A ligand that has ONLY ONE donor atom (which forms a single bond to the central metal) is called

1. syllabus classifies ligands by the number of donor atoms. 2. MONODENTATE: one donor atom per ligand. Examples: $\mathrm{H_2O}$ (donor O), $\mathrm{NH_3}$ (donor N), $\mathrm{Cl^-}$, $\mathrm{CN^-}$ (donor C usually). 3. BIDENTATE: two donor atoms per ligand. Example: ethylenediamine 'en' ($\mathrm{H_2N{-}CH_2{-}CH_2{-}NH_2}$). 4. POLYDENTATE (option A): more than one donor atom. Includes bi-, tri-, tetra-, etc. EDTA (hexadentate) is the most famous. 5. CHELATING (option D): a special case of polydentate where the ligand wraps around the metal to form a ring. 6. AMBIDENTATE (option B): can bind through EITHER of two donor atoms (e.g. $\mathrm{NO_2^-}$ via N or O), but bonds through only ONE at a time. _Source: syllabus Chemistry Part 1, Ch 5, §5.2 (Ligand classification), p. 2–3._