Page 88 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-5.4 C O M P O S I T I O N A L N O M E N C L A T U R E

Examples:
thallium tris(iodide), or thallium(III) iodide,
1. TlI 3
or thallium(3þ) i odide
2. Tl(I 3 ) t hallium triiodide(1 ), or thallium(I) ( triiodide),
or thallium(1þ) ( triiodide)

Both compounds in Examples 1 a nd 2 h ave the overall formula T lI 3 and both could be
named by the simple s toichiometric name t hallium triiodide. However, it is possible, and
usually desirable, to convey more information in the name.
The compound in Example 1 c onsists of iodide, I , a nd thallium, i n t he proportion 3:1,

whereas t he compound in Example 2 c onsists of triiodide(1 ), I 3 , a nd thallium in
the proportion 1:1. In the first name for the first compound, then, the multiplicative prefix
‘tris’ is used to make it completely clear that three iodide ions are involved rather than
one triiodide ion. The a lternative names use the oxidation number III for thallium and the
charge number 3þ, r espectively, to convey indirectly the proportions of the constituents.
In the first name in Example 2, it is clear that the electronegative constituent is a
homopolyatomic e ntity with charge 1. The next two names convey this indirectly by adding
the oxidation number or the charge number to the name t hallium; including the parentheses
around t he name of the electronegative part reinforces that it is a h omopolyatomic e ntity.
For both compounds, f ully explicit names including the charge number for the thallium
ion, although partly redundant, are also acceptable. Thus, thallium(3þ) t ris(iodide) and
thallium(1þ) t riiodide(1 ), for Examples 1 a nd 2 r espectively, may be preferable in
systematic contexts s uch as indexes and registries.

Examples:

mercury dichloride, or mercury(II) chloride,
3. HgCl 2
or mercury(2þ) c hloride
dimercury dichloride, o r ( dimercury) dichloride,
4. Hg 2 Cl 2
or dimercury(2þ) c hloride

In Example 4, the first name i s p urely stoichiometric, whereas the second n ame contains more
information in indicating that the compound contains a h omodiatomic cation. In the last name,
where the charge of the dication is specified, the prefix ‘di’ f or ‘chloride’ is not necessary.

Examples:

disodium (trisulfide) (this indicates the presence of the
5. Na 2 S 3
polyatomic anion),
or sodium trisulfide(2 ) ( with the charge on the anion i ndicated,
the multiplicative prefix on the cation name is not necessary)
6. Fe 2 S 3 diiron tris(sulfide), or iron(III) sulfide

2 2
Salts which contain anions that are S n chains, as well as those c ontaining several S
anions, are both referred to as ‘polysulfides’ but, as demonstrated, names may be given that
provide a d istinction between t hese c ases.

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