Page 8 - 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

2. Pentagonal bipyramid (PBPY-7)
3
2
1

3 PBPY-7-34-12342 (not 12432)
2
4

4
IR-9.3.3.7 T-shaped systems ( TS-3)
The configuration index f or T-shaped systems follows t he polyhedral symbol and consists of
a s ingle digit, the priority number of the ligating atom on the stem of the T ( as opposed t o t he
crosspiece of the T).
IR-9.3.3.8 See-saw systems ( SS-4)
The configuration index for see-saw systems consists of two digits, the priority numbers of
the two ligating atoms separated by the largest angle. The number of lower numerical value
is cited first.
Examples:
1. 2.

1 1

2 3
M largest angle M largest angle
3 1

1 2

SS-4-11 SS-4-12

IR-9.3.4 Describing absolute configuration – d istinguishing between enantiomers
IR-9.3.4.1 General

There are two well-established, but fundamentally d ifferent, s ystems for distinguishing
between t wo enantiomers (stereoisomers that are mirror images of one another). The first,
based on the chemical constitution of the compound, involves the R/S convention used for
describing tetrahedral centres and the closely r elated C/A convention used for other
polyhedra. The R/S and C/A conventions use the priority sequence referred to in Section IR-
9.3.3.2, and detailed in Section IR-9.3.5, where the ligating atoms are assigned a p riority
number based (usually) on their atomic number and their substituents.
The second i s b ased on the geometry of the molecule and makes use of the skew-lines
convention; it is usually applied only to octahedral complexes. The two enantiomers are
identified by the symbols D and L in this system. The C/A nomenclature is not r equired f or
those chelate complexes where the skew-lines convention is completely unambiguous (see
Sections IR-9.3.4.11 to 9.3.4.14).

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