Naming Coordination Compounds

Coordination compounds are made of central metal atom/ion(s), surrounded by a specified number of ligands. The ligands can be atoms, ions, or molecules and are attached to the metal through a coordinate covalent bond. The nomenclature of coordination compounds is based on recommendations of the Ionic Nomenclature Committee of the International Union of Pure and Applied Chemistry (IUPAC). The various aspects of naming coordination compounds are:

1. Order of listing ions

The first rule is that the names of neutral coordination compounds are given without spaces. For ionic coordination complexes, the cations are named before the anions, like all ionic substances. For instance, NaCl is called sodium chloride, and K2[PtCl6] will be named Potassium hexachloroplatinate (IV).

2. Sequence of nomenclature

The ligands are named first, before the metal atom/ion. Afterward, the name of the metal is listed. Finally, a parenthesis containing the oxidation state of the metal is written. The oxidation state is always enclosed in a parenthesis and can be determined from the overall charges on the ligands, and that on the coordination sphere. In the case of K2[CuBr4], the oxidation state of copper is two, so its name will be written as Potassium tetrabromocuprate(II). 

3. Names of ligands

Neutral ligands are named like the respective molecule, without any spaces. Water and ammonia molecules, when acting as ligands, are called aqua and ammine, respectively.

Moreover, positive ligands end with -ium whereas negative ligands end with -o. For example, H2NCH2CH2NH2 is ethylenediamine, O2- is called oxo, and NH2NH3+ is referred to as hydrazinium. 

If the name of the uncoordinated anion ends with “ate”, it is changed to “ato” when it acts as a ligand e.g., sulfate ion will be called sulfato when acting as a ligand.

Alternatively, in case the name of the uncoordinated ion ends in “ite”, it is changed to “ito”. E.g., nitrite ion will be called nitrito, as a ligand.

Moreover, the name of the uncoordinated ions that end with “ide”, the ligand name will be “ido”. For example, azide ion when acts as a ligand, is termed azido.

An exception are the organic groups, which although considered anions, are allotted their regular names without an “o” at the end. The methyl group will still be referred to as methyl.

Negative, Positive and Neutral Ligands

4. Ligand order

Ligands having a negative charge are named first, followed by neutral ligands, and finally those with a positive charge in their rare occurrence.

Within each of the charge groups, the ligands have their names written in alphabetical order, ignoring the prefixes identifying the number of each of the ligands. 

In the example that follows, chloro and nitro are negative ligands, and ammine is a neutral ligand. So the complex [Pt(NH3)4(NO2)Cl]SO4 will be called Chloronitrotetraammineplatinum(IV) sulfate.

5. Number of ligands

The number of similar ligands is given the prefixes di for two, tri for three, tetra for four, and so on. Hence, the term hexaaqua will be included in the name of the complex if six water ligands are present. Furthermore, the number of chelate ligands is designated using prefixes of bis for two, tris for three, tetrakis for four, etc. This is demonstrated in the following examples:

[Co(NH3)4Cl2]Cl is called Dichlorotetraamminecobalt(III) chloride.

[Co(en)2Cl2]Cl is called Dichlorobis(ethylenediamine)cobalt(III) chloride.

6. Oxidation State of Metals

The oxidation state of the central metal atoms/ ions is specified by the corresponding Roman numbers (I, II, III, IV,….) in parenthesis after the end of the name of the complex. On the contrary, a minus sign is used to indicate a negative oxidation state:

Na[Mn(CO)5] is termed Sodium pentacarbonylmanganate(-I).

Ni(CO)4 is called Tetracarbonylnickel(O).

K4[Fe(CN)6] will be named Potassium hexacyanoferrate(II).

7. Termination of names

For anionic complexes, -ate is added at the end along with the name of the metal. On the other hand, cationic and neutral complexes have their names as given, without any additions:

Na[Co(CO)4] is named Sodium tetracarbonylcobaltate(-I).

[Ni(DMG)2] is called Bis(dimethylglyoximato)nickel(II).

8. Linkage isomerism

Particular ligand molecules may have more than one point of attachment to the central metal atom/ion. Therefore, it must be indicated while naming the complexes. In the case of thiocyanate and nitrite ions, the following names should be used:

Nitro for -NO2, and nitrito for -ONO.

Thiocyanato for -SCN, and isothiocyanato for -NCS.

9. Other isomers

Optical isomers are given the symbols Δ or Λ. In addition, geometrical isomers are identified by cis or trans and mer or fac, with the latter two standing for meridional or facial, respectively.

10. Bridging ligands

The bridging groups (attached to two metal atoms/ions) present in the complexes are specified using the Greek letter ‘μ’ before naming the ligands. In case there are two bridging ligands of the same type, di-μ- will be used as a prefix.

If the molecule is symmetrical, a more compact name can be stated by writing the name of the bridging ligand first.

Concepts Berg

What are the rules for naming coordination complexes?

There are several rules for naming coordination complexes which include the sequence of nomenclature, order of listing ions, names of ligands, order of writing ligand names, referring to number of ligands of the same type, oxidation state of the central metal atom/ion, termination of names, and referring to bridging ligands and isomers.

How do you name a ligand?

Neutral ligands are named exactly like their molecule, with any spaces omitted. Ligands that are positive have their names end with -ium. On the other hand, negative ligands have -o added at the end of their name.

How do you name coordination compounds?

If the coordination compound is neutral or cationic, its name appear as it is. For an anionic coordination compound, its name ends with -ate. Similar to normal ionic compounds, the cation is named before the anion.

What is the IUPAC name of the complex K3[Fe(CN)6]?

The IUPAC name of the complex K3[Fe(CN)6] is Potassium hexacyanoferrate.

Is it Chloro or Chlorido for naming of Cl- ligand nomenclature of coordination compounds?

In the modern nomenclature, it is preferred to write chlorido when the chloride ion is acting as a ligand. Chloro, being a part of the older nomenclature, can be written when the chloride ion is not included in the coordination sphere.

What are some tough IUPAC naming of coordination compounds?

Dichloridobis(ethylenediammine)cobalt(III) chloride for [Co(en)2Cl2]Cl.

Pentacarbonyltriphenylphosphinechromium(O) for [Cr(PPh3)(CO)5].

Tetrapyridineplatinum(II) tetrachloridoplatinate(II) for [Pt(py)4][PtCl4].

What naming should be given to coordination compounds having 4 methyl groups?

A coordination compound having four methyl groups will have tetramethyl included in its name.

What are the basic applications of coordination compounds?

Coordination compounds are used as catalysts for the manufacture of organic substances. Additionally, they are used for hydrometallurgical processes for the extraction of several metals from their ores. Moreover, coordination complexes have long been used as pigments and dyes. Furthermore, the complexes are an integral part of hemoglobin, chlorophyll and vitamin B-12.


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