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.
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.
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
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.