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The Naming of Complexes

The Naming of Complexes

The nomenclature of the complexes is patterned after a system suggested by Alfred Werner, a Swiss chemist and Nobel laureate, whose outstanding work more than 100 years ago laid the foundation for a clearer understanding of these compounds. The following five rules are used for naming complexes:

  1. If a coordination compound is ionic, name the cation first and the anion second, in accordance with the usual nomenclature.
  2. Name the ligands first, followed by the central metal. Name the ligands alphabetically. Negative ligands (anions) have names formed by adding –o to the stem name of the group. For examples, see the table below. For most neutral ligands, the name of the molecule is used. The four common exceptions are aqua (H2O), ammine (NH3), carbonyl (CO), and nitrosyl (NO). For example, name [Pt(NH3)2Cl4] as diamminetetrachloroplatinum(IV).
    Table 1: Examples of Anionic Ligands
    Anionic LigandName
  3. If more than one ligand of a given type is present, the number is indicated by the prefixes di– (for two), tri– (for three), tetra– (for four), penta– (for five), and hexa– (for six). Sometimes, the prefixes bis– (for two), tris– (for three), and tetrakis– (for four) are used when the name of the ligand already includes di-, tri-, or tetra-, or when the ligand name begins with a vowel. For example, the ion bis(bipyridyl)osmium(II) uses bis- to signify that there are two ligands attached to Os, and each bipyridyl ligand contains two pyridine groups (C5H4N).

When the complex is either a cation or a neutral molecule, the name of the central metal atom is spelled exactly like the name of the element and is followed by a Roman numeral in parentheses to indicate its oxidation state (Table 2 and Table 3). When the complex is an anion, the suffix -ate is added to the stem of the name of the metal, followed by the Roman numeral designation of its oxidation state (Table 4). Sometimes, the Latin name of the metal is used when the English name is clumsy. For example, ferrate is used instead of ironate, plumbate instead leadate, and stannate instead of tinate.

The oxidation state of the metal is determined based on the charges of each ligand and the overall charge of the coordination compound. For example, in [Cr(H2O)4Cl2]Br, the coordination sphere (in brackets) has a charge of 1+ to balance the bromide ion. The water ligands are neutral, and the chloride ligands are anionic with a charge of 1− each. To determine the oxidation state of the metal, we set the overall charge equal to the sum of the ligands and the metal: +1 = −2 + x, so the oxidation state (x) is equal to 3+.

Table 2: Examples in Which the Complex Is a Cation
[Co(NH3)6]Cl3hexaamminecobalt(III) chloride
[Pt(NH3)4Cl2]2+tetraamminedichloroplatinum(IV) ion
[Ag(NH3)2]+diamminesilver(I) ion
[Cr(H2O)4Cl2]Cltetraaquadichlorochromium(III) chloride
[Co(H2NCH2CH2NH2)3]2(SO4)3tris(ethylenediamine)cobalt(III) sulfate
Table 3: Examples in Which the Complex Is Neutral
Table 4: Examples in Which the Complex Is an Anion
[PtCl6]2−hexachloroplatinate(IV) ion
Na2[SnCl6]sodium hexachlorostannate(IV)


Do you think you understand naming coordination complexes? You can look over more examples and test yourself with online quizzes at the University of Sydney’s site.


Coordination Numbers and Oxidation States

Determine the name of the following complexes and give the coordination number of the central metal atom.

(a) Na2[PtCl6]

(b) K3[Fe(C2O4)3]

(c) [Co(NH3)5Cl]Cl2


(a) There are two Na+ ions, so the coordination sphere has a negative two charge: [PtCl6]2−. There are six anionic chloride ligands, so −2 = −6 + x, and the oxidation state of the platinum is 4+. The name of the complex is sodium hexachloroplatinate(IV), and the coordination number is six.

(b) The coordination sphere has a charge of 3− (based on the potassium) and the oxalate ligands each have a charge of 2−, so the metal oxidation state is given by −3 = −6 + x, and this is an iron(III) complex. The name is potassium trisoxalatoferrate(III) (note that tris is used instead of tri because the ligand name starts with a vowel). Because oxalate is a bidentate ligand, this complex has a coordination number of six.

(c) In this example, the coordination sphere has a cationic charge of 2+. The NH3 ligand is neutral, but the chloro ligand has a charge of 1−. The oxidation state is found by +2 = −1 + x and is 3+, so the complex is pentaamminechlorocobalt(III) chloride and the coordination number is six.

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