Question

(i) Write the IUPAC name of the following coordination compounds: (a) K₃[Cr(C₂O₄)₃] (b) [Co(NH₃)₅(CO₃)]Cl. (ii) Write the electronic configuration of the following: (a) Cr³⁺ (b) Cu.

Hint:

Name cation before anion and use -ate for an anionic complex; recall the anomalous d-shell stabilities, so Cr³⁺ is [Ar]3d³ and Cu is [Ar]3d¹⁰4s¹.

Answer 1

Step 1: Rules used.
The cation is named first, then the anion. Inside the complex, ligands are named in alphabetical order before the metal, negative ligands take an -o ending, and the metal oxidation state is shown by a Roman numeral. If the complex ion is an anion, the metal name ends in -ate.
Step 2: (a) K₃[Cr(C₂O₄)₃].
Each oxalate ligand C₂O₄ carries a 2- charge and there are three, total 6-. Three K⁺ give 3+, so chromium must be +3. The complex ion is an anion, so we use chromate. Name: potassium trioxalatochromate(III).
Step 3: (b) [Co(NH₃)₅(CO₃)]Cl.
NH₃ is neutral (ammine) and carbonate CO₃ is 2-. The outer Cl is 1-, so the complex ion is 1+. Hence cobalt is +3. Ammine comes before carbonato alphabetically. Name: pentaamminecarbonatocobalt(III) chloride.
Step 4: (a) Electronic configuration of Cr³⁺.
Chromium (Z = 24) is [Ar]3d⁵4s¹. To make Cr³⁺ remove three electrons, first the 4s electron then two 3d electrons: Cr³⁺ = [Ar]3d³.
Step 5: (b) Electronic configuration of Cu.
Copper (Z = 29). A fully filled 3d shell gives extra stability, so instead of [Ar]3d⁹4s² the actual configuration is Cu = [Ar]3d¹⁰4s¹.