Question

The IUPAC name of the complex [Co(NH\textsubscript{3})\textsubscript{2}(H\textsubscript{2}O)\textsubscript{4}]Cl\textsubscript{3 is}

• A. Diaminetetraaquacobalt(III) trichloride
• B. Diaminetetraaquacobalt(II) chloride
• C. Diaminetetraaquacobalt(III) chloride
• D. Tetraaquadiaminecobalt(III) trichloride
• E. Tetraaquadiaminecobalt(II) chloride

Hint:

In IUPAC naming, the number of counter ions (the ones outside the bracket) is never indicated by prefixes like 'di' or 'tri'. The name "trichloride" (Options A and D) is an immediate disqualifier!

Answer 1

The Correct Option is C

Concept: IUPAC nomenclature for coordination compounds follows strict rules regarding ligand naming, alphabetical order, and metal oxidation state.
• Ligand Order: Ligands are named in alphabetical order. NH\textsubscript{3} is "ammine" and H\textsubscript{2}O is "aqua".
• Oxidation State: Calculated based on the charge of the counter ions and neutral ligands.
• Counter Ion: Named last, without numerical prefixes (like "tri").

Step 1: Calculate the oxidation state of Cobalt (Co). Ammine (\(NH_3\)) and Aqua (\(H_2O\)) are neutral ligands (charge 0). The three chloride ions (\(Cl_3\)) outside the bracket have a total charge of \(-3\). To balance this, the coordination sphere must be \(+3\). \[ x + 2(0) + 4(0) = +3 \implies x = +3 \] So, Cobalt is in the (III) state.

Step 2: Construct the name. 1. Alphabetical ligands: Ammine comes before Aqua. 2. Count: Two ammines \(\rightarrow\) diamine; four aquas \(\rightarrow\) tetraaqua. 3. Metal: Since the complex is a cation, the name is simply cobalt. 4. Anion: chloride.
(Note: We never say "trichloride" for the counter ion). Combined: Diaminetetraaquacobalt(III) chloride.