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IUPAC Naming of Coordination Compounds

IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals.

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Student-friendly explanation

Ligands have special names such as ammine for NH3, aqua for H2O, chloro for Cl-, hydroxo for OH-, cyano for CN-, and nitro or nitrito depending on the donor atom. Prefixes such as di, tri, tetra, penta, and hexa show the number of identical ligands. Alphabetical order is decided by ligand names, not by numerical prefixes. If the complex ion is anionic, the metal name usually ends in -ate, such as ferrate, cuprate, cobaltate, or platinate.

How to write this in exams

  1. 1

    Start with the exact idea

    IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals.

  2. 2

    Then show how to use it

    Identify cation and anion. Isolate the coordination entity. Calculate metal oxidation state. Convert ligand formulae into ligand names. Arrange ligand names alphabetically. Add metal name with Roman numeral, then name counter ions.

  3. 3

    Add one concrete example

    [Co(NH3)5Cl]Cl2 is named pentaamminechlorocobalt(III) chloride. The complex ion is cationic, NH3 is named ammine, Cl- inside the sphere is named chloro, and cobalt has oxidation state +3.

  4. 4

    Avoid this incomplete answer

    A common wrong answer for K4[Fe(CN)6] is potassium hexacyanoiron(II), missing the anionic complex ending ferrate.

Definition

IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals.

Example

[Co(NH3)5Cl]Cl2 is named pentaamminechlorocobalt(III) chloride. The complex ion is cationic, NH3 is named ammine, Cl- inside the sphere is named chloro, and cobalt has oxidation state +3.

Rule to remember

Naming order rule: cation first, anion second. Inside coordination entity: ligand names in alphabetical order followed by metal name and oxidation state in Roman numerals. For an anionic complex, use the metal name with -ate ending where applicable.

Memory hook

Name outside charge partners first, then name the bracket from ligand alphabet to metal Roman numeral.

Examples and method

Worked example

Name K4[Fe(CN)6]. Potassium is the cation. In [Fe(CN)6]4-, each CN- is -1, so x - 6 = -4 and x = +2. Six cyano ligands give hexacyano. Since the complex ion is anionic, iron becomes ferrate. Name: potassium hexacyanoferrate(II).

Method to apply

Identify cation and anion. Isolate the coordination entity. Calculate metal oxidation state. Convert ligand formulae into ligand names. Arrange ligand names alphabetically. Add metal name with Roman numeral, then name counter ions.

Diagram support

No structural diagram is required. A rule-flow chart for naming can help students avoid order and oxidation-state errors.

How CBSE asks it

Exam questions may give a formula and ask for the IUPAC name, or give a name and ask for the formula. They may also test ligand names, oxidation state, and alphabetical order.

Avoid common mistakes

Common confusion

A frequent mistake is writing ammonia instead of ammine, water instead of aqua, or placing ligands according to formula order instead of alphabetical order.

Common wrong answer

A common wrong answer for K4[Fe(CN)6] is potassium hexacyanoiron(II), missing the anionic complex ending ferrate.

Exam tip

Do the charge calculation before naming the metal. The Roman numeral is not the coordination number; it is the oxidation state of the metal.

Quick check

Name the ligand H2O and the ligand NH3 in coordination compound nomenclature.

H2O is named aqua and NH3 is named ammine.

Answer writing and exam use

1-mark answer

IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals.

2-mark answer

IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals. Naming order rule: cation first, anion second. Inside coordination entity: ligand names in alphabetical order followed by metal name and oxidation state in Roman numerals. For an anionic complex, use the metal name with -ate ending where applicable. [Co(NH3)5Cl]Cl2 is named pentaamminechlorocobalt(III) chloride. The complex ion is cationic, NH3 is named ammine, Cl- inside the sphere is named chloro, and cobalt has oxidation state +3.

3-mark answer

Ligands have special names such as ammine for NH3, aqua for H2O, chloro for Cl-, hydroxo for OH-, cyano for CN-, and nitro or nitrito depending on the donor atom. Prefixes such as di, tri, tetra, penta, and hexa show the number of identical ligands. Alphabetical order is decided by ligand names, not by numerical prefixes. If the complex ion is anionic, the metal name usually ends in -ate, such as ferrate, cuprate, cobaltate, or platinate. Naming order rule: cation first, anion second. Inside coordination entity: ligand names in alphabetical order followed by metal name and oxidation state in Roman numerals. For an anionic complex, use the metal name with -ate ending where applicable. Name K4[Fe(CN)6]. Potassium is the cation. In [Fe(CN)6]4-, each CN- is -1, so x - 6 = -4 and x = +2. Six cyano ligands give hexacyano. Since the complex ion is anionic, iron becomes ferrate. Name: potassium hexacyanoferrate(II). Exam questions may give a formula and ask for the IUPAC name, or give a name and ask for the formula. They may also test ligand names, oxidation state, and alphabetical order. A common wrong answer for K4[Fe(CN)6] is potassium hexacyanoiron(II), missing the anionic complex ending ferrate.
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