Page 10 - 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-9.3.4.4 The C/A convention for other polyhedral centres
The R/S convention makes use of priority numbers for the determination of chirality at
tetrahedral centres, as detailed above. The same principles are readily extendable to
geometries other than tetrahedral. 14 However, in order to avoid c onfusion, and to emphasize
the unique a spects of the priority sequence systems as applied to coordination polyhedra, the
symbols R and S are replaced b y t he symbols C and A when applied to other polyhedra.
The procedure for arriving at ligating atom priorities is detailed in Section IR-9.3.5.
Once these priorities have been assigned, t he reference axis (and direction) a ppropriate t o
the geometry is identified. The priority numbers of the ligating atoms coordinated in the
plane perpendicular to the reference axis are then considered, viewing from the axial ligating
atom of higher priority.
Beginning with the highest priority atom in the plane perpendicular to the reference axis,
the clockwise and anticlockwise sequences of priority numbers are compared, and that with
the lower number at the first point of difference is chosen. If the chosen s equence results
from a c lockwise reading of the priority numbers, t hen the structure is given the chirality
symbol C, o therwise it is given the symbol A.

IR-9.3.4.5 The C/A convention for trigonal bipyramidal centres
The procedure is similar to that used for tetrahedral systems i n t he R/S convention, b ut it is
modified because of the presence of a u nique r eference axis (running through the two axial
donor atoms and the central atom).
The structure is oriented so that the viewer looks down the reference axis, with the more
preferred donor atom (having a p riority number with lower numerical value) closer to the
viewer. Accordingly, the axial donor atom with the lower priority lies beyond the central
atom. Using this orientation, the priority sequence of the three ligating atoms in the trigonal
plane is examined. If the sequence proceeds from the highest priority to the lowest priority
in a c lockwise fashion, the chirality symbol C is assigned. C onversely, if the sequence from
highest to lowest priority (from lowest numerical index to highest numerical index) i s
anticlockwise, the symbol A is assigned.

Examples:
1. 2.

1 1
4 2

M 5 M 5

3 4
2 3

Chirality symbol ¼ C Chirality symbol ¼ A

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