Question

Write the IUPAC name of the following complex : \[ K_3[Cr(C_2O_4)_3] \]

Hint:

Important naming rules for coordination compounds:

• Negative complex ions use metal names ending in ``-ate''.

• Oxalate ligand is named as ``oxalato''.

• For complex ligand names, use prefixes such as: bis, tris, tetrakis instead of di, tri, tetra

• Oxidation state of the metal is always written in Roman numerals.

Answer 1

Step 1: Identify the outer ions and the complex ion.
$K_3[Cr(C_2O_4)_3]$ gives $3K^+$ as the outer cation and $[Cr(C_2O_4)_3]^{3-}$ as the complex anion (three $K^+$ charges balance the $-3$ charge on the complex).
Step 2: Name the ligand and find the oxidation state of Cr.
The ligand $C_2O_4^{2-}$ is oxalato. Three such ligands need the prefix tris (used for polysyllabic names). Oxidation state of Cr: $x + 3(-2) = -3 \Rightarrow x = +3$.
Step 3: Build the complete IUPAC name.
Since the complex ion is anionic, the metal takes the suffix -ate: chromate(III). Naming the outer cation first, then the complex: Potassium tris(oxalato)chromate(III).