Question

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

Hint:

Coordination number equals the total number of donor bonds, not the number of ligand molecules -- for a monodentate ligand like $\text{NH}_3$ each one contributes exactly one bond, but a bidentate ligand such as ethylenediamine (en) or oxalate contributes two, so always multiply ligand count by denticity.

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 \[ \boxed{6} \]