Question

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

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 (ox) or ethylenediamine (en) form two bonds per ligand! center minipage0.4 Coordination Geometry Summary tabular|c|c| Coordination No. & Geometry
2 & Linear
4 & Tetrahedral / Sq. Planar
6 & Octahedral
tabular minipage center

Answer 1

Step 1: Read the question. 
We must find the coordination number of cobalt in $[\text{Co(NH}_3)_6]^{3+}$. The choices in the text are 6, 4, 3 and 2.
Step 2: What is coordination number. 
The coordination number is the count of ligand atoms that bond straight to the central metal. So we look at the ligands around cobalt.
Step 3: Identify the ligand. 
The ligand here is ammonia $\text{NH}_3$. Each ammonia gives one nitrogen atom to bond with cobalt, so it is monodentate, meaning one bond each.
Step 4: Count the ligands. 
The formula shows six $\text{NH}_3$ groups around the cobalt.
Step 5: Find the total bonds. 
Each ammonia makes one bond, and there are six of them, so\[ 6 \times 1 = 6 \text{ bonds} \]
Step 6: State the answer. 
So the coordination number of cobalt is \[ \boxed{6} \]