Page 19 - 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

Bis(tridentate) complexes (i.e. octahedral complexes containing two identical linear
tridentate ligands) m ay exist in three stereoisomeric forms, and there will be more i f t he
tridentate ligands do not themselves contain some s ymmetry elements. The three isomers
of the simplest case are represented below (Examples 1, 2 a nd 3), along w ith their
polyhedral symbols (Section IR-9.3.2.1) and configuration indexes (Section IR-9.3.3.4).
Complexes o f N-(2-aminoethyl)ethane-1,2-diamine and iminodiacetate c an be described by
these diagrams.
O O
H H
N − N −
H 2 N NH 2 O O

N-(2-aminoethyl)ethane-1,2-diamine, iminodiacetate, or
0
or 2,2 -azanediylbis(ethan-1-amine) 2,2 -azanediyldiacetate
0
The need for the priming convention can be seen by considering what the configuration
indexes of Examples 1 a nd 3 w ould be in the absence of the priming convention. The two
ligands are identical and consist of two similar fragments fused together. If the primes are
ignored, the two complexes have the same distributions of ligating atoms (four d onors of
priority 1 i n a square plane, and two of priority 2 trans to one another). They would
therefore have the same configuration index, even though t hey are clearly d ifferent
complexes.
One way to highlight the difference between these two examples is to note that, in
Example 1, all the donor atoms are trans to donors that are part of the other ligand. This is
not true i n E xample 3. Using primes to indicate the groupings of donor atoms in particular
ligands allows t hese two stereoisomers t o b e d istinguished from one another by their
configuration indexes.

Examples:

1. 2. 3.
2 2 1'
1' 1 1' 1 2' 1

1' 1 2' 1 1 2
2' 1' 1'
0 0
0 0
OC-6-1 1 OC-6-2 1 OC-6-11 0
The priority numbers on one of the ligands are arbitrarily primed. The primed
number is assigned lower priority than the corresponding unprimed number, but a h igher
priority than the next higher u nprimed number. Thus 1 has lower priority than 1, but
0
higher than 2.
The technique also d istinguishes between s tereoisomers for complexes of higher
polydentate ligands as indicated in Examples 4, 5 a nd 6 f or linear tetradentate ligands such
as N,N -bis(2-aminoethyl)ethane-1,2-diamine. In this case, the donor atom priority numbers
0
in half of the tetradentate ligand have been primed.

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