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

following sections give details for particular geometries. Commonly u sed terms are included
for each geometry discussed.

IR-9.3.3.2 Conﬁguration index

Once the coordination geometry has been speciﬁed by the polyhedral s ymbol, it becomes
necessary t o i dentify which ligands (or donor atoms) occupy particular c oordination
positions. This is achieved through t he use of the conﬁguration index which is a s eries of
digits identifying the positions of the ligating atoms on the vertices of the coordination
polyhedron. The conﬁguration index has the property t hat it distinguishes between
diastereoisomers. It appears within the parentheses enclosing the polyhedral symbol (see
Section IR-9.3.2.1), following that symbol and separated from it by a h yphen.
Each donor atom must be assigned a p riority number based on the rules developed
by Cahn, Ingold and Prelog (the CIP rules). 13 These priority numbers are then used to
form the conﬁguration index for the compound. The application of the CIP rules to
coordination compounds is discussed in detail in Section IR-9.3.5 but, in general, d onor
atoms that have a h igher a tomic number have higher priority than those that have a l ower
atomic number.
The presence of polydentate ligands may require the use of primes o n s ome o f t he
numbers in the conﬁguration index. The p rimes are used to indicate either that donor atoms
are not part of the same polydentate ligand as those t hat have unprimed priority numbers, or
that the donor atoms belong to different parts o f a polydentate ligand that are related by
symmetry. A p rimed priority number means that that donor atom has lower priority than the
same kind of donor atom without a p rime on the priority number. More detail on the
‘priming convention’ can be found in Section IR-9.3.5.3.

IR-9.3.3.3 Square planar c oordination systems (SP-4)

The terms cis and trans are used commonly as preﬁxes t o d istinguish between s tereoisomers
in square planar systems of the form [Ma 2 b 2 ], where M i s t he central atom, and ‘a’ and ‘b’
are different types o f d onor atom. Similar donor atoms occupy coordination sites adjacent to
one another in the cis isomer, a nd opposite to one another in the trans isomer. The cis-trans
terminology is not adequate to distinguish between t he three isomers o f a s quare p lanar
coordination entity [ Mabcd], b ut could be used, in principle, for an [Ma 2 bc] system (where
the terms cis and trans would r efer to the relative locations of the similar donor atoms). T his
latter use is not recommended.
The conﬁguration index f or a s quare p lanar system is placed after the polyhedral s ymbol
(SP-4). It is the single digit which is the priority number for the ligating atom trans to the
ligating atom of priority number 1, i.e. the priority number of the ligating atom trans to the
most preferred ligating atom.

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