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: Fix the metal oxidation number by charge balance.
For K₃[Cr(C₂O₄)₃] the three potassium ions carry +3, so the whole complex ion is 3-. Each oxalate is 2-, three make 6-, therefore x - 6 = -3, giving Cr = +3.
Step 2: Assemble the anionic name.
Because the complex is negatively charged its metal takes the -ate suffix (chromate). Three oxalato groups give the prefix tri. Reading cation then anion: potassium trioxalatochromate(III). The tris(oxalato) form is also accepted.
Step 3: Charge balance for the cobalt salt.
In [Co(NH₃)₅(CO₃)]Cl one chloride sits outside, so the bracketed ion is +1. Five neutral ammines add nothing and carbonate is -2, so x - 2 = +1, x = +3. Ligands in alphabetical order give pentaammine then carbonato: pentaamminecarbonatocobalt(III) chloride.
Step 4: Build Cr³⁺ by counting electrons.
Neutral Cr has 24 electrons in the anomalous [Ar]3d⁵4s¹ arrangement. Removing three electrons leaves 21. The 4s empties first, then two 3d electrons leave, giving three in the d set: [Ar]3d³.
Step 5: Build Cu.
Copper has 29 electrons. The stability of a complete 3d shell shifts one 4s electron into 3d, so the ground state is [Ar]3d¹⁰4s¹ rather than 3d⁹4s².