IUPAC Nomenclature of Mononuclear Coordination Compounds

The IUPAC (International Union of Pure and Applied Chemistry) system provides a standardized way to name coordination compounds, which are molecules or ions containing one or more coordinate covalent bonds. For JEE students, mastering the rules of IUPAC nomenclature is essential for accurately identifying and writing the formulas of these compounds. This guide focuses on the rules for mononuclear coordination compounds, which contain a single central metal atom.

1.0Mononuclear Coordination Compound

A coordination compound consists of a central metal atom or ion bonded to a surrounding group of molecules or ions called ligands. These ligands are connected to the central metal via coordinate covalent bonds. A mononuclear coordination compound has only one central metal atom.

The coordination entity is typically enclosed in square brackets, e.g., $[Co(N{H}_3{)}_6{]}^{3+}$. This entity consists of the central metal ion (Co) and the ligands (NH_3). The overall charge on the complex is balanced by counter-ions outside the brackets.

2.0Basic Rules for Naming

The IUPAC name of a coordination compound is written as a single word, following a specific order:

1. Cation before Anion: The cation is named first, followed by the anion. This is a general rule of chemical nomenclature.
   
   Example: In $K_4[Fe(CN{)}_6]$, the cation is Potassium and the anion is the complex $[Fe(CN{)}_6{]}^{4-}$
2. Ligands before Metal: Within the coordination sphere (inside the brackets), the ligands are named first, followed by the central metal ion.
3. Alphabetical Order of Ligands: If there are multiple different ligands, they are named in alphabetical order, regardless of their charge.
4. Prefixes for Multiple Ligands: Prefixes like di-, tri-, tetra-, etc., are used to indicate the number of identical ligands. If the ligand name itself contains a prefix (e.g., ethylenediamine, triethylamine), then prefixes like bis-, tris-, tetrakis are used instead.

3.0Naming Ligands

The name of a ligand depends on whether it is neutral, negative, or positive.

Ligand Charge Nomenclature 

4.0Naming the Central Metal Ion

The name of the central metal ion depends on the charge of the coordination sphere.

• Cationic or Neutral Complex: The name of the metal remains unchanged.
  
  Examples: Cobalt, Chromium, Iron, Copper.
• Anionic Complex: The ending of the metal name is changed to -ate. In some cases, the Latin name of the metal is used.
• -ate (e.g., cobaltate, chromate, nickelate)
• Iron → ferrate
• Copper → cuprate
• Lead → plumbate
• Silver → argentate
• Gold → aurate

Oxidation State: The oxidation state of the central metal is indicated immediately following its name using a Roman numeral enclosed in parentheses.

    ◦ Calculation: The oxidation state is calculated by summing the charges of the ligands and counter ions, then solving for the metal's charge to satisfy the overall neutrality or charge of the complex. (For K4​[Fe(CN)6], the Fe is +2, resulting in Potassium hexacyanidoferrate(II)).

5.0Putting it All Together

Let's name a few compounds using these rules.

Example 1: $[Co(N{H}_3{)}_6]C{l}_3$

1. Identify Cation and Anion: The complex $[Co(N{H}_3{)}_6{]}^{3+}$is the cation, and Cl^− is the anion.
2. Name the Ligand: NH3​ is ammine. There are six of them, so use the prefix hexa-.
3. Name the Metal: The complex is a cation, so the metal name is cobalt.
4. Calculate Oxidation State: The overall charge is balanced by three Cl− ions, so the complex has a +3charge. The ammine ligand is neutral.
   
   Oxidation state of $Co+(6\times 0)=+3⟹Co$ is in the +3 oxidation state.
5. Final Name: The ligands are named first, followed by the metal and its oxidation state. The anion is named last.
   
   Hexaamminecobalt(III) chloride

Example 2: $K_4[Fe(CN{)}_6]$

1. Identify Cation and Anion: K+ is the cation, and the complex $[Fe(CN{)}_6{]}^{4-}$ is the anion.
2. Name the Ligand: CN^− is a negative ligand, so its name is cyanido. There are six of them, so use the prefix hexa-.
3. Name the Metal: The complex is an anion, so the metal name is changed to ferrate (from Iron).
4. Calculate Oxidation State: The overall charge is balanced by four K+ ions, so the complex has a −4charge. The cyanido ligand has a −1 charge.
5. Oxidation state of $Fe+(6\times -1)=-4⟹Fe$ is in the +2 oxidation state.
6. Final Name: The cation is named first, followed by the complex anion.
7. Potassium hexacyanidoferrate(II)

6.0Writing Formulas from IUPAC Names

Writing the formula from the name is the reverse process.

Example: Sodium hexanitrocobaltate(III)

1. Identify Cation and Anion: The cation is Sodium (Na+). The complex anion is hexanitrocobaltate(III).
2. Determine Metal and Ligands: The metal is Cobalt (Co). The ligands are six nitro (NO2−​) groups. The oxidation state of Co is +3.
3. Determine Charge on Complex: Charge of complex = (Oxidation state of Co) + (Charge of 6 nitro ligands)
   
   Charge of complex = $(+3)+(6\times -1)=-3$
4. Balance Charges: To balance the −3 charge of the complex $[Co(N{O}_2{)}_6{]}^{3-}$, we need three Na+ ions.
5. Final Formula: $N{a}_3[Co(N{O}_2{)}_6]$

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