Nomenclature of Coordination Compounds: Rules and Explanation

Coordination compounds are an important chapter of inorganic chemistry. Transition metals of the periodic table usually make complex compounds. The metal atoms in these compounds are bound to multiple anions or neutral molecules. These compounds are a different type of compound known as coordination compounds.In many coordination compounds, there is a metallic element as the central atom. That is why they are also known as metal complexes. As mentioned above, they generally contain a transition element as the central atom. It should be noted here that the central atom in these complexes is called the coordination centre.

Keyterms: Coordination Compounds, Transition metals, Anion, Neutral Molecules, Atom, Lewis Acids

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Important Terms Involving Coordination Compounds

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The explanation of a few vital terms in the chemistry of coordination compounds is mentioned below.

1. Coordination Entity

When there is bonding between a central metal atom or ion with a fixed number of ions or molecules, it is known as a coordination entity. Examples include [CoCl₃(NH₃)₃] and [Fe(CN)₆]^4-

2. Central Atoms and Central Ions

As it is mentioned, the atoms and ions to which a set variety of atoms, molecules, or ions are bound are referred to as the central atoms and therefore the central ions.

In coordination compounds, the central atoms or ions are usually Lewis Acids and might, therefore, act as electron-pair acceptors.

3. Ligands

The atoms, molecules, or ions that are bound to the coordination center or the central atom/ion are termed as ligands.

These ligands will either be a simple ion or molecule (such as Cl^– or NH₃) or in the type of giant molecules, like ethane-1,2-diamine (NH₂-CH₂-CH₂-NH₂).

4. Coordination number

It is the amount of ligand donor atoms, to which the metal is directly bonded. For example, within the coordination complex given by [Ni(NH₃)₄]²⁺, the coordination number of nickel is four.

5. Coordination Sphere

The non-ionizable part of a complex compound that consists of central transition metal ions encircled by neighboring atoms or groups encircled in square brackets.

The coordination centre, the ligands connected to the coordination centre, and therefore the net charge of the chemical compound as a full, form the coordination sphere when written along.

This coordination sphere is typically accompanied by a counter ion (the ionizable groups that attach to charged coordination complexes).

Example: [Co(NH₃)₆] – coordination sphere

6. Coordination polyhedron

The geometric shape, formed as a result of bonding of ligands and coordination centre is the coordination polyhedron.

Examples of such spatial arrangements in coordination compounds embrace tetrahedral and square planar.

7. Oxidation number

It is defined as the charge that the central atom would carry, if all the ligands along with their shared electrons are removed from the central atom.

For example, the oxidation number of the platinum atom in the complex [PtCl₆]^2- is +4.

The video below explains this:

IUPAC Nomenclature of Coordination Compounds Detailed Video Explanation:

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Why will we need to name the compounds?

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How would your teacher call you and your ally if you both didn’t have any names? In a similar manner, it is vital to have a set of rules which assign a unique name and formula to each coordination compound. 

Just like we all know that nomenclature of organic compounds is vital, naming the varied coordination or complex compounds is additionally important. In this chapter, we are going to see how the varied complex compounds get their names. We will look into the few easy rules to follow for the same.

Also, it becomes important while you deal with isomers. We tend to follow many rules of the International Union of Pure and Applied Chemistry (IUPAC) system while naming these compounds. in a very shell, the principles are as follows:

In Coordination Compounds,

• Within the coordination entities, we tend to list down the central atom/ion 1st followed by the ligands.
• After this, we tend to list down the ligands in alphabetical order.
• We write the formula of the coordination entity in square brackets.
• Ligand abbreviations are to be boxed in in parentheses.
• We should not keep any space between the ligands and the metal inside a coordination sphere.
• The charge on the cation(s) ought to be adequate to the charge of the anion(s).

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Rules of Complex Nomenclature

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Rule 1- While naming a coordination compound, we have to always name the cation before the anion. This rule doesn't count the very fact of whether or not the complexion is cation or anion. Let us perceive this with the assistance of an example.

Na[Co(NH)₄(Cl)₂] → sodium is to be named first followed by [Co(NH)₄(Cl)₂]

Sodium tetraamminedichlorocobalt (I)

[Co(NH)₄(Cl)₂]SO₄ → [Co(NH)₄(Cl)₂] is to be named first followed by SO₄

Tetraamminedichlorocobalt (0) sulfate.

Rule 2- When we see that there are multiple kinds of ligands present in any coordination compound, we have to name the ligands in alphabetic order by the name of the central metal atom/ion.

• Name of the anionic ligands ends with ‘o’.

For example, Chloride → Chlorido, Nitrate → Nitrito

• For neutral ligands, their common name is employed as such e.g.

H₂NCH₂CH₂NH₂ → ethylenediamine

H₂O → aqua

NH₃ → amine

CO → carbonyl

N₂ → dinitrogen

O₂ → dioxygen

Rule 3- If the names of the ligands have a numerical prefix, then we have to use the terms like bis, tris, and tetrakis. this will represent the matter to which they refer being placed in parentheses. For instance, we have a tendency to name [NiCl₂(PPh₃)₂] as dichloridobis(triphenylphosphine) nickel (II)

Rule 4- After naming the ligand in alphabetic order, we have to name the central metal atom/ion.

If the advanced ion is an anion, the name of metal ends with the suffix -ate for Latin name.

Rule 5- We provide the oxidation state of the metal within the complex as a roman numeral in parentheses.

Rule 6- We name the neutral complex molecule the same as that of the complex cation.

Rule 7- There are some ligands like NO₂, CN that are hooked up to the central metal atom/ion through totally different atoms. We have to name them as follows:

Thus, M-NO₂ → nitro

M-ONO → nitrito

M-SCN → thiocyanato

M-NCS → isothiocyanato

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