Identify the complex ion

The compound is $\mathrm{K}_{3}[\mathrm{Co}(\mathrm{C}_2\mathrm{O}_4)_3]$

Here, the coordination entity is: $[\mathrm{Co}(\mathrm{C}_2\mathrm{O}_4)_3]^{3-}$

Find the oxidation state of cobalt

Let the oxidation state of Co be $x$.

Each oxalato ligand $\left(\mathrm{C}_2\mathrm{O}_4\right)$ has charge $-2$. Since there are 3 oxalato ligands, total ligand charge is: $3 \times (-2) = -6$

The complex ion has charge $-3$, so: $x - 6 = -3$ $x = +3$

Thus, cobalt is in the $+3$ oxidation state.

Name the ligand

The ligand $\mathrm{C}_2\mathrm{O}_4^{2-}$ is called oxalato.

Since there are 3 such ligands, the ligand part becomes: $\text{tris(oxalato)}$

We use tris instead of tri because the ligand name is polydentate/complex and already contains a numerical pattern.

Name the metal

The complex ion is anionic ($-3$ charge), so the metal name ends with -ate.

Therefore, cobalt becomes: $\text{cobaltate}$

Including oxidation state: $\text{cobaltate(III)}$

Name the cation first

The cation is $\mathrm{K}^+$, so the name begins with: $\text{Potassium}$

Complete IUPAC name

Therefore, the correct IUPAC name is: $\boxed{\text{Potassium tris(oxalato)cobaltate(III)}}$

Check options

• A: Potassium tris(oxalato)cobalt(III) ❌
  Incorrect because anionic complex should use cobaltate.

• B: Potassium tris(oxalato)cobaltate(III) ✅
  Correct.

• C: Potassium trioxalatocobaltate(III) ❌
  Incorrect IUPAC style; should be tris(oxalato), not trioxalato.

• D: Potassium trioxalatocobalt(III) ❌
  Incorrect for both reasons above.

Comparison with stored answer

My derived answer is B, but the stored correct answer is C.

According to standard IUPAC nomenclature for coordination compounds, B is correct and C is not the preferred name.