Nomenclature of Coordination Compounds: Molecules having atoms \({\rm{C}}\) and \({\rm{O}}\) can be named carbon monoxide and carbon dioxide, and molecules having \({\rm{C}}\) and \({\rm{H}}\) can be named methane, ethane, propene, hexyne, etc. There are various molecules that have the same atoms, but they have different names. This is because different atoms combine in different ratios and form different compounds.

Coordination Compounds

A compound in which the transition metal is linked to two or more groups of ions or neutral molecules called ligands through coordinate bonds is called a coordination compound. Such a compound is also called a complex compound or Werner Complex. Coordination compounds do not lose their identity in the solution, and the transition metal cation remains bonded to ligands.

Examples of Coordination Compounds

\({\left[ {{\rm{Fe}}{{({\rm{CN}})}_6}} \right]^{3 – }},\,{\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]^{2 + }},\,{\left[ {{\rm{Mn}}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_6}} \right]^{2 + }}\)

Biologically important compounds such as chlorophyll, haemoglobin, and vitamin \({\rm{B}} – 12\) are coordination compounds of magnesium, iron, and cobalt.

Ligands

The groups attached to the central metal ion/atom in a complex are called ligands. The ligands may be anionic \({\rm{C}}{{\rm{N}}^ – },\,{\rm{C}}{{\rm{l}}^ – },\,{{\rm{C}}_{\rm{2}}}{\rm{O}}_4^{2 – }\) or neutral molecules like \({{\rm{H}}_2}{\rm{O}},\,{\rm{N}}{{\rm{H}}_3},\,{\rm{CO}}\) of their nature, all types of ligands have lone pairs of electrons on one or more atoms.

Types- On the basis of the number of donor atoms available, the ligands have been classified as unidentate, bidentate, tridentate, polydentate, and chelating ligands.

Nomenclature of Coordination Compounds

A large number of coordination compounds of various types are synthesized every year in different laboratories. If these compounds were named randomly, all the chemists of the world might not have effective communication with each other. Therefore, it is essential to adopt a uniform system of naming these complex compounds.

A systematic method of naming coordination compounds was officially formulated by the International Union of Pure and Applied Chemistry (IUPAC).

IUPAC Rules of Nomenclature of Coordination Compounds

1. In a coordination compound, first, the name of the cation is written, followed by the name of the complex ion. If there is an anion, its name is written in the last.
2. In naming a complex ion, first, the names of the ligands are written, followed by the name of the central metal atom or metal ion.

Different types of ligands are named differently.
a. Negative Ligands ending with -ide are named, replacing ‘ide’ or only ‘e’ with the suffix ‘o’. For example,

Anion	Symbol	Ligand name
Chloride	\({\rm{C}}{{\rm{l}}^ – }\)	Chloride
Bromide	\({\rm{B}}{{\rm{r}}^ – }\)	Bromide
Cyanide	\({\rm{C}}{{\rm{N}}^ – }\)	Cyanido
Amide	\({\rm{NH}}_2^ – \)	Amido
Imide	\({\rm{N}}{{\rm{H}}^{2 – }}\)	Imido
Nitride	\({{\rm{N}}^{2 – }}\)	nitrido
Oxide	\({{\rm{O}}^{2 – }}\)	oxo
Peroxide	\({\rm{O}}_2^{2 – }\)	peroxo
Hydroxide	\({\rm{O}}{{\rm{H}}^ – }\)	hydroxo
Phosphide	\({{\rm{P}}^{3 – }}\)	phosphido
Sulphide	\({{\rm{S}}^{2 – }}\)	sulphido
Hydrogen sulphide	\({\rm{H}}{{\rm{S}}^ – }\)	Hydrogen sulphido

Anionic ligands whose names end in -ite or -ate are named, replacing ‘e’ with ‘o’. For example,

