Page 11 - 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-9.3.4.6 The C/A convention for square pyramidal centres
A p rocedure similar to that described in Section IR-9.3.4.4 is used for square pyramidal
structures. In the case of SPY-5 s ystems, the polyhedron is oriented so that the viewer looks
along the formal C 4 axis, from the axial ligand toward t he central atom. The priority
numbers of the ligating atoms in the perpendicular plane are then considered, beginning with
the highest priority atom (the one h aving the priority number of lowest numerical value).
The clockwise and anticlockwise sequences of priority numbers are compared, and the
structure is assigned the symbol C or A according to whether the clockwise (C) o r
anticlockwise (A) s equence is lower at the first point of difference.
The chirality of an SPY-4 system is defined in a s imilar way. In this case, the viewer
looks along the formal C 4 axis in such a w ay that the ligands are further away than the
central atom. The priority numbers are then used to assign the symbol C or A, a s f or the SPY-5
system.

Examples:
1. 2.

1 1
3 4 2 5
M M
2 5 3 4

Chirality symbol ¼ C Chirality symbol ¼ A

IR-9.3.4.7 The C/A convention for see-saw centres
The absolute c onfigurations of see-saw complexes can be described using t he C/A system.
The configuration index f or see-saw systems c onsists of two digits, t he priority numbers of
the two ligands separated by the largest angle. The higher p riority ligand of these t wo is
identified and used as a p oint from which to view the two ligands not involved in the
configuration index. If moving from the higher priority ligand to the lower (through the
smaller angle) entails making a c lockwise motion, the absolute c onfiguration is assigned C.
An anticlockwise direction results in the absolute configuration A.

Example:

1.

1

3
M anticlockwise looking from the top
1

2
SS-4-12-A

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