Project
Chemical
Nomenclature and Structure Representation Division (VIII)
Number: 2003-025-1-800
Title: Extension of IUPAC rules for stereo descriptors to include
coordination numbers 7-12
Task Group
Chairman: G. J. Leigh
Members:
E. Hey-Hawkins,
R. Sommer, and R.
M. Hartshorn
Completion Date: 2007 - project completed
Objective:
The set of stereo descriptors in the Red
Book extend only as far as coordination number 6. Hey-Hawkins and
Sommer have recently described how these might be extended to face-capped
octahedra, and this project is an attempt to provide a unified method
for descriptors for all higher coordination numbers 7-12.
Description:
Hey-Hawkins and Sommer (European Journal of Inorganic Chemistry,
to be submitted) have recently described a procedure for assigning stereo
descriptors to 7-coordinate complexes that can be regarded as capped
octahedra. This relies on the established nomenclature processes of
defining a principal axis (through the capping position and the central
atom), assigning the ligand positions to a series of planes perpendicular
to the principal axis and then numbering the ligand positions in sequence,
starting at the "highest" plane. The chirality is then assigned
on the basis of the numbering sequence. Currently the Red Books only
deal with coordination numbers up to 6, plus pentagonal bipyramidal
structures, so that the increasing numbers of complexes with higher
coordination number cannot be treated by established rules.
The project aims to present a unified treatment for higher coordination
numbers. This involves defining appropriate idealised polyhedra with
rules on how to assign them to real structures, determining unambiguously
principal axes, deriving a consistent method for defining planes, and
finally deciding rules for numbering. The whole would need to be consistent
with the principles laid down for lower coordination numbers, and would
also have implications for boron nomenclature and possibly for cluster
nomenclature, which is almost non-existent The absence of rules for
higher coordination numbers is a gap in the current nomenclatures, and
the project would also enable us to determine how far the current rules
can be extended. This is not currently stated.
Progress:
Feb 2007 - The task group completed the draft of a technical
report titled 'Representation of configuration in Coordination polyhedra
and the extension of current methodology to coordination numbers greater
than six'. The technical report has been accepted and is now scheduled
for publication in October 2007.
Project completed - IUPAC Technical Report published in Pure
Appl. Chem.
79(10), 1779-1799, 2007
Abstract:
Established IUPAC recommendations for the representation of configuration
are reviewed briefly and then methodologies are examined for dealing
with regular polyhedra that are typically used when working with coordination
numbers greater than six. More than one method is available and can
be used for many such geometries, but the deviations from regular polyhedral
geometries that are typically found in such molecules can make it difficult
to arrive reproducibly at the same description for a complex using purely
objective criteria. Recommendations are made for some geometries of
coordination numbers seven, eight, and nine, but not for higher coordination
numbers.
