Q.Write the IUPAC names of the following coordination compounds:

Concept: Werner Coordination Theory — the central metal ion has a fixed primary valence (oxidation state) and a variable secondary valence (coordination number), which determines the complex’s geometry and naming.

Reasoning steps:

1. Identify the complex ion (inside square brackets) and the counter ions outside.
2. For the complex ion: name ligands in alphabetical order (ignoring prefixes like di-, tri-), then the metal with its oxidation state in Roman numerals in parentheses.
3. For anionic complexes, add the suffix -ate to the metal name. Cationic or neutral complexes use the metal name as is.

IUPAC names:

1. Hexaamminecobalt(III) chloride
2. Pentaamminechloridocobalt(III) chloride
3. Potassium hexacyanidoferrate(III)
4. Potassium trioxalatoferrate(III)
5. Potassium tetrachloridopalladate(II) …

Werner’s coordination theory is the foundation here. He showed that metal ions have two types of valency: primary (ionic, satisfied by counter ions) and secondary (coordinate, satisfied by ligands). In naming, we treat the coordination sphere (the metal + its attached ligands) as a single entity. The name of the complex ion is built by listing ligands in alphabetical order (ignoring prefixes like di-, tri-), then the metal, then its oxidation state in Roman numerals in parentheses. If the complex ion is an anion, we add “-ate” to the metal’s name. Counter ions are named last.

Let’s go through each compound step by step.

1. $[Co(NH_3)_6]Cl_3$

   The coordination sphere is $[Co(NH_3)_6]^{3+}$ — a cation. Ligands: six ammonia molecules, which are neutral. The ligand name is “ammine” (note the double m). So “hexaammine” (hexa- for six). Metal: cobalt. Oxidation state: each NH$_3$ is neutral, so Co must be +3 to balance the 3 Cl$^-$ ions outside. Hence cobalt(III). The counter ion is chloride (three of them).

   Name: Hexaamminecobalt(III) chloride.

2. $[Co(NH_3)_5Cl]Cl_2$

   Complex ion: $[Co(NH_3)_5Cl]^{2+}$. Ligands: five ammines and one chloride. Alphabetical order: “ammine” before “chlorido” (IUPAC uses “chlorido” for Cl$^-$ as a ligand, not “chloro”). So “pentaamminechlorido”. Metal: cobalt. Oxidation state: five neutral NH$_3$ + one Cl$^-$ ligand (charge -1) gives total ligand charge = -1. Complex charge is +2, so Co must be +3 (since +3 – 1 = +2). Hence cobalt(III). Counter ions: two chlorides.

   Name: Pentaamminechloridocobalt(III) chloride.

3. $K_3[Fe(CN)_6]$

   Complex ion: $[Fe(CN)_6]^{3-}$ — an anion. Ligands: six cyanide ions, named “cyanido” (IUPAC prefers “cyanido” over “cyano”). So “hexacyanido”. Metal: iron. Since it’s an anionic complex, we use “ferrate” (Latin root). Oxidation state: each CN$^-$ is -1, total ligand charge = -6. Complex charge is -3, so Fe must be +3 (since +3 – 6 = -3). Hence ferrate(III). Counter ion: potassium (three).

   Name: Potassium hexacyanidoferrate(III).

4. $K_3[Fe(C_2O_4)_3]$

   Complex ion: $[Fe(C_2O_4)_3]^{3-}$. Ligands: three oxalate ions ($C_2O_4^{2-}$), which are bidentate. The ligand name is “oxalato”. So “trioxalato”. Metal: iron → ferrate. Oxidation state: each oxalate is -2, total ligand charge = -6. Complex charge = -3, so Fe must be +3 (since +3 – 6 = -3). Hence ferrate(III). Counter ion: potassium.

   Name: Potassium trioxalatoferrate(III).

