Page 2 - 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

IR-9.3.2.2 Choosing between closely related geometries
For real molecules or ions, the stereochemical descriptor should be based on the nearest
idealized g eometry. However, some i dealized geometries are closely related [e.g. square
planar (SP-4), four-coordinate square pyramidal (SPY-4), see-saw (SS-4), and tetrahedral
(T-4); T -shaped (TS-3), trigonal p lanar (TP-3), and trigonal p yramidal (TPY-3)] and care
may therefore be required i n m aking the choice.
The following approach is useful in determining the polyhedral s ymbol for four-
coordinate structures. The key is to consider the locations of the central atom and the
coordinating atoms in relation to each other. If all five atoms are in (or are close to being i n)
the same plane, then the molecule should be treated as square planar. If the four
coordinating atoms are in a p lane, but the central atom is significantly displaced from the
plane, then the square p yramidal geometry is appropriate. I f t he four coordinating atoms do
not lie in (or close to) a p lane, then a p olyhedron can be defined by joining a ll four
coordinating atoms together w ith lines. If the central atom lies inside this polyhedron t he
molecule should b e r egarded as tetrahedral, otherwise, it should be regarded as having a
see-saw structure.
T-shaped and trigonal p lanar molecules both have a c entral atom that lies in (or close to)
the plane d efined by the coordinating atoms. They differ in that the angles between the three
coordinating atoms are approximately the same i n t he trigonal planar structure, w hile one
angle is much l arger than the other two in a T -shaped molecule. The central atom lies
significantly out o f t he plane in a t rigonal p yramidal structure.

IR-9.3.3 Describing configuration – d istinguishing between diastereoisomers

IR-9.3.3.1 General

The placement of ligands around t he central atom must b e d escribed in order to identify
a p articular diastereoisomer. T here are a n umber of common terms (e.g. cis, t rans, mer
and fac) u sed to describe the relative locations of ligands in simple s ystems. However,
they can be used only when a particular g eometry is present (e.g. octahedral or square
planar), a nd when there are only two kinds of donor atom present (e.g. Ma 2 b 2 in a
square planar complex, where M i s a central atom and ‘a’ and ‘b’ are types o f d onor
atom).
Several methods have been used to distinguish between diastereoisomers in more
complex s ystems. Thus, stereoisomers resulting from the coordination of linear tetradentate
ligands have often been identified as trans, cis-a, o r cis-b, 10 and those resulting from
coordination of macrocyclic tetradentate ligands have their own system. 11 The scope of
most of these nomenclatures is generally quite limited, but a p roposal with wider
application in the description of complexes of polydentate ligands has been made more
recently. 12
Clearly a g eneral method is required in order to distinguish between d iastereoisomers
of compounds in which either other g eometries or more than two k inds of donor atoms
are present. T he configuration index has been developed for this purpose. The next section
outlines the method by which a c onfiguration index i s o btained for a c ompound, and the

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