Project Details Alignment of principles for specifying ligands and substituent groups across various areas of nomenclature

Project No.:
Start Date:
08 November 2017
End Date:


This project intends to provide the basis for completion of several projects with different states of progress:
Boron hydride nomenclature (2012-045-1-800)
Metallacycles nomenclature (2013-030-1-800)
Preferred names for inorganic compounds (kappa document) (2006-038-1-800)
Blue Book revision and extension (2015-052-1-800)
Inorganic and coordination polymers (2011-035-1-800)

by reaching a consensus on
– the grouping of substituents or ligands with different kinds of modifications
– the alphabetical order of substituents or ligands with different kinds of modifications
– the positioning of locants for substituents and ligands in chemical names