Anion	Symbol	Ligand name
Acetate	\({\rm{C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ – }\)	acetato
Crabonate	\({\rm{CO}}_3^{2 – }\)	carbonate
Nitrate	\({\rm{NO}}_3^ – \)	Nitrato
Nitrite	\({\rm{ON}}{{\rm{O}}^ – }\)	Nitro or nitrito \( – {\rm{O}}\)
Nitrite	\({\rm{NO}}_2^ – \)	Nitro or nitrito \( – {\rm{N}}\)
Sulphate	\({\rm{SO}}_4^{2 – }\)	Sulphato
Sulphite	\({\rm{SO}}_3^{2 – }\)	Sulphito
Oxalate	\({{\rm{C}}_2}{\rm{O}}_4^{2 – }\)	Oxalate
Thiosulphate	\({{\rm{S}}_2}{\rm{O}}_3^{2 – }\)	thiosulphate

b. Positive Ligands, end with -ium. For example,

Symbol	Ligand name
\({\rm{NO}}_2^ + \)	nitronium
\({\rm{N}}{{\rm{O}}^ + }\)	nitrosonium
\({\left[ {{\rm{N}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}} \right]^ + }\)	hydrazinium

c. Neutral Ligands have no special words in the ending. For example,

Neutral Ligands	Names
\({\rm{N}}{{\rm{H}}_3}\)	ammine
\({{\rm{H}}_2}{\rm{O}}\)	aqua
\({\rm{NO}}\)	Nitrosyl
\({\rm{CO}}\)	Carbonyl
\({{\rm{O}}_2}\)	dioxygen
\({{\rm{N}}_2}\)	dinitrogen
\({\rm{CSHSN}}\)	pyridine
\({{\rm{H}}_2}{\rm{NC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{N}}{{\rm{H}}_2}\)	ethylenediamine

d. Organic Ligands are given their own names. For example,

\({\rm{C}}{{\rm{H}}_3}{\rm{ ( methyl), }}{{\rm{C}}_2}{{\rm{H}}_5}{\rm{ (ethyl), }}{{\rm{C}}_6}{{\rm{H}}_5}{\rm{ (phenyl), P}}{\left( {{{\rm{C}}_6}{{\rm{H}}_5}} \right)_3}{\rm{ (triphenylphosphine)}}{\rm{. }}\)

3. Order of naming the ligands- Ligands are named in alphabetical order of preference without any consideration of charge and prefixes di, tri, etc.

For example, the complex \(\left[ {{\rm{PtCl}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}\left( {{\rm{N}}{{\rm{O}}_2}} \right)} \right]{\rm{S}}{{\rm{O}}_4}\) is named Tetraamminechloridonitroplatinum \(\left( {{\rm{IV}}} \right)\) sulphate.

4. Numerical prefixes to indicate the number of ligands- If there are several ligands of the same type, the prefixes like di, tri, tetra, penta, etc., are used to indicate the number of ligands of that type.

For example,

\(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]{\rm{C}}{{\rm{l}}_3}\) is named Hexaamminecobalt \(\left( {{\rm{III}}} \right)\) chloride.

\(\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right]\) is named Tetracarbonylnickel \(\left( 0 \right).\)

If the ligand is complex and is repeated two, three or four times, then the word bis, tris, tetrakis, etc., are used.

For example,

\(\left[ {{\rm{Co}}{{({\rm{en}})}_3}} \right]{\rm{C}}{{\rm{l}}_3}\) is named Tris-(ethane\( – 1,\,2 – \)diamine) cobalt \(\left( {{\rm{III}}} \right)\) chloride.

5. The naming of the complex ion- Positive and neutral complexes have no special ending. In such cases, ligands are named alphabetically and without separation or hyphen. The name of the central metal is written as such, followed by its oxidation state indicated by a Roman numeral \(\left( {{\rm{II,}}\,{\rm{III,}}\,{\rm{IV}}} \right).\) For example,

\(\left[ {{\rm{Ag}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{2}}}} \right){\rm{Cl}}\) is named Diamminesilver \(\left( {\rm{I}} \right)\) chloride

\(\left[ {{\rm{PtC}}{{\rm{l}}_{\rm{4}}}{{\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)}_{\rm{2}}}} \right]\) is named Diamminetetrachloridoplatinum \(\left( {{\rm{IV}}} \right)\)

\(\left[ {{\rm{Ni(dmg}}{{\rm{)}}_{\rm{2}}}} \right]\) is named as Bis-(dimethylyglyoximato) nickel \(\left( {{\rm{II}}} \right)\)

In the case of anionic complexes, the Latin name of the central metal atom ends in “-ate” followed by the oxidation number in Roman numeral in the bracket.

