Q.From the ligands $\text{Cl}^-$, $\text{NH}_3$, ethylenediamine ($\text{en}$), and EDTA$^{4-}$, identify which ones are chelating ligands. Explain briefly why chelate complexes such as $[\text{Ni}(\text{en})_3]^{2+}$ are generally more thermodynamically stable than the corresponding complex with an equal number of monodentate ligands, such as $[\text{Ni}(\text{NH}_3)_6]^{2+}$.

False. Oxalato ($C_2O_4^{2-}$) is a bidentate ligand, donating through two oxygen atoms.

The oxalate ion, $^{-}OOC-COO^{-}$, has two carboxylate groups. It coordinates to a central metal ion through one oxygen atom from each of its two carboxylate groups, forming a five-membered chelate ring. Since it uses exactly two donor atoms to bind the metal, it is classified as a bidentate ligand, not tridentate.

True — ethylenediamine (en), $H_2N-CH_2-CH_2-NH_2$, has two donor nitrogen atoms and is a classic bidentate ligand.

Ethylenediamine has a lone pair of electrons on each of its two terminal $-NH_2$ nitrogen atoms. Both nitrogens can simultaneously coordinate to the same metal centre, forming a stable five-membered chelate ring (e.g. in $[Co(en)_3]^{3+}$). Because it donates through exactly two atoms, it is a textbook example of a bidentate chelating ligand.

$NH_4^+$ cannot act as a ligand because its nitrogen has no lone pair left to donate — all four bonds and the lone pair are already used in bonding to hydrogens.

A ligand must have at least one atom with a lone pair of electrons available to donate to the central metal ion, forming a coordinate (dative) bond. In $NH_4^+$, nitrogen's lone pair has already been used to bond the fourth hydrogen, giving nitrogen a formal positive charge and no remaining lone pair — so it cannot coordinate to a metal. In contrast, $NO$ (via its N or O lone pair), $NH_2CH_2CH_2NH_2$ (ethylenediamine, two N lone pairs — bidentate) and $CO$ (lone pair on C) all have available lone pairs and are well-known ligands.

A monodentate ligand attaches to the central metal ion through only one donor atom; among the options, only methanamine (CH3NH2) has just one donor site.

• Methanamine (CH3NH2): has a single lone pair on nitrogen available to donate to the metal — one donor atom, so it is monodentate.

• Ethylenediamine (H2N-CH2-CH2-NH2): has two nitrogen donor atoms that can simultaneously bind the same metal ion, forming a five-membered chelate ring — it is bidentate.

• EDTA (ethylenediaminetetraacetate): has two N atoms and four carboxylate O atoms available to bind — it is hexadentate, forming very stable chelate complexes.

• Glycinate ion (H2N-CH2-COO-): has both an amino N and a carboxylate O that can bind the metal — it is bidentate.

Denticity = number of donor atoms that bind the metal. Ethylene diamine binds through two nitrogen atoms → bidentate.

• Ethylene diamine (en), H2N-CH2-CH2-NH2, has two donor N atoms → bidentate.
• EDTA has 2 N + 4 O donor sites → hexadentate.
• Acetate ion (CH3COO-) normally binds through one O → monodentate.
• Pyridine binds through one N → monodentate.

Hence the bidentate ligand is ethylene diamine.

Ligands donate an electron pair to the metal, so they are Lewis bases.

A ligand is an ion or molecule that binds to a central metal atom/ion by donating one or more lone pairs of electrons into empty metal orbitals, forming a coordinate (dative) bond.

By definition an electron-pair donor is a Lewis base and an electron-pair acceptor (the metal) is a Lewis acid. Since every ligand donates its lone pair, all ligands are Lewis bases.