Question

What is IUPAC name of $[\text{Co}(\text{H}_2\text{O})(\text{NH}_3)_5]\text{I}_3$ ?

• A. Pentaammineaquacobalt (III) iodide
• B. Pentaammineaquocobalt iodide
• C. Monoaquapentaammine triiodo cobaltate
• D. Pentaammineaquatriiodo cobaltate

Hint:

Be very careful with alphabetical ordering for ligands: "ammine" (a-m) comes strictly before "aqua" (a-q). Also note that the prefix "penta-" does not affect the alphabetical ordering!

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question requires us to determine the correct systematic IUPAC name for the coordination complex compound $[\text{Co}(\text{H}_2\text{O})(\text{NH}_3)_5]\text{I}_3$.

Step 2: Key Formula or Approach:
According to systematic IUPAC coordination chemistry naming rules:
1. Name the cation part first, followed by the anion. Here, the coordination sphere is the cation.
2. Inside the coordination sphere, name ligands in alphabetical order regardless of charge prefixes.
3. Neutral ligand $\text{NH}_3$ is named "ammine" and $\text{H}_2\text{O}$ is named "aqua".
4. State the oxidation number of the central metal atom using Roman numerals in parentheses immediately after its name.

Step 3: Detailed Explanation:
Let's find the oxidation state of the central Cobalt atom ($x$):
$\text{NH}_3$ and $\text{H}_2\text{O}$ are neutral ligands with a charge of 0.
Each iodide counter-ion carries a charge of $-1$, meaning three iodides contribute a total charge of $-3$.
$$x + 5(0) + 1(0) + 3(-1) = 0 \implies x = +3$$ The metal name is written as "cobalt(III)" because the complex carries a net positive charge.
Now let's order the ligands alphabetically:
Five $\text{NH}_3$ groups $\rightarrow$ "pentaammine" (starts with 'am')
One $\text{H}_2\text{O}$ group $\rightarrow$ "aqua" (starts with 'aq')
Since "ammine" comes before "aqua" alphabetically, we arrange them as:

pentaammineaquacobalt(III).
Finally, append the name of the outer counter-anion:

iodide. Combining these components gives "Pentaammineaquacobalt (III) iodide", matching option (A).

Step 4: Final Answer:
The correct IUPAC name is Pentaammineaquacobalt (III) iodide, which corresponds to option (A).