Nomenclature

Naming metal complexes

Before we look at the rules and steps involved in naming metal complexes consider the three formulae given below. Each of these examples is an ionic salt or a coordination salt made up of positive and negative ions as you would expect from an ionic compound. The metal complex is shown in square brackets and the other ion/ions present are there to simply balance the charge on the metal complex.

[Co(NH₃)₅Cl]Cl₂

K₄[Fe(CN)₆]

[Co(en)₃]Cl₃

In the first example hopefully you will notice that there are two chloride ion (Cl^-) outside the square brackets, now from the given formula we can see that overall the salt has no charge; it is neutral, this means that since there is a charge of 2^- from the two chloride ions the metal complex must have a charge of 2⁺ and since ammonia is a neutral ligand the cobalt ion in the complex must have a charge of 2⁺. The metal complex is also a cation since it has a positive charge.

In the second example there are four potassium ions (K⁺) outside the square brackets of the metal complex, so the metal complex must have a charge of 4^- to balance the 4⁺ charge from the four potassium ions, that is the metal complex is an anion.

cartoon image to remind students how to name salts containing metal complexes

cartoon image to remind students how to name salts containing metal complexes

While in the third example the ligand in the metal complex is the bidentate ligand ethylenediamine (NH₂CH₂CH₂NH₂) which hopefully you will recall is given the shorthand symbol "en". This salt or coordination compound contains three chloride ions (Cl^-), this means that the complex has a charge of 3⁺ and since the ethylenediamine ligands are neutral ligands the cobalt ion in the complex has a charge of 3⁺. The complex is therefore cationic.

Naming complexes

The rules for naming metal complexes are very straightforward to apply e.g.

Rule 1- When naming salts that contain metal complexes the name of the cation present is given before the name of the anion.

Rule 2- When naming the metal complex (the part inside square brackets [ ]) the names of the ligands present are given first followed by the name of the metal present. When there are two or more different ligands present in the complex their names are given in alphabetical order.

Rule 3- The Greek prefixes: di, tri, tetra, penta and hexa are used to indicate the number of each type of ligand present. If there are two or more different ligands then the names of the ligands are written in alphabetical order ignoring any Greek prefixes.

Rule 4- If the ligands have a negative charge; that is they are anionic then the names of these ligands ends in the letter -o; e.g.

-ide → -o (e.g. chloride → chloro )
-ate → -ato (e.g. nitrate → nitrato)
-ite → -ito (e.g. nitrite → nitrito)

The table below gives a more complete list on how to name common anionic ligands:

Anionic ligand	Ligand name
bromide (Br^-)	bromo
chloride (Cl^-)	chloro
fluoride (F^-)	fluoro
hydroxide (OH^-)	hydroxo
oxide(O^2-)	oxo
sulfate (SO₄^2-)	sulfato
carbonate(CO₃^2-)	carbonato
cyanide(CN^-)	cyano
oxalate(C₂O₄^2-)	oxalato

* Note- The oxalate ion (C₂O₄^2-) is also known as ethanodioate. You will need to consider this when you come to name complexes containing this bidentate ligand, you will need to use the prefixes bis, tris if more than one oxalate ion is present.

Rule 5- Neutral ligands are usually just given the name of molecule; however there are a few exceptions, these are listed in the table below:

Ligand	Ligand name
ammonia (NH₃)	ammine
water (H₂O)	aqua
carbon monoxide(CO)	carbonyl

Rule 6- The oxidation number of the metal present in the complex is written in Roman numerals in parenthesis after the metal name.

Worked example 1

So based on the rules above how would we go about naming the three complexes mentioned at the start of this page? Worked example on how to name the salt pentamminechlorocobalt(III)

Worked example on how to name the salt pentamminechlorocobalt(III)

Worked example 2

The second salt we had to name from the examples above has the formula K₄[Fe(CN)₆], this time the complex [Fe(CN)₆]^4- will have an overall charge of 4^-, we know this because there are four potassium ions (K⁺) in the formula for the salt and each of these ions has a +1 charge. Now the complex also contains six cyanide ions (CN^-) each of which has a 1^- charge, this obviously means that the six cyanide ions will contribute an overall charge of -6, so in order for the complex to have an overall charge of 4^- the iron ion present must have a charge of 2⁺ (Fe²⁺).

Rule 7- The metal complex in this example has a negative charge, that is it is an anion, the metal ion present in these anionic complexes is named by adding the letters -ate to the end of the metal ion in the complex. The Latin name of the metal (except mercury) is also used when one is available, for example the table below lists the Latin name for some common metals and how these are named when they are present in an anionic complex. The oxidation state of the metal as before is also indicated using Roman numerals.

Metal name	Latin name	Metal name in anionic complex
iron	ferrum	ferrate
copper	cuprum	cuprate
gold	aurum	aurate
tin	stannum	stannate
silver	argentum	argentate
manganese	manganese	manganate
vanadium	vanadium	vanadate

So to name the salt K₄[Fe(CN)₆] simply follow the rules outlined above, taking into account that the complex in this example is anionic, the method used is outlined below:

Worked example on how to name the salt potassium hexacyanoferrate(II)

Worked example 3

To name the final complex [Co(en)₃]Cl₃ requires one more rule, now we stated in rule 3 that the Greek prefixes di, tri, tetra, penta and hexa were used to indicate the number of ligands attached to the central metal ion in a complex, however if the name of the attached ligand already uses one of these Greek prefixes; as is the case with ethylenediamine then an alternative system is used to indicate the number of attached ligands, here the number of attached ligands is indicated using the prefixes bis (2), tris (3), tetrakis (4), pentakis (5) and hexakis (6). So the salt [Co(en)₃]Cl₃ will be named: tris(ethylenediamine)cobalt(III) chloride.

Nomenclature

Naming metal complexes

Naming complexes

Worked example 1

Worked example 2

Worked example 3

Practice questions

Check your understanding - Questions on naming complexes.

Check your understanding - Additional questions on naming complexes.

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