5. $K_2[PdCl_4]$

   Complex ion: $[PdCl_4]^{2-}$. Ligands: four chloride ions → “tetrachlorido”. Metal: palladium → “palladate” (anionic form). Oxidation state: each Cl$^-$ is -1, total = -4. Complex charge = -2, so Pd must be +2 (since +2 – 4 = -2). Hence palladate(II). Counter ion: potassium (two). …

Method: IUPAC Nomenclature of Coordination Compounds

This follows the Additive Nomenclature method under IUPAC rules (Red Book). The core principle: name the cation first, then the anion — and within the coordination sphere, name ligands alphabetically before the central metal (with its oxidation state in Roman numerals in parentheses).

Steps

1. Identify the cation and anion of the compound.
2. For the coordination sphere (inside square brackets):
   • Name ligands in alphabetical order (ignoring prefixes like di-, tri-).
   • Anionic ligands end in -o, in the modern -ido style (e.g., chlorido, cyanido, oxalato).
   • Neutral ligands keep their name (e.g., ammine, water).
   • Write the central metal name last.
   • Add the oxidation state of the metal in Roman numerals in parentheses.
3. If the complex is an anion, the metal name ends in -ate.
4. Outside the sphere: name counter-ions normally.

(i) $[Co(NH_3)_6]Cl_3$

• Cation: $[Co(NH_3)_6]^{3+}$ — hexaamminecobalt(III)
• Anion: three chloride ions → chloride
• Full name: Hexaamminecobalt(III) chloride

(ii) $[Co(NH_3)_5Cl]Cl_2$

• Cation: $[Co(NH_3)_5Cl]^{2+}$ — ligands: ammine (×5) + chlorido Alphabetical order: ammine before chlorido → Pentaamminechloridocobalt(III) (chlorido is IUPAC for Cl⁻ ligand)
• Anion: two chloride ions → chloride
• Full name: Pentaamminechloridocobalt(III) chloride

(iii) $K_3[Fe(CN)_6]$

• Cation: three potassium ions → potassium
• Anion: $[Fe(CN)_6]^{3-}$ — six cyanido ligands → hexacyanido Metal is iron, but as anion → ferrate Oxidation state: Fe is +3 → (III)
• Full name: Potassium hexacyanidoferrate(III)

(iv) $K_3[Fe(C_2O_4)_3]$

• Cation: potassium
• Anion: $[Fe(C_2O_4)_3]^{3-}$ — oxalate ligand ($C_2O_4^{2-}$) is oxalato Three → trioxalato Metal: iron → ferrate, Fe is +3 → (III)
• Full name: Potassium trioxalatoferrate(III)

(v) $K_2[PdCl_4]$

• Cation: potassium
• Anion: $[PdCl_4]^{2-}$ — four chlorido → tetrachlorido Metal: palladium → palladate, Pd is +2 → (II) …

Here are the common mistakes students make when naming coordination compounds under Werner’s theory, using your six examples as case studies. Each mistake is followed by a clear how-to-avoid tip.

1. Wrong order: Naming the anion before the cation

Mistake: Writing the name of the complex ion first, then the counter ion — especially when the complex is anionic (e.g., $K_3[Fe(CN)_6]$).

Example of error:

“Potassium hexacyanidoferrate(III)” is correct, but some write “hexacyanidoferrate(III) potassium” or “tripotassium hexacyanidoferrate(III)”.

How to avoid:

• Always name the cation first, then the anion — just like in simple salts (NaCl = sodium chloride).
• For $K_3[Fe(CN)_6]$: cation = $K^+$ → “potassium”, anion = $[Fe(CN)_6]^{3-}$ → “hexacyanidoferrate(III)”.
• For $[Co(NH_3)_6]Cl_3$: cation = $[Co(NH_3)_6]^{3+}$ → name first, then “chloride”.

2. Forgetting to change the suffix for anionic complexes

Mistake: Using the metal’s English name (e.g., “iron”, “cobalt”) even when the complex is an anion.

Example of error:

$K_3[Fe(CN)_6]$ named as “potassium hexacyanoiron(III)” instead of “potassium hexacyanidoferrate(III)”.

How to avoid:

• If the complex ion is negative, replace the metal name’s ending with -ate.
  • Iron → ferrate
  • Cobalt → cobaltate
  • Copper → cuprate
  • Silver → argentate
  • Tin → stannate
  • Lead → plumbate
• Exception: Some metals use Latin roots (e.g., Fe → ferrate, Cu → cuprate). Memorise the common ones.

