Page 172 - 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

of specifying which ligating atoms bind to which central atom. In order to do this, the central
atoms must b e i dentified, i.e. by assigning n umbers to these a toms according to the order in
which they appear in the central atom list. (The later the central atom elements appear i n
Table VI, the lower the numbers they are assigned.)
Additional rules are needed when t here is more than one central atom of the same
element (see Sections IR-9.2.5.5 and IR-9.2.5.6) except i f t he presence of symmetry in
the structure makes two or more of the central atoms equivalent (see, for example, Section
IR-9.2.5.4) and the name eventually generated is independent of the numbering.
Thecentralatom n umbers a re then used a s l ocants f ortheligatingatoms andare p lacedtothe
left o f e ach k appa symbol. I ndividual kappa designators, i.e. kappa symbols w ith a numerical
superscript (as applicable), central atom locant and ligator a tom s ymbol, a re separated b y
commas.

Examples:

7. 1 2
½ðOCÞ ReCoðCOÞ
4
5
4
5
nonacarbonyl-lk C,2k C-rheniumcobalt
8. 1 2
2
½Cl 4 ReReCl 4
4
4
octachlorido-lk Cl,2k Cl-dirhenate(2 )
In these t wo examples, structural i nformation indicated by the formulae is not
communicated b y t he names. In fact, any polynuclear complex must either contain at
least one ligand binding to more than one central atom (a bridging l igand) or contain a b ond
between t wo central atoms. In order to specify these aspects of the structure in names,
further devices are needed. These are introduced in the following two sections.

IR-9.2.5.2 Bridging ligands
Bridging ligands, as far as they can be specified, are indicated by the Greek letter m appearing
before the ligand symbol or name a nd separated from it by a h yphen; the conventions applied
were briefly introduced in IR-9.1.2.10. In names, the whole term, e.g. m-chlorido, is separated
from the rest of the name b y h yphens, a s i n a mmine-m-chlorido-chlorido, etc., unless the
bridging l igand name i s c ontained within its own set of enclosing marks. If the bridging ligand
occurs more than once, multiplicative prefixes a re employed, as in tri-m-chlorido-chlorido, or
as in bis(m-diphenylphosphanido), if more c omplex ligand names are involved.
Bridging ligands are listed in alphabetical order together w ith the other ligands, b ut in
names a b ridging l igand is cited before a c orresponding non-bridging ligand, as in di-m-
chlorido-tetrachlorido. I n f ormulae, bridging ligands are placed after terminal ligands of the
same kind. Thus, in both names and formulae bridging ligands are placed further away from
the central atoms than are terminal ligands of the same kind.

Example:
1. [Cr 2 O 6 (m-O)] 2 m-oxido-hexaoxidodichromate(2 )

The bridging index n, t he number of coordination centres connected by a b ridging ligand,
is placed as a r ight subscript. The b ridging i ndex 2 i s n ot normally indicated. Multiple

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