Question

The coordination number of cobalt in $[\text{Co(NH}_3)_6]^{3+}$ is}

• A. 6
• B. 4
• C. 3
• D. 2

Hint:

Remember that the coordination number is equal to the total number of coordinate bonds formed, not just the number of ligands. For example, bidentate ligands like oxalate ($\text{ox}$) or ethylenediamine ($\text{en}$) form two bonds per ligand! {0.4\textwidth} Coordination Geometry Summary {|c|c|} Coordination No. & Geometry
2 & Linear
4 & Tetrahedral Sq. Planar
6 & Octahedral

Answer 1

The Correct Option is A

Step 1: Concept
The coordination number of a metal ion in a complex refers to the number of ligand donor atoms directly bonded to it. In coordination complexes, ligands are molecules or ions that donate electron pairs to form coordinate covalent bonds with the central metal ion.


Step 2: Meaning
In $[\text{Co(NH}_3)_6]^{3+}$, cobalt ($\text{Co}$) is the central metal ion and ammonia ($\text{NH}_3$) molecules act as the ligands. The superscript $3+$ indicates that the complex ion carries an overall positive charge of three.


Step 3: Analysis
To determine the coordination number, we count how many donor atoms from the ligands are directly bonded to the cobalt ion in the complex $[\text{Co(NH}_3)_6]^{3+}$. The chemical formula shows that there are six $\text{NH}_3$ molecules surrounding the cobalt ion. Ammonia ($\text{NH}_3$) is a monodentate ligand because each molecule contains a single nitrogen donor atom that can form exactly one coordinate covalent bond with the metal center. Since there are six monodentate $\text{NH}_3$ ligands directly bonded to the cobalt ion, the total number of coordinate bonds is: \[6 \times 1 = 6\]


Step 4: Conclusion
Therefore, the coordination number of cobalt in this complex is 6, which correctly matches option A.
Final Answer: (A)