In the early days of chemical research, the chemists used to give individual names to each new compound on the basis of its source, properties possessed by that compound, etc. This led to confusion, because, in many cases, the same compound had more than one name. Example: Methane was named as marsh gas as well as damp fire. But with rapid progress in the field of chemistry and chemical research, the number of known compounds increased vastly and consequently, it became very difficult to give individual names to each compound being discovered, and also to remember these names.
In 1967, the International Union of Pure and Applied Chemistry (IUPAC) was formed and it brought out a scheme for giving systematic names to chemical compounds. This system of nomenclature is known as IUPAC nomenclature. This nomenclature gives an insight into the structure of the compound and its chemical composition.
The rules are formulated separately for organic compounds, inorganic compounds and other miscellaneous compounds.
- The basic carbon skeleton is indicated by the root word.
- These root words differ based on the number of carbons in main chain.
- C1 = meth; C2 = eth; C3 = prop; C4 = buta; C5 = penta; C6 = hexa; C7 = hepta; C8 = octa; C9 = nona; C10 = deca.
- A primary suffix is added to the root word to indicate the saturation or unsaturation in the carbon chain.
- Name of Carbon Chain Primary Suffix Name
- Saturated [C-C] -ane Alkane
- Unsaturated [C=C] -ene Alkene
- Unsaturated [C$\equiv $C] -yne Alkyne
A secondary suffix indicates the functional group present in the carbon compound. Usually the terminal ‘e’ of the primary suffix is replaced by the second suffix.
Functional group Secondary Suffix
- Alcohol -ol
- Aldehyde -al
- Ketone -one
- Carboxylic acid -oic acid
- The group which contains one hydrogen less than the corresponding alkane is called as the alkyl group.
- The group which contains one hydrogen less than the corresponding alkene is called as the alkenyl group.
- The group which contains one hydrogen less than the corresponding alkyne is called as the alkynyl group.
According to the latest IUPAC conventions, the names of ligands surrounding the central metal atom are written in alphabetical order of preference irrespective of whether they are negative or neutral. For example, in the complex [Co(NH3)4Cl2(NO2)]
, the ligands are named in the order amine, chloro and nitro.
A roman numeral or zero is indicated in parentheses next to the central metal to denote its oxidation number (like copper (II) for cupric ion).
Example: [Ag)NH3)2]Cl - Diamminesilver (I) chloride
If the complex is an anion, the ending 'ate' i
s employed. If the complex is a cation or neutral molecule, the name is not changed. For some elements, the ion name is based on the Latin name of the metal (for example- argentate for silver).
Examples are, K3
] - Potassium trioxalatechromate (III) and K4
] - Potassium hexacyanoferrate (II).
Some examples of complexes are
- [Co(en)3]Cl3: Tris (ethylenediamine) cobalt (III) chloride.
- [Ni(NH3)6]Cl2: Hexaamminenickel (II) chloride.
- [Mn(H2O)6 ]2+: Hexaaquamanganese (II) ion.
Examples of Inorganic compounds are:
- Mg3N2 - Magnesium Nitride.
IUPAC Nomenclature of Coordination Compounds
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The first comprehensive system of nomenclature was suggested by A. Werner. Though this has been modified by the inorganic nomenclature committee of the International Union of Pure and Applied Chemistry (IUPAC), the fundamental rules suggested by Werner essentially remain the same.
The inorganic nomenclature committee formulated certain rules for naming coordination complexes. They are as follows.
- The positive part of a coordination compound or the cation is named first, followed by the negative part or the anion.
- The ligands (the groups attached to the central metal atom of a complex) are named first, followed by the central metal atom. The prefixes, di, tri, tetra, etc. are used to indicate the number of each kind of ligand present. The prefixes bis (two ligands), tris (three ligands), etc. are used when the ligand name is complicated.
- The ligands are named in alphabetical order. Names of the anionic ligands end in 'o', those of the cationic ligands end with an 'ium'. Neutral ligands have their regular names except that H2O is named 'aqua', NH3 'ammine', NO- 'Nitrosyl' and CO 'carbonyl'.
- The oxidation state of the central metal is indicated in Roman numerals in parenthesis.
- When a complex species has negative charge, the name of the central metal ends in -ate. For some elements, the ion name is based on the Latin name of the metal (for example aggregate for silver).
- A ligand bound to two metal atoms to form a bridge is indicated by prefixing it with µ.
- For poly nuclear complexes, containing two or more metal atoms, the ligands binding two metal atoms are termed as bridging ligands. In the naming of such ligands, the word µ is added before the name of such ligands and is separated from the rest of the complex by a hyphen (-). For more than one bridging group, the word is repeated before each bridging group.
is named as bis (ethylenediamine)cobalt (III) - µ-imido -µ - hydroxobis (ethylenediamine) cobalt (III) ion.
The naming of coordination complex, resembles the naming of a normal inorganic salt, where the positive part is named first followed by the negative part, or the anion. The Greek prefixes, mono, bi, tri, tetra, penta, etc
are introduced to indicate the number of each ligand surrounding the central metal atom. The use of mono is avoided, since the single number is understood as such.
An inorganic compound is named based on the bond present in it. In an ionic compound, the naming is as follows.
Ionic compound contains two parts, cation or the positive ion and anion or the negative ion.
- Simple cations borrow the name from the element itself. So, they are named as such. Example: Na+ = Sodium, Cu2+ = Copper.
- For the negative part, or the anion, the name is formed from the element itself, but with a suffix, -IDE. Example: Cl- : Chloride, O2- = Oxide, etc.
- The number depicted on an anion or cation is not mentioned in the name, because, the electro neutrality principle means that the total positive charge would be equal to the total negative charge. Thus, Na2O would simply be sodium oxide and not disodium oxide.
- A simple binary inorganic compound, can, thus be named with the positive ion first and then the negative ion.
- Also, for transition metal cations and the elements with multiple oxidation states, the oxidation state of the cation is written in the brackets, to indicate the oxidation state. Example - CuCl2 - Copper (II) chloride.
For a covalent compound, the naming is slightly different. The electro neutrality principle does not work in the case of covalent compounds.In a covalent compound, the central atom is named first, borrowing the name of the element itself. The second part, or the surrounding atoms are named as the negative part in an ionic compound, with -ide as a suffix.
For example: CO2
= Carbon dioxide, PCl3
- Phosphorus tri chloride. Thus, the second part of a covalent compound is named with a suffix of bi, tri, tetra, penta, etc.