Question

The coordination number of cobalt in [Co(NH$_3$)$_6$]Cl$_3$ is:

• A. 3
• B. 4
• C. 6
• D. 8

Hint:

Do not confuse coordination number with the oxidation state of the metal.
In $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$, the coordination number is 6, while the oxidation state of cobalt is $+3$.
Only count the ligands inside the square brackets; counter-ions outside the brackets do not contribute to the coordination number.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question is from the topic "Coordination Compounds."
The goal is to determine the coordination number of the cobalt metal ion in the complex compound $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$.

Step 2: Key Formula or Approach:
The coordination number (CN) of a central metal ion is defined as the number of ligand donor atoms to which the metal is directly bonded.
It is calculated as:
\[ \text{Coordination Number} = \text{Number of ligands} \times \text{Denticity of the ligand} \]

Step 3: Detailed Explanation:

• Let us analyze the formula of the complex compound: $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$.

• The square brackets enclose the coordination sphere: $[\text{Co}(\text{NH}_3)_6]^{3+}$.

• The chloride ions ($\text{Cl}^-$) lie outside the coordination sphere as counter-ions and do not form coordinate bonds with the cobalt ion.

• Inside the coordination sphere, we have the Cobalt ion surrounded by ammonia ($\text{NH}_3$) ligands.

• Ammonia ($\text{NH}_3$) is a monodentate ligand because it contains a single donor nitrogen atom with a lone pair of electrons.

• There are exactly 6 ammonia molecules coordinated to the central Cobalt ion.

• Using the formula:
\[ \text{CN} = 6\ (\text{ligands}) \times 1\ (\text{denticity}) = 6 \]

• This coordinate bonding forms an octahedral geometry around the central cobalt ion.



Step 4: Final Answer:
The coordination number of cobalt in $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$ is 6, which is option (C).