Page 184 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-9.3 C O O R D I N A T I O N C O M P O U N D S

that differ only in the spatial d istribution of the components are known a s s tereoisomers.
Stereoisomers that are mirror images of one another are called e nantiomers (sometimes
these have been called optical isomers), w hile those t hat are not are called
diastereoisomers ( or geometrical isomers). T his is an important distinction in chemistry
as, in general, diastereoisomers e xhibit different physical, chemical and spectroscopic
properties from one another, while enantiomers exhibit identical properties (except in the
presence of other chiral entities). It is instructive to consider an everyday analogy in order
to establish how the configuration of a m olecule (and the embedded spatial r elationships)
can be described.
Using the terminology introduced above, left and right hands may be regarded as
enantiomers of one another, since t hey are different (non-superimposable), but they are
mirror images of each other. In both cases the thumbs are adjacent to the index fi nger, and
the components o f e ach hand are similarly disposed relative to all the other p arts of that
hand. If the thumb and index fi nger of a r ight hand were to be exchanged, the resulting hand
could be considered to be a d iastereoisomer of the normal right hand (and it too would have
an enantiomer, resulting from a s imilar exchange on a l eft hand). The key point is that the
relative positions of the components o f d iastereoisomers (the normal right hand and the
modified one) are different.
In order to describe the hand fully the components ( four fingers, one t humb and the
central part of the hand) must be identified, the points of attachment available on the hand,
and the relative positions of the fingers and thumb around t he hand, must b e d escribed and
whether t he hand is ‘left’ or ‘right’ must b e s pecified. The last three steps deal with the
configuration of the hand.
In the case of a c oordination compound, the name a nd formula describe t he ligands
and central atom(s). Describing the configuration of such a c oordination compound requires
consideration of three factors:

(i) coordination geometry – i dentification of the overall shape of the molecule;
(ii) relative configuration – d escription of the relative positions of the components o f t he
molecule, i.e. where the ligands are placed around t he central atom(s) in the identified
geometry;
(iii) absolute c onfiguration – i dentification of which enantiomer is being specified (if the
mirror images are non-superimposable).

The next three sections deal with these steps in turn. A m ore detailed discussion of the
configuration of coordination compounds can be found elsewhere. 9

IR-9.3.2 Describing the coordination geometry

IR-9.3.2.1 Polyhedral symbol

Different geometrical arrangements of the atoms attached to the central atom are possible
for all coordination numbers greater than one. Thus, two-coordinate s pecies may involve a
linear or a b ent disposition of the ligands and central atom. Similarly, three-coordinate
species may be trigonal p lanar or trigonal pyramidal, and four-coordinate species may be

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