Page 5 - 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-9.3

IR-9.3.3.4 Octahedral coordination systems (OC-6)
The terms cis and trans are used commonly as prefixes t o d istinguish between s tereoisomers
in octahedral systems of the form [Ma 2 b 4 ], where M i s t he central atom, and ‘a’ and ‘b’ are
different types o f d onor atom, and in certain similar systems. The ‘a’ donors occupy adjacent
coordination sites in the cis isomer, and opposite coordination sites in the trans isomer
(Example 1 ).
The terms mer (meridional) and fac (facial) are used commonly to distinguish between
stereoisomers of complexes of the form [Ma 3 b 3 ]. In the mer isomer (Example 2 ) t he two
groups of three similar donors each lie on a m eridian of the coordination octahedron, in
planes that also contain the central atom. In the fac isomer (Example 3 ) t he two groups of
three similar donors each occupy coordination sites on the corners of a f ace of the
coordination octahedron.
The configuration index of an octahedral system follows the polyhedral symbol (OC-6)
and consists of two digits.
The first digit is the priority number of 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. If there i s m ore than one ligating atom of priority 1, then the first digit is the
priority number of the trans ligand with the highest numerical value ( remembering that a
primed n umber will be of higher n umerical value than the corresponding unprimed number).
These two ligating atoms, the priority 1 a tom and the (lowest priority) atom trans to it,
define the reference axis of the octahedron.
The second d igit of the configuration index i s t he priority number of the ligating atom
trans to the most preferred ligating atom in the plane that is perpendicular t o t he reference
axis. If there is more t han one such ligating atom in that plane, the priority number of the
trans atom having the largest numerical value is selected.

Examples:
1. a 1

b b 2 2

b b 2 2
OC-6-12
a 1

b 1
b a 2 1

b a 2 2 OC-6-12

b 2

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