\({{\rm{K}}_{\rm{3}}}\left[ {{\rm{Fe(CN}}{{\rm{)}}_{\rm{6}}}} \right]\) is named as Potassium hexacyanoferrate \(\left( {{\rm{III}}} \right){\rm{.}}\)

6. Point of attachment- When a ligand can coordinate through more than one atom, the point of attachment is indicated by placing the symbol of the donor atom attached, after the name of the ligand separated by a hyphen. The attachment can also be indicated by using different names (e.g., thiocyanato and isothiocyanato). For example,

If \( – {\rm{N}}{{\rm{O}}_{\rm{2}}}\) coordinates through \( – {\rm{N,}}\) it is called nitrito \( – {\rm{N}}\) (or nitro) and if through \( – {\rm{O,}}\) it is called nitrito\( – {\rm{O,}}\) (or nitrito).

If \( – {\rm{SCN}}\) coordinates through \( – {\rm{S}},\) it is called thiocyanato or thiocyanato\( – {\rm{S}}\) and if through \( – {\rm{N,}}\) It is called thiocyanato \( – {\rm{N}}\) or isothiocyanato.

For example,

\(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}({\rm{ONO}})} \right]{\rm{S}}{{\rm{O}}_4}\) is named Pentaamminenitritocobalt\(\left( {{\rm{III}}} \right)\)sulphate.

\({\left( {{\rm{N}}{{\rm{H}}_4}} \right)_3}\left[ {{\rm{Cr}}{{({\rm{SCN}})}_6}} \right]\) is named Ammonium hexathiocyanato \( – {\rm{S}} – \)chromate\(\left( {{\rm{III}}} \right)\) or Ammonium hexathiocyanatochromate\(\left( {{\rm{III}}} \right).\)

7. The naming of bridging groups- For ligands that acts as a bridge between two metal ions, the Greek letter \({\rm{\mu }}\left( {{\rm{mu}}} \right)\) is written before their names and is separated from the rest of the complex by a hyphen.

For example,

Solved Examples on Nomenclature of Coordination Compounds

Q.1. Using IUPAC norms, write the systematic names of the following:
Ans:
1. \(\left[ {{\rm{CoCl}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}\left( {{\rm{N}}{{\rm{O}}_2}} \right)} \right]{\rm{Cl}}\)
Solution: Tetraamminechloridonitrocobalt \(\left( {{\rm{III}}} \right)\) chloride.
2. \(\left[ {{\mathop{\rm PtCl}\nolimits} {{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_2}\left( {{\rm{N}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}} \right)} \right]{\rm{Cl}}\)
Solution: Diamminechloro(methylamine)platinum \(\left( {{\rm{II}}} \right)\) chloride.
3. \({\left[ {{\rm{Co}}{{({\rm{en}})}_3}} \right]^{3 + }}\)
Solution: Tris-(ethylenediamine) cobalt \(\left( {{\rm{III}}} \right)\) ion.
4. \(\left[ {{\rm{CoCl}}{{({\rm{en}})}_2}\;{{\rm{F}}_2}} \right]{\rm{Cl}}{{\rm{O}}_4}\)
Solution: Bis-(ethylenediamine) difluoridocobalt \(\left( {{\rm{III}}} \right)\)perchlorate.