3. Omitting the oxidation state of the metal

Mistake: Leaving out the Roman numeral for the metal’s oxidation state, or writing it in the wrong place.

Example of error:

$[Co(NH_3)_5Cl]Cl_2$ named as “pentaamminechloridocobalt chloride” — missing the (III).

How to avoid:

• Always calculate the oxidation state of the central metal before naming.
  • For $[Co(NH_3)_5Cl]Cl_2$:
    • $NH_3$ = neutral, $Cl^-$ inside complex = $-1$, two outer $Cl^-$ = $-2$ total.
    • Let Co = $x$: $x + 0 + (-1) + (-2) = 0 \Rightarrow x = +3$.
  • Write it as cobalt(III).
• Place the Roman numeral in parentheses right after the metal name (or metal-ate name), with no space before the parenthesis.

4. Incorrect ligand order (alphabetical vs. charge-based)

Mistake: Listing ligands by charge (anionic first, then neutral) instead of alphabetical order (ignoring prefixes like di-, tri-).

Example of error:

$[Pt(NH_3)_2Cl(NH_2CH_3)]Cl$ named with the ligands out of alphabetical order (e.g. “chlorido” placed before “ammine”) — the correct order is ammine, then chlorido, then methanamine.

How to avoid:

• Alphabetise ligands by their name (not by symbol or charge).
  • “ammine” (A), “chlorido” (C), “methanamine” (M).
  • Correct order: diamminechlorido(methanamine) — ammine (A) comes before chlorido (C), which comes before methanamine (M).
• Prefixes like di-, tri-, tetra- are ignored when alphabetising. Only the ligand root counts.
• Exception: If two ligands start with the same letter, then use the full name.

5. Wrong use of numerical prefixes (di, tri, tetra, etc.)

Mistake: Using “di”, “tri” for complex ligands like ethylenediamine (en) or oxalate ($C_2O_4^{2-}$) — or forgetting them entirely.

Example of error:

$K_3[Fe(C_2O_4)_3]$ named as “potassium trioxalatoferrate(III)” — correct. But some write “potassium oxalatoferrate(III)” (missing “tri”) or “potassium tris(oxalato)ferrate(III)” (using “tris” incorrectly for a simple ligand).

How to avoid:

• For simple ligands (e.g., $Cl^-$, $NH_3$, $CN^-$): use di, tri, tetra, penta, hexa.
• For ligands whose own name already contains a multiplying prefix (e.g., ethylenediamine, which contains “di”): use bis, tris, tetrakis, pentakis and put the ligand name in parentheses, to avoid ambiguity — e.g. $[Co(en)_3]^{3+}$ → tris(ethylenediamine)cobalt(III).
  • Oxalato does not need bis/tris — NCERT’s own convention simply uses tri (e.g. potassium trioxalatoaluminate(III), potassium trioxalatochromate(III)). So $[Fe(C_2O_4)_3]^{3-}$ → trioxalatoferrate(III), matching the correct answer above. (The 2004 IUPAC recommendations also treat tris(oxalato)ferrate(III) as an equally unambiguous style, but NCERT — and the answer to this question — uses trioxalato; in the board exam, write the NCERT form.)
• Check the ligand: only use bis/tris/tetrakis when the ligand’s own name already contains di/tri/tetra, or when a simple prefix would be genuinely ambiguous.

6. Forgetting to enclose complex ligands in parentheses

Mistake: Writing “methanamine” without parentheses when it’s a complex ligand, or using parentheses for simple ligands.

Example of error:

$[Pt(NH_3)_2Cl(NH_2CH_3)]Cl$ named as “diamminechloridomethanamineplatinum(II) chloride” — no parentheses around methanamine.

How to avoid:

• Always put parentheses around the name of a ligand that is itself a molecule with a systematic name (e.g., methanamine, ethylenediamine, oxalate).
• Simple ligands like $NH_3$ (ammine), $Cl^-$ (chlorido), $CN^-$ (cyanido) do not need parentheses. …