Nomenclature Of Coordination Compounds

For naming of coordination compounds just follow the rules briefly explained below.

1) Oxidation state: Firstly, (oxidation state O.S.) of metal must be known. Oxiadtion states can be found in many ways. One of the method to find it is the given formula:

Charge on sphere = Mⁿ⁺ + L

Charge on ligand is fixed but, charge on metal is different. Let’s find O.S. of Cobalt. Consider a cobalt complex [Co(NH₃)₄(OH)₂]⁺¹, its oxidation state will be:

Charge on sphere = Mⁿ⁺ + L

For the above complex, O.S. of NH₃ is 0, as it is a neutral ligand. O.S. of OH = -1, and charge on sphere = +1. So,

+1 = Mⁿ⁺ + (-2)

+1 +2 = Mⁿ⁺

+3 = Mⁿ⁺

Hence O.S. for Cobalt is +3. Take some other examples.

[Ni (H₂O)₄]⁰, here oxidation state of Nickel will be 0, as water is neutral ligand. [Ni (H₂O)₄]Cl₂, here oxidation state of Nickel will be +2, as Chlorine has -2 oxidation state.

Oxidation state of metal is written in roman numbers within brackets. Oxidation state of metal is written at last of metal name.

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2) Order of ionic species: Cationic species (+ve) are named first. Anionic species (-ve) are named later like Sodium Chloride Na⁺Cl^-. Take an example for coordination compound

Na₂[Co(Cl)₆H₂O]^-2

It is named Sodiumaquapentachlorcobaltate III. Here Na₂, a cation, is named before anion. Take another example

[CoCl(H₂O)₅]⁺²Cl₂

This complex is named as Pentaaquachlorocobalt III chloride. Here chloride, an anion, is named after cation. Atoms/species out of the sphere are not ligands.

3) Complex name: The first letter of complex should be capital and all others small. There should be no space between any letters in the name of complex. Only cationic and anionic part should have space.

4) Ligand name: if more than one ligands are present then they are named alphabetically. For example, for the complex [Cr (H₂O)(Cl)(en)₂]SO₄ ligand aqua will come first, then chloro and then ethylenediamine and the named is:

Aquachlorobis(ethylenediamine)chromium(III) sulpahte

If same ligand has more than molecules then they are written as di, tri, tetra, penta, hexa etx. For example: Cl₅ will be Pentachloro, (NH₃)₂ will be diamino. (di, tri, tetra, penta etc are not taken into consideration while naming alphabetically. Common names of molecules are used for naming them as ligands. For example

Cl^-	chloro
OH^-	Hydroxo
H₂O	Aqua
NH₃	Ammino
CO	Carbonyl

5) Chelating agents: They are abbreviated. For example, pyridine as “Py” and ethylenediamine as “en”. If more than one chelating agents are present then they are named as “bis” for two, “tris” for three, “tetrakis” for four, “pentakis” for five and so on. The chelating agents are written in brackets as explained in point number 4.

6) Termination the complex name:

K₃[Fe(CN)₅CO]^-3

Potassium carbonylpentacyanoferrate(II)

If sphere have negative charge, the name of metal end at "ate" e.g. Na₂SO₄ is named as Sodium sulpahte. If sphere has negative charge, then the name of metal cobalt will be “cobaltate”. But, if the sphere has positive charge then the name of metal cobalt will be as it is.

7) Naming salts: salts are not numerized. For above example we will not write tripotassium but only potassium. Similarly, FeCl₂ will be “ferric chloride” not ferric dichloride. And AlCl₃ will be “Aluminum chloride” not aluminum trichloride.

We can see the above example following all rules of nomenclature. Cationic specie (potassium) named first and anionic (cyano) named later. More than two ligands have been named here alphabetically as carbonyl first and then cyano. Five molecules of cyano are written as pentacyano. The oxidation state is written at the end of metal in roman numbers within bracket. Potassium not written as ripotassium. And finally, metal name ends with “ate” as the sphere has negative charge.

For more comprehension follow the given brief formula while naming a complex.