Page 17 - 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

The following sections outline t he method used to arrive at the priority numbers for a
given set of donor atoms, and the ways that the basic rules have to be modified in order to
describe a dequately systems that include polydentate ligands. T hese modifications, which
are collectively referred to as the priming convention, make u se of primes o n t he priority
numbers to indicate which donor atoms are grouped together w ithin a p articular polydentate
ligand.

IR-9.3.5.2 Priority numbers
The procedure for assigning priorities in mononuclear coordination systems i s b ased on the
standard s equence rules developed for chiral carbon compounds by Cahn, Ingold and
Prelog. 13 (See also Section P-91 of Ref. 1.) These CIP rules can be used quite generally for
assigning priorities to groups attached to a c entral atom.
The essence of these rules, when a pplied to coordination compounds, i s t hat the ligands
attached to the central atom are compared to one another, beginning with the donor atom and
then moving outwards in the structure. The comparison is made on the basis of atomic
number and then, i f r equired (e.g. when isotopes a re being specified), a tomic mass. Other
properties may be used for subsequent comparisons, but the need for them is sufficiently rare
that they need not be detailed here.
Once the ligands have been compared, the priority numbers are assigned as follows:

(i) identical ligands are assigned the same priority,
(ii) the ligand(s) with highest priority is (are) assigned the priority number 1; those w ith the
next highest priority, 2; and so on.

Examples:
1.
5
O
1 C 2
Br Cl
3 3
Ph P PPh 3
3
NMe 3
4

Priority sequence: Br4Cl4PPh 3 , P Ph 3 4NMe 3 4CO
Priority numbers sequence: 14243, 34445

2.
3 1
H 3 N OH
3 Pt 2
CH 3
H 3 N N N
H

H
In Example 2, the heterocyclic ligand is given priority 2 s ince i t h as a l ower atomic number
donor atom than OH, and the substitution of the nitrogen d onor ranks it above the ammine
ligands.

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