Page 12 - 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-9.3.4.8 The C/A convention for octahedral c entres
The absolute c onfigurations of some o ctahedral complexes can be described using either the
skew-line r eference system (Section IR-9.3.4.11) or the C/A system. The fi rst is used more
commonly, but the C/A system is more general and may be used for most c omplexes. The
skew-line r eference system is only applicable to tris(bidentate), bis(bidentate) and closely
related systems.
The reference axis for an octahedral centre is that axis containing the ligating atom of
CIP priority 1 a nd the trans ligating atom of lowest possible p riority (highest numerical
value) (see Section IR-9.3.3.4). The a toms in the coordination plane p erpendicular to the
reference axis are viewed from the ligating atom having that highest priority (CIP priority 1)
and the clockwise and anticlockwise sequences of priority numbers are compared. The
structure is assigned the symbol C or A, a ccording to whether t he clockwise (C) o r
anticlockwise (A) s equence is lower at the first point of difference.
Examples:
1. 2. 3.
1 1 1
4 5 3 6 5 4
M M M
3 6 4 5 3 6

2 2 2
Chirality symbol ¼ C Chirality symbol ¼ A Chirality symbol ¼ C
Example 4 s hows t he compound [CoBr 2 (en)(NH 3 ) 2 ] which has the polyhedral symbol OC-6
þ
and the configuration index 32. The chirality symbol is C.

Example:
4.
+ 1
H 2 Br 2 3
N NH
H C 3 M
2
H 2 C Co
N Br
H 2 2 1
NH 3
3
Example 5 s hows the complex [ Ru(CO)ClH(PMe 2 Ph) 3 ] w hich has the descriptor
OC-6-24-A. T he chloride ligand has priority 1.
Example:
1
5. Cl
OC PMe 2 Ph 3 2
Ru M
H PMe 2 Ph
4 2
PMe 2 Ph
2

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