Page 162 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-9.2 C O O R D I N A T I O N C O M P O U N D S

9. Na[PtBrCl(NH 3 )(NO 2 )]
sodium amminebromidochloridonitrito-kN-platinate(1 )

10. [Fe(CNMe) 6 ]Br 2
hexakis(methyl isocyanide)iron(II) bromide

11. [Co(en) 3 ]Cl 3
tris(ethane-1,2-diamine)cobalt(III) trichloride

IR-9.2.3 Formulae of coordination compounds

A ( line) formula of a c ompound is used to provide basic information about the constitution of
the compound in a c oncise and convenient manner. Different applications may require
flexibility in the writing of formulae. Thus, on occasion it may be desirable to violate t he
following guidelines in order to provide more information about the structure of the compound
that the formula represents. In particular, this is the case for dinuclear compounds where a
great deal of structural i nformation can be provided by relaxing the ordering principles
outlined in Section IR-9.2.3.1. (See also Section IR-9.2.5, particularly S ection IR-9.2.5.5.)

IR-9.2.3.1 Sequence of symbols within the coordination formula
(i) The central atom symbol(s) is (are) listed first.

(ii) The ligand symbols (line formulae, abbreviations or acronyms) are then listed in
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alphabetical order (see Section IR-4.4.2.2). Thus, CH 3 CN, MeCN and NCMe would
be ordered under C, M a nd N r espectively, and CO precedes Cl because single letter
symbols precede two letter symbols. The placement of the ligand in the list does not
depend on the charge of the ligand.

(iii) More information is conveyed by formulae that show ligands with the donor atom
nearest the central atom; this p rocedure is recommended w herever possible, even for
coordinated water.

IR-9.2.3.2 Use of enclosing marks

The formula f or the entire coordination entity, whether c harged or not, is enclosed in square
brackets. When ligands are polyatomic, their formulae are enclosed in parentheses. Ligand
abbreviations are also usually enclosed in parentheses. The n esting order of enclosing marks
is as given i n S ections IR-2.2 and IR-4.2.3. Square brackets are used only to enclose
coordination entities, and parentheses and braces are nested alternately.
Examples 1–11 in Section IR-9.2.2.4 illustrate the use of enclosing marks in formulae.
Note also that in those e xamples there i s n o s pace between r epresentations of ionic species
within a f ormula.

IR-9.2.3.3 Ionic charges and oxidation numbers
If the formula o f a charged coordination entity is to be written w ithout that of any counter-
ion, the charge is indicated outside the square bracket as a r ight superscript, with the number
before the sign. The oxidation number of a c entral atom may be represented by a R oman
numeral, which should be placed as a r ight superscript on the element symbol.

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