Question

The IUPAC name of the complex shown below is \( \text{K}_3[\text{Co}(\text{ox})_3] \):

• A. Trispotassium trioxalatocobaltate (III)
• B. Potassium trioxalate cobaltate(III)
• C. Potassium trioxalatecobalt(III)
• D. Potassium trioxalatocobaltate(III)

Hint:

To eliminate options in coordination nomenclature questions instantly, always check the suffix of the metal! If the complex is preceded by an active metal counter-ion (like \(\text{K}\), \(\text{Na}\)), the complex sphere is guaranteed to be an anion, meaning the metal name must end in -ate!

Answer 1

The Correct Option is D

Concept: According to the IUPAC nomenclature rules for coordination compounds:
• Order of Naming: The cation is named first, followed by the anion, regardless of which one is complex. No numerical prefixes (like di, tri) are used for simple counter-ions outside the coordination sphere.
• Naming Coordination Sphere: Ligands are named first in alphabetical order, followed by the central metal atom.
• Prefixes for Ligands: For simple anionic ligands like oxalate (\(\text{ox}^{2-}\)), numerical prefixes like 'tri-' are used to denote quantity. The suffix of the anionic ligand changes from '-ate' to '-ato'. Thus, oxalate becomes trioxalato.
• Name of Central Metal: If the coordination complex ion is an anion (negatively charged sphere), the name of the central metal atom ends with the suffix -ate. Cobalt becomes cobaltate.
• Oxidation State: The oxidation state of the central metal atom is calculated and indicated by a Roman numeral in parentheses directly after the metal name without any spaces.

Step 1: Determining the charge of the coordination sphere and the oxidation state of Cobalt.
The formula of the complex is \( \text{K}_3[\text{Co}(\text{ox})_3] \). When dissociated in water, it yields: \[ \text{K}_3[\text{Co}(\text{ox})_3] \rightarrow 3\text{K}^+ + [\text{Co}(\text{ox})_3]^{3-} \] The coordination sphere carries a net charge of \(-3\), making it an anionic complex. Oxalate (\(\text{ox}\)) is a bidentate ligand with a charge of \(-2\). Let the oxidation state of Cobalt (\(\text{Co}\)) be \(x\): \[ x + 3(-2) = -3 \] \[ x - 6 = -3 \quad \Rightarrow \quad x = +3 \] Thus, the oxidation state of cobalt is \(\text{III}\).

Step 2: Assembling the complete IUPAC name according to the structural rules.

• Cation: \(\text{K}^+\) is named simply as Potassium (no prefix 'tri').
• Anionic Sphere: Three oxalate ligands give trioxalato. Since the complex is anionic, cobalt is named as cobaltate. Adding the oxidation state gives cobaltate(III). Combining the cation and anion names with a space separating them gives: \[ \text{Potassium trioxalatocobaltate(III)} \] This corresponds precisely to option (D).