Page 9 - 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-9.3.4.2 The R/S convention for tetrahedral centres
The convention used to describe t he absolute c onfigurations of tetrahedral centres was
originally developed for carbon atom centres (see Ref. 13 and Section P-91 of Ref. 1) but
can be used for any tetrahedral centre. There is no need to alter the rules in treating
tetrahedral metal complexes.
The symbol R is assigned if the cyclic sequence of priority numbers, p roceeding from
highest priority, is clockwise when t he viewer is looking down the vector from the
tetrahedral centre to the least preferred substituent ( the substituent h aving the priority
number with the highest numerical value, i.e. 4). An anticlockwise c yclic sequence is
assigned the symbol S.
1 1

M M
3 2 2 3
R S
This system is most often used in conjunction with configuration internally in ligands but
can be applied equally well to tetrahedral metal centres. It has also been useful for
pseudotetrahedral organometallic complexes when, for example, cyclopentadienyl ligands
are treated as if they were monodentate ligands of high priority.

Example:

1. 1

Fe 4
CO
I
2 PPh 3
3
T-4-S

IR-9.3.4.3 The R/S convention for trigonal pyramidal centres

Molecules containing a t rigonal pyramidal centre (TPY-3) may exist as a p air of
stereoisomers. The configuration of this centre can be described in a s imilar way to that
of a t etrahedral centre. This is achieved through notional placement of a ‘ phantom atom’ of
low priority in the coordination site that would c reate a t etrahedral centre from a t rigonal
pyramidal centre. The centre can then be identified as R or S by the methods d escribed
above.
The use of some b onding theories leads to the placement of a l one pair on a t rigonal
pyramidal centre. If this is done, the absolute c onfiguration of the centre is also described by
the R/S convention, in this case by placing the ‘phantom atom’ in the site that is occupied by
the lone pair. Examples of this practice may be found in the description of absolute
configurations for sulfoxides in which the alkyl substituents are different.

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