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Nomenclature of Coordination Complexes
Nomenclature is important in Coordination Chemistry because of the existence of isomers. In 1970, the International Union of Pure and Applied Chemistry (IUPAC), recommended changes to the existing 1957 rules of Inorganic Nomenclature. However, many textbooks do not use these newer rules and should NOT be followed for guidance. For example, the 3rd Edition of "Basic Inorganic Chemistry" by F.A. Cotton, G. Wikinson and P.L. Gaus, John Wiley and Sons, Inc, 1995 page 181, is once again incorrect (the 2nd Edition had it right!).

The rules to follow are outlined below:
1. In naming the entire complex, the name of the cation is given first and the anion second (just as for sodium chloride), no matter whether the cation or the anion is the complex species.

2. In the complex ion, the name of the ligand or ligands precedes that of the central metal atom. (This procedure is reversed for writing formulae.)

3. Ligand names generally end with 'o' if the ligand is negative ('chloro' for Cl-, 'cyano' for CN-, 'hydrido' for H-) and unmodified if the ligand is neutral ('methylamine' for MeNH2).
Special ligand names are 'aqua' for water, 'ammine' for ammonia, 'carbonyl' for CO, 'nitrosyl' for NO.

4. A Greek prefix (mono, di, tri, tetra, penta, hexa, etc.) indicates the number of each ligand (mono is usually omitted for a single ligand of a given type). If the name of the ligand itself contains the terms mono, di, tri, eg triphenylphosphine, then the ligand name is enclosed in parentheses and its number is given with the alternate prefixes bis, tris, tetrakis instead.
For example, Ni(PPh3)2Cl2 is named dichlorobis(triphenylphosphine)nickel(II).

To avoid the confusion as to whether "dimethylamine" means two separate methylamines or the single ligand dimethylamine, then for the former case it should be named as bis(methylamine). Some texts suggest that if a ligand is "complicated" then use the bis, tris multipliers. What constitutes "complicated" is not spelled out however, so a simpler approach is to use them if the name of the ligand is three or more syllables long.

5. A Roman numeral or a zero in parentheses is used to indicate the oxidation state of the central metal atom.

6. If the complex ion is negative, the name of the metal ends in 'ATE' for example, ferrate, cuprate, nickelate, cobaltate etc.

7. If more than one ligand is present in the species, then the ligands are named in alphabetical order regardless of the number of each. For example, NH3 (ammine) would be considered an 'a' ligand and come before Cl- (chloro). (This is where the 1971 rules differ from the 1957 rules. Some texts still say that ligands are named in the order: neutral then anionic).

Some additional notes.
(i) Some metals in anions have special names
      B     Borate    Au    Aurate    Ag    Argentate   Fe    Ferrate
      Pb    Plumbate  Sn    Stannate  Cu    Cuprate
(ii) Use of brackets or enclosing marks.
Square brackets are used to enclose a complex ion or neutral coordination species.
note that it is not necessary to enclose the halogens in brackets.

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Created and maintained by Dr. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created March 1996. Links checked and/or last modified 15th January 2001.