Naming inorganic compounds is usually a routine operation, but one problem has long defied solution - how to define the chirality of certain octahedral complexes containing chelates (ligands with more than one point of attachment). Santiago Herrero from the Complutense University of Madrid, Spain, and his colleague have now proposed a way of doing this.

The general idea when defining the chirality of such complexes is to ignore the identity of the donor atoms on the ligands and any chiral centres on the chelates, as this can be dealt with by standard stereochemical symbols. Instead, the focus is the topology of the chelate network. This is how the six atoms attached to the central metal atom - the vertices of the octahedron - are joined together. The chelates are first drawn as straight lines joining the vertices. These lines are then compared, and the complex assigned as  (lambda) if they define a left-handed helix, and  (delta) if they define a right-handed helix. This is fine in simple complexes such as A, but things get difficult in cases like B, because there is more than one possible pair of lines to compare. Herrero and Usón have now devised a heirarchy of rules that enable such systems to be dealt with.

The new system works by first (if necessary) artificially linking the chelates together so that the resulting line has the greatest number of changes of plane. A set of rules then allows you to pick a pair of links that you can use to define the complex as  or . Further rules, involving consideration of all the vertices that are not directly joined, allow the designation of chirality in even stranger complexes.

Although the chirality symbols in several cases are different under the new system, says Herrero, there are fewer rules to contend with. In conclusion, he says that their method 'could be useful for keeping and searching precise structural information about octahedral compounds'.

David Barden