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Nomenclature of Complex Ions

This page explains how to name some common complex metal ions. Although the names of complex ions can look long and worrying, the formulae are simply being coded in much the same way that organic names are coded. Once you have sorted out that code, the names are entirely descriptive.

Naming the ligands

The table shows some common ligands and the code for them in the name of a complex ion. The old names sometimes differ by a letter or so, but never enough for it to be confusing.

 

ligand coded by (old name)
H2O aqua aquo
NH3 ammine ammino
OH- hydroxo hydroxy
Cl- chloro  
F- fluoro  
CN- cyano  

 

Take care with the code for ammonia as a ligand - it has 2 "m"s in its name. If you miss one of these out so that you are left with "amine" or "amino", you are refering to the NH2 group in an organic compound. This is probably the only point of confusion with these names.

Coding for the number of ligands

The normal prefixes apply if there is more than one ligand.

 

no of ligands coded by
2 di
3 tri
4 tetra
5 penta
6 hexa

Putting this together

For a complex ion containing only one type of ligand, there is no problem. For example:

[Cu(H2O)6]2+ is called the hexaaquacopper(II) ion.

(Don't worry about the copper(II) bit for the moment.) The fact that there are two "a"s next to each other in the name is OK.

With more than one type of ligand in an ion, the ligands are named in alphabetical order - ignoring the prefixes. For example:

[Cu(NH3)4(H2O)2]2+ is called the tetraamminediaquacopper(II) ion.

The "ammine" is named before the "aqua" because "am" comes before "aq" in the alphabet. The "tetra" and "di" are ignored.

Naming the metal

You might have thought that this was fairly obvious, but it isn't necessarily. It depends on whether the complex ion ends up as positively or negatively charged.

For positively charged complex ions

A positively charged complex ion is called a cationic complex. A cation is a positively charged ion. The metal in this is named exactly as you would expect, with the addition of its oxidation state. Going back to a previous example, [Cu(H2O)6]2+ is called the hexaaquacopper(II) ion because the copper's oxidation state is +2.

Copper's oxidation is +2 because the original uncomplexed ion was Cu2+ - NOT because the complex carries 2+ charges.

The oxidation state is frequently left out if a metal only ever has one oxidation state. For example, in its compounds aluminium always has an oxidation state of +3. [Al(H2O)6]3+ is usually just called the hexaaquaaluminium ion rather than the hexaaquaaluminium(III) ion.

For negatively charged complex ions

A negatively charged complex ion is called an anionic complex. An anion is a negatively charged ion. In this case the name of the metal is modified to show that it has ended up in a negative ion. This is shown by the ending -ate.

With many metals, the basic name of the metal is changed as well - sometimes drastically! Common examples include:

 

metal changed to
cobalt cobaltate
aluminium aluminate
chromium chromate
vanadium vanadate
copper cuprate
iron ferrate

 

So, for example, suppose you bond 4 chloride ions around a Cu2+ ion to give [CuCl4]2-.

The name shows the 4 (tetra) chlorines (chloro) around a copper in an overall negative ion (cuprate). The copper has on oxidation state of +2. This is the tetrachlorocuprate(II) ion.

Similarly, [Al(H2O)2(OH)4]- is called the diaquatetrahydroxoaluminate ion. Take the name to pieces so that you can see exactly what refers to what. Don't forget that the two different ligands are named in alphabetical order - aqua before hydroxo - ignoring the prefixes, di and tetra. The oxidation state of the aluminum could be shown, but is not absolutely necessary because aluminum only has the one oxidation state in its compounds. The full name is the diaquatetrahydroxoaluminate(III) ion.

Contributors

Jim Clark (Chemguide.co.uk)