Question

According to IUPAC coordination nomenclature rules, what is the correct official name for the coordination compound represented by \( [\text{Co}(\text{NH}_3)_5(\text{Cl})]\text{Cl_2 \)?}

• A. Chloridopentaamminecobaltic chloride
• B. Pentaaminechloridocobalt(II) dichloride
• C. Pentaaminechloridocobalt(III) chloride
• D. Pentaamminedichloridocobalt(III) chloride

Hint:

Pay close attention to spelling details during the exam: the coordinated neutral ammonia ligand must always be spelled with a double 'm' (ammine), whereas organic amine compounds use a single 'm' (amine).

Answer 1

The Correct Option is C

Concept: Naming coordination compounds requires applying systematic IUPAC guidelines:

• Name the ligands inside the coordination bracket first, in alphabetical order, before naming the central metal ion.
• Stated numerical prefixes (di, tri, tetra, penta) do not alter the alphabetical priority order.
• Specify the oxidation state of the central metal using Roman numerals in parentheses immediately after its name.

Step 1: Identify and alphabetize the ligands inside the coordination sphere.
The complex bracket contains two distinct types of ligands:

• Five neutral ammonia molecules, named as ammine.
• One anionic chloride ion, named as chloridocon or chlorido.

Arranging them alphabetically places "ammine" (starts with 'a') before "chlorido" (starts with 'c'), which gives: pentaamminechlorido.

Step 2: Calculate the oxidation state of the central Cobalt metal.
Let the unknown oxidation state of Cobalt be \( x \). The neutral ammine molecules carry a charge of 0, while each chloride ion carries a charge of \(-1\): \[ x + 5(0) + 1(-1) + 2(-1) = 0 \quad \Rightarrow \quad x - 1 - 2 = 0 \quad \Rightarrow \quad x = +3 \] The metal is named as cobalt(III) because the complex ion carries a net positive charge.

Step 3: Combine the components into a single name.
Combine the alphabetized ligands, the metal with its oxidation state, and finish with the counter-anion name outside the bracket: \[ \text{Pentaamminechloridocobalt(III) chloride} \]