Questions: Coordination Compounds and Nomenclature

Question 1

What is the correct IUPAC name for [Co(NH₃)₅Cl]Cl₂?

• A. Cobalt pentaammine chloride dichloride
• B. Pentaamminechloridocobalt(III) chloride
• C. Chloropentaamminecobalt(II) dichloride
• D. Pentaamminecobalt(III) trichloride

Answer: (B)

The IUPAC rules require ligands listed alphabetically (ammine before chlorido), then the metal with oxidation state in Roman numerals. Inside the bracket: five NH₃ (pentaammine) and one Cl⁻ (chlorido) coordinated to Co. The cobalt oxidation state is +3 (the complex ion has charge +2 from Co, offset by one Cl⁻ inside, and two Cl⁻ outside balance the +2 ion charge: total Co charge must satisfy x + 0×5 + (−1) = +2, so x = +3). The two Cl⁻ outside the bracket are the counter ions, named as 'chloride.' Option A uses incorrect ligand ordering and format. Option C has the wrong oxidation state. Option D ignores the inner-sphere chloride.

Question 2

In IUPAC nomenclature for coordination compounds, anionic ligands receive the suffix '-ido' (or '-o') while neutral ligands keep their usual names, with specific exceptions like aqua (H₂O), ammine (NH₃), carbonyl (CO), and nitrosyl (NO).

• T. True
• F. False

Answer: True

This is a core naming convention. Anionic ligands like Cl⁻ become chlorido, CN⁻ becomes cyanido, and OH⁻ becomes hydroxido. Neutral ligands generally use their molecular name, but four have special names by convention: water is aqua, ammonia is ammine (with double-m to distinguish from the organic amine), carbon monoxide is carbonyl, and nitric oxide is nitrosyl. These exceptions must be memorized because they appear constantly in coordination chemistry.

Question 3

When naming the complex ion [Fe(CN)₆]⁴⁻, the metal is given the suffix '-ate' because it is in an anionic complex.

• T. True
• F. False

Answer: True

When the overall complex ion carries a negative charge, the metal name receives the '-ate' suffix. Iron becomes ferrate (using the Latin root), copper becomes cuprate, and so on. So [Fe(CN)₆]⁴⁻ is hexacyanidoferrate(II). The oxidation state of iron is +2 because six CN⁻ ligands contribute −6 total, and the overall charge is −4: x + (−6) = −4, so x = +2. If the complex were cationic or neutral, the normal metal name would be used without the '-ate' ending.

Question 4

The compound K₃[Fe(C₂O₄)₃] contains three bidentate oxalate ligands. Explain how the prefixes differ when naming multiple monodentate versus polydentate ligands, and give the correct IUPAC name for this compound.

Model answer: For monodentate ligands, Greek prefixes (di-, tri-, tetra-, penta-, hexa-) indicate the number. For polydentate ligands whose names already contain Greek prefixes or are complex, multiplicative prefixes (bis-, tris-, tetrakis-) are used, with the ligand name in parentheses. Oxalate (C₂O₄²⁻) is a polydentate ligand, so three of them uses 'tris' rather than 'tri.' The correct name is potassium tris(oxalato)ferrate(III). Iron's oxidation state: 3(−2) from oxalate + x = −3 overall charge balanced by 3 K⁺, so x = +3.

The bis/tris/tetrakis system exists specifically to avoid ambiguity. If you wrote 'trioxalatoferrate,' it could be misread as three 'oxalato' groups or one 'trioxalato' entity. Parentheses with multiplicative prefixes eliminate this confusion.

Question 5

Why does IUPAC nomenclature list ligands in alphabetical order rather than by charge or donor atom type?

Model answer: Alphabetical ordering provides a single unambiguous convention that works regardless of the ligands involved. Ordering by charge would create ties (multiple anionic or neutral ligands), requiring a secondary sorting rule. Ordering by donor atom type would be ambiguous for ambidentate ligands. Alphabetical order eliminates all such ambiguity: every chemist arrives at the same name for the same compound. The alphabetical ordering applies to the ligand name itself (ammine before chlorido because 'a' precedes 'c'), ignoring multiplicative prefixes like di-, tri-, or bis-.

This convention is purely practical — it ensures one-to-one correspondence between compound and name. The key subtlety is that prefixes indicating number (di, tri, bis, tris) are NOT considered when alphabetizing, so 'dichlorido' would be alphabetized under 'c', not 'd'.