Page 16 - 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

shows the same structure oriented to emphasize the skew-line relationship between a p air of
chelate r ings that can be used to define chirality. Figure I R-9.3(c) shows t hat the same
convention can be used for the cis-bis(bidentate) complex. The two chelate rings define the
two skew-lines that, in turn, define the helix and the chirality of the substance. The
procedure is precisely the same as that described for the tris(bidentate) c ase, but only a
single pair of chelate rings is available.

(a) (b) (c)
Figure IR-9.3. Two orientations of a t ris(bidentate) structure, (a) and (b), to show the chiral
relationship between these two species and the bis(bidentate) structure (c).

IR-9.3.4.14 Application of the skew-lines convention to conformations of chelate rings
In order to assign the chirality of a r ing conformation, the line AA in Figure I R-9.2 is defined
as that line joining t he two ligating atoms of the chelate ring. The o ther line BB is that
joining t he two ring a toms which are neighbours to each of the ligating atoms. These two
skew-lines define a h elix in the usual way. T he tangent describes a r ight-handed ( d) o r a left-
handed (l) h elix with respect to the axis and thereby defines the conformation in terms of the
convention given in Figure I R-9.1. T he relationship between t he convention of Figure I R-9.2
and the usual representation of chelate ring conformation may be seen by comparing Figures
IR-9.2 and IR-9.4.

N CH 2 N CH 2
M M CH 2
N CH 2 N CH 2
(a) (b)

Figure IR-9.4. d-Conformation of chelate rings: (a) five-membered; (b) six-membered.

IR-9.3.5 Determining ligand priority

IR-9.3.5.1 General

The methods for differentiating between s tereoisomers o utlined earlier in this Chapter require
the assignment of priorities for the ligand atoms attached to the central atom (i.e. t he donor
atoms). These priority numbers are then used in the configuration index, which describes the
relative positions of the ligands, a nd in the assignment of the absolute configuration of the
compound.

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