Question

What is the coordination number of central metal ion in \([\text{Co(en)}_3]^{3+}\) complex ?

• A. \(2\)
• B. \(4\)
• C. \(6\)
• D. \(8\)

Hint:

Each en is bidentate and donates two pairs, so 3 x 2 = 6.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Coordination number is the number of donor atoms directly bonded to the central metal ion, not the number of ligand molecules.

Step 2: Key Point:
Ethylenediamine (en), \(\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2\), is a bidentate ligand. It binds the metal through two nitrogen atoms.

Step 3: Detailed Explanation:
In \([\text{Co(en)}_3]^{3+}\) there are 3 en ligands, each attached through 2 donor atoms:
\[ \text{Coordination number} = 3 \times 2 = 6 \]
The geometry is octahedral.

Step 4: Why the other options are wrong.
The value 2 or 4 would apply to a complex with fewer donor atoms, such as \([\text{Ag(NH}_3)_2]^+\) or \([\text{Ni(CN)}_4]^{2-}\). The value 8 is not found here since only six donor atoms are present.

Final Answer:
The coordination number is 6, option (C). \[ \boxed{6} \]