Nomenclature of Coordination Compounds

Full Notes

Formulas of Mononuclear Coordination Entities

A mononuclear coordination entity contains a single central metal atom/ion surrounded by ligands. The following rules must be followed for writing correct formulas.

Rules for Writing Formulas

1. The central atom is listed first.
2. Ligands are listed next to the central atom in alphabetical order, based on the name of the ligand, not its prefix.
   NH₃ (ammine) comes before Cl⁻ (chloro)
3. The formula of each ligand is enclosed in parentheses only when the ligand itself contains a subscript.
   ethylenediamine (en) is written without parentheses as it contains no subscript.
4. The formula of the entire coordination entity is enclosed in square brackets [ ], with no space between the metal and ligand names.
5. If the compound is ionic, the cation is written first, followed by the anion, each outside the coordination brackets as needed.
6. There should be no space between parts of the formula (e.g., [Co(NH₃)₄Cl₂]Cl).
7. The overall charge on the complex ion or compound must be balanced with the counter-ions.

Examples Formulas

[Co(NH₃)₄Cl₂]Cl

K₄[Fe(CN)₆]

[Ni(CO)₄]

Naming of Mononuclear Coordination Compounds

The systematic naming of coordination compounds follows a separate set of rules. A mononuclear coordination compound contains a single metal centre. The name must reflect the nature, number, and charge of ligands and metal, in a precise order.

Rules for Naming Mononuclear Coordination Compounds

1. The name of the cation (whether it is a simple ion or complex ion) is written before the name of the anion.
2. Within a coordination entity, the names of the ligands are written first, followed by the name of the central atom/ion.
3. Ligands are named in alphabetical order (ignoring prefixes like di-, tri-, etc., used to indicate quantity).
4. The names of anionic ligands end in –o, while neutral ligands are generally written using their common names, with a few important exceptions:

5. When more than one ligand of the same kind is present, prefixes are added:
   di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6)
   For ligands that already include a numerical prefix (e.g., ethylenediamine), the following are used:
   bis- (2), tris- (3), tetrakis- (4)
   Ligand names following these special prefixes are placed in parentheses.

Examples Prefix usage

[Cr(en)₃]³⁺ → tris(ethylenediamine)chromium(III) ion

[Co(NH₃)₆]³⁺ → hexaamminecobalt(III) ion

6. The name of the central metal atom/ion depends on the charge of the coordination entity:
   • For cationic or neutral complexes, the usual name of the metal is used.
   • For anionic complexes, the name of the metal ends in –ate. For some metals, Latin-derived names are used in the anionic form:

7. The oxidation number of the central atom is written in Roman numerals in parentheses, immediately following its name.

Examples Naming complete compounds

[Co(NH₃)₆]Cl₃
Ligands: 6 NH₃ = hexaammine
Central metal: cobalt (cationic complex)
Oxidation state: +3
hexaamminecobalt(III) chloride

K₄[Fe(CN)₆]
Ligands: 6 CN⁻ = hexacyano
Central metal: Fe (anionic complex → ferrate)
Oxidation state: +2
potassium hexacyanoferrate(II)

[Pt(NH₃)₂Cl₂]
Ligands: 2 NH₃ (ammine), 2 Cl⁻ (chloro)
Metal: platinum
Oxidation state: +2
diamminedichloroplatinum(II)

[Fe(CO)₅]
Ligands: 5 CO (carbonyl)
Metal: Fe
Oxidation state: 0
pentacarbonyliron(0)

Summary

• Write cation first then anion and place coordination entities in brackets.
• List ligands alphabetically before the metal name.
• Use –o endings for anionic ligands and special names for key neutral ligands.
• Use di/tri… or bis/tris/tetrakis for multiple ligands as appropriate.
• Use –ate for anionic metal names and show oxidation state in Roman numerals.