Page 153 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

C O O R D I N A T I O N C O M P O U N D S IR-9.1

IR-9.1 I N T R O D U C T I O N

IR-9.1.1 General

This Chapter p resents t he definitions and rules necessary for formulating and naming
coordination compounds. Key terms such as coordination entity, coordination polyhedron,
coordination number, chelation and bridging l igands are first defined and the role of additive
nomenclature explained (see also Chapter IR-7).
These definitions are then used to develop rules for writing the names and formulae of
coordination compounds. The rules allow the composition of coordination compounds to be
described in a w ay that is as unambiguous as possible. The names and formulae provide
information about the nature of the central atom, the ligands that are attached to it, and the
overall charge on the structure.
Stereochemical d escriptors are then introduced as a m eans of identifying or
distinguishing between t he diastereoisomeric or enantiomeric structures that may exist for
a c ompound of any particular composition.
The description of the configuration of a c oordination compound requires fi rst that the
coordination geometry be specified using a p olyhedral symbol ( Section IR-9.3.2.1). Once
this is done the relative positions of the ligands around t he coordination polyhedron a re
specified using the configuration index (Section IR-9.3.3). The configuration index is a
sequence of ligand priority numbers produced by following rules specific to each
coordination geometry. If required, the chirality of a c oordination compound can be
described, again u sing l igand priority numbers (Section IR-9.3.4). The ligand priority
numbers used in these descriptions are based on the chemical composition of the ligands.
A d etailed description of the rules by which they are obtained is provided in Section P-91 of
Ref. 1, but an outline is given in Section IR-9.3.5.

IR-9.1.2 Definitions

IR-9.1.2.1 Background

The development of coordination theory and the identification of a c lass of compounds
called c oordination compounds b egan with the historically significant concepts of primary
and secondary valence.
Primary valencies were obvious from the stoichiometries of simple compounds such as
NiCl 2 , F e 2 (SO 4 ) 3 and PtCl 2 . H owever, new materials were frequently observed when o ther,
independently stable substances, e.g. H 2 O, NH 3 or KCl, w ere added to these s imple
compounds giving, for example, NiCl 2 ·4H 2 O, Co 2 (SO 4 ) 3 ·12NH 3 or PtCl 2 ·2KCl. S uch
species were called complex c ompounds, in recognition of the stoichiometric complications
they represented, a nd were considered characteristic of certain metallic elements. The
number of species considered to be added to the simple compounds gave rise to the concept
of secondary valence.
Recognition of the relationships between t hese complex compounds l ed to the
formulation of coordination theory and the naming of coordination compounds using
additive n omenclature. Each coordination compound either is, or contains, a c oordination
entity (or complex) that consists of a c entral atom to which other groups are bound.

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