Nomenclature of Coordination Compounds | Chemistry Class 12 - NEET PDF Download

Why do we need to name the compounds?

Naming coordination compounds provides a clear, unambiguous way to represent their composition and structure. A systematic name allows chemists to identify the central metal, the type and number of ligands, the charge and oxidation state of the metal, and where necessary the mode of bonding (linkage). This is especially important when distinguishing isomers and when communicating results in examinations, laboratory work or literature.

Basic IUPAC conventions (summary)

• Name the cationic part before the anionic part when giving the full compound name.
• Within the name of a coordination entity (complex ion or neutral complex), list ligands first and then the central metal atom/ion.
• List ligands in alphabetical order (alphabetisation is done on ligand names ignoring multiplicative prefixes such as di-, tri-, bis-, tris-).
• Write the formula of a coordination entity inside square brackets, e.g. [M(L)n].
• Enclose ligand abbreviations in parentheses when used (for clarity in names use full ligand names; abbreviations appear in formulas).
• Do not put any space between the metal symbol and the ligands inside the coordination sphere in the formula.
• Overall charge neutrality must be maintained: the total positive charge(s) = total negative charge(s) for an ionic compound.

Detailed rules of nomenclature with explanations and examples

1. Cation before anion:

   When naming an ionic coordination compound, the cationic part is named first and the anionic part second, regardless of whether the coordination entity itself is a cation or anion.

   Examples:

   Na[Co(NH₃)4Cl₂] - sodium tetraammine dichloridocobaltate(I)

   [Co(NH₃)₆]Cl₃ - hexaamminecobalt(III) chloride

2. Order of ligands and ligand name endings:

   Ligands are named before the metal. Anionic ligands use an "o" or "ido" ending in many conventional names (for class 12 / NCERT style, anionic ligands generally end in -o, e.g. chloro, cyano), while neutral ligands are given their common names (with some classical IUPAC names: H₂O → aqua, NH₃ → ammine, CO → carbonyl, NO → nitrosyl). The multiplicative prefixes (di, tri) are not used for alphabetisation; alphabetisation ignores these prefixes.

   
   
   Question for Nomenclature of Coordination Compounds

   Try yourself:Identify the correct formula for hexaaquamanganese(II) ion.

   • A.

     [Mn(H₂O)₆]²⁺
   • B.

     [Mn(H₂O)₆]^2-
   • C.

     [Mn₂(H₂O)₆]⁺
   • D.

     [Mn(H₂O)₆]⁺²

   Explanation

   The parenthesis after manganese represents its oxidation state (+2) which is indicated outside the square bracket with sign following the number. The H₂O molecule is a neutral ligand named as aqua.

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3. Multiplicative prefixes and complex ligand names:

   When a ligand name itself contains a numeral or when a ligand name is polydentate or complex (for example, triphenylphosphine or ethylenediamine), use the terms bis, tris, tetrakis etc., and place the ligand name in parentheses. This avoids ambiguity.

   Example: [NiCl₂(PPh₃)₂] is named dichloridobis(triphenylphosphine)nickel(II).

   
4. Naming the central metal:

   After listing ligands, name the central metal. If the coordination entity is a cation or neutral complex, give the metal name as the element (e.g. cobalt). If the coordination entity is an anion, the metal name ends with the suffix -ate and where customary the Latin form of the metal name may be used in older traditional names (e.g. ferrate for iron, cuprate for copper). Give the oxidation state of the metal as a Roman numeral in parentheses immediately after the metal name.

   
   Question for Nomenclature of Coordination Compounds

   Try yourself:The correct way of naming Co in [CoCl₂(en)₂]⁺ is ________

   • A.

     cobalt(III)
   • B.

     cobalt(II)
   • C.

     cobaltate(III)
   • D.

     cobaltate(II)

   Explanation

   The complex ion given is cationic and hence the metal is named the same as the element. Also, the oxidation number of cobalt is +3 as (en) is a neutral ligand and Cl has a -1 charge. Thus, the correct naming for Co is cobalt(III).

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5. Oxidation state:

   Indicate the oxidation number of the central metal in parentheses using Roman numerals immediately after the metal name, e.g. cobalt(III), nickel(II). For anionic complexes, this appears after the metal root with the -ate ending, e.g. ferrate(II) in K₄[Fe(CN)₆] → potassium hexacyanoferrate(II).

6. Neutral complexes:

   Neutral coordination complexes are named in the same way as complex cations: ligands first (alphabetical), then the metal with its oxidation state in parentheses where required.

7. Linkage (ambidentate) isomers:

   Some ligands can bind through different atoms (ambidentate ligands). The name must specify the donor atom when necessary. Common examples:

   M-NO2 (bound through N) → nitro

   M-ONO (bound through O) → nitrito

   M-SCN (bound through S) → thiocyanato

   M-NCS (bound through N) → isothiocyanato

8. Solvent of crystallisation / lattice water (hydrates):

   If the crystalline compound contains uncoordinated solvent molecules (for example, water of crystallisation), these are indicated after the main compound name using Arabic numerals for the number of such molecules, e.g. "12-water" for 12 molecules of water. The solvent is usually named as "water" or "solvent" in this context.

   Example: AlK(SO₄)₂·12H₂O - aluminium potassium sulfate 12-water

   (a) Complex cations	IUPAC name
   (b) Complex anions
   (c) Organic groups
   (d) Bridging groups
   (e) Hydrates AlK(SO4)2·12H2O	aluminium potassium sulfate 12-water

9. Writing the formula of a coordination compound:

   When writing formulas follow these conventions:

   • Enclose the entire coordination entity in square brackets: [M(L)n].
   • Inside the brackets, the central metal atom is usually written first followed by ligands.
   • List ligands inside the formula in the order: negative ligands, neutral ligands, positive ligands. Within each group, list ligands alphabetically by the first element or symbol of the ligand (or by ligand name), and use multiplicative prefixes to show numbers (di, tri, etc.).
   • Use Arabic numerals (1, 2, 3, ...) for any solvent-of-crystallisation or lattice components outside the coordination sphere.
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