Q.Which of the following species is not expected to be ligand?

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 cla …

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 exa …

$NH_4^+$ 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.

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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. …

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. …

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.

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