Derivation of Formula of a Complex from its IUPAC Name

The rules followed by IUPAC to write the formula of a complex in a systematic way are:
1. First, the symbol of the outer cation (if any) is written.
2. After this, the symbol of the central metal atom or ion is written, followed by the symbol and number of the ligands.
3. Heteroatomic ligands are enclosed by \(\left( {} \right)\) and their number is indicated by a numeral outside the parentheses.
4. The central atom and ligands are enclosed by square brackets \(\left[ { } \right].\)
5. The charge on each ligand and central metal ion is computed, and a suitable subscript is used to indicate the number of outer cations or anions so that the total charge on the complex molecule becomes zero.
6. In a cationic complex molecule, first, the formula of complex ion is written, then that of the outer anions.

Solved Examples

Q.1. Write the formulae of the following complex compounds:
Ans:
1. Iron \(\left( {{\rm{III}}} \right)\) hexacyanoferrate \(\left( {{\rm{II}}} \right)\)
Solution- \({\rm{F}}{{\rm{e}}_4}{\left[ {{\rm{Fe}}{{({\rm{CN}})}_6}} \right]_3}\)
2. Tetraammineaquachloridocobalt \(\left( {{\rm{III}}} \right)\) chloride
Solution- \(\left[ {{\mathop{\rm CoCl}\nolimits} \left( {{{\rm{H}}_2}{\rm{O}}} \right){{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{C}}{{\rm{l}}_2}\)
3. Tetracarbonylnickel \(\left( 0 \right)\)
Solution- \(\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right]\)
4. Dichloridobis-(ethane \( – 1,\,2 – \)diamine)cobalt \(\left( {{\rm{III}}} \right)\) ion
Solution- \({\left[ {{\rm{CoC}}{{\rm{l}}_2}{{({\rm{en}})}_2}} \right]^ + }\)

Summary

Through this article, we studied in detail the IUPAC rule to write the names of the coordination compounds. Also, we studied how to write the formula of the coordination compounds from its IUPAC name.

FAQs on Nomenclature of Coordination Compounds

Let’s look at some of the commonly asked questions about nomenclature of coordination compounds:

Q1. How do you write the formula for a coordination compound?
Ans: The rules followed by IUPAC to write the formula of a complex in a systematic way are:
1. First, the symbol of the outer cation (if any) is written.
2. After this, the symbol of the central metal atom or ion is written, followed by the symbol and number of the ligands.
3. Heteroatomic ligands are enclosed by \(\left( \, \right)\) and their number is indicated by a numeral outside the parentheses.
4. The central atom and ligands are enclosed by square brackets \(\left[ \, \right].\)
5. The charge on each ligand and central metal ion is computed, and a suitable subscript is used to indicate the number of outer cations or anions so that the total charge on the complex molecule becomes zero.
6. In a cationic complex molecule, first, the formula of complex ion is written, then that of the outer anion.

Q2. What are the speciality of coordination compounds? Write any four points.
Ans: Some of the importance of coordination compounds are as follows-

1. Coordination compounds are useful in analytical chemistry to detect the presence of cations, to estimate the amount in volumetric analysis.
2. Coordination compounds are useful as a catalyst in industrial processes.
3. Coordination compounds are useful as enzymes in biological processes and as medicines.
4. Coordination compounds find their applications in metallurgy, water softening and electroplating.

Q3. How do you name coordination compounds?
Ans: The naming of the coordination compounds can be done by following rules that are as follows-
1. The name of the cation is given first and the anion second.
2. In the complex ion, the name of the ligand or ligands precedes that of the central metal atom.
3. The names of the ligands generally end with ‘o’ if the ligand is negative and unmodified if the ligand is neutral.
4. A Greek prefix (mono, di, tri, tetra, penta, hexa, etc.) indicates the number of each ligand. If the name of the ligand itself contains the terms mono, di, tri, then the ligand name is enclosed in \(\left( \, \right)\) and its number is given with the alternate prefixes bis, tris, tetrakis instead.
6. If the complex ion is negative, the name of the metal ends with ‘ate’.
7. If more than one ligand is present, then the ligands are named in alphabetical order regardless of the number of each.

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