoms available to bind one metal centre simultaneously. The most important example is EDTA$^{4-}$ (ethylenediaminetetraacetate), which is hexadentate — it offers six donor atoms at once: the two nitrogen atoms of its central ethylenediamine backbone, plus the four oxygen atoms of its four carboxylate ($-\text{COO}^-$) arms. A single EDTA$^{4-}$ ion can therefore completely occupy all six coordination positions of an octahedral metal ion like $\text{Ca}^{2+}$ or $\text{Pb}^{2+}$ by itself, wrapping around it like a claw — which is exactly why EDTA is used both to measure water hardness and to treat heavy-metal poisoning (Section 5.18). …