Question

(i) What is meant by ambidentate ligand? Give examples also. (2)
(ii) Write the IUPAC names of following compounds:
a) [Co(Br)₂(en)₂]⁺
b) K₃[Fe(C₂O₄)₃] (1+1)

Hint:

Ambidentate = one ligand, two possible donor atoms (e.g. NO₂^-, SCN^-). For naming, work out the metal oxidation state from the overall charge, list ligands alphabetically, and add -ate to the metal only when the complex is an anion.

Answer 1

Part (i): Ambidentate ligand
Step 1 (Definition): A ligand that has two (or more) different donor atoms and can coordinate to the central metal atom through either one of them (but only one at a time) is called an ambidentate ligand.
Step 2 (Examples):
• Nitrite ion, NO₂^-: bonds through nitrogen (nitrito-N, written -NO₂) or through oxygen (nitrito-O, written -ONO).
• Thiocyanate ion, SCN^-: bonds through sulphur (thiocyanato-S, -SCN) or through nitrogen (isothiocyanato, -NCS).
• Cyanide ion, CN^-: bonds through carbon (cyano, -CN) or through nitrogen (isocyano, -NC).

Part (ii): IUPAC names
a) [Co(Br)₂(en)₂]⁺
Step 1 (Oxidation state): en (ethane-1,2-diamine) is neutral; each Br is -1, so 2 Br give -2. Overall charge is +1, therefore \( x - 2 = +1 \Rightarrow x = +3 \); Co is in the +3 state.
Step 2 (Naming rules): Ligands are named alphabetically: bromido (b) before ethane-1,2-diamine (e). The prefix bis is used for the chelating en to avoid confusion with di.
Name: Dibromidobis(ethane-1,2-diamine)cobalt(III) ion.

b) K₃[Fe(C₂O₄)₃]
Step 1 (Oxidation state): Three K⁺ give +3, so the complex ion carries -3. Each oxalate C₂O₄^2- is -2, and three give -6. Thus \( x - 6 = -3 \Rightarrow x = +3 \); Fe is in the +3 state.
Step 2 (Naming rules): It is an anionic complex, so the metal ends in -ate: ferrate. The chelating oxalato ligand takes the prefix tris.
Name: Potassium tris(oxalato)ferrate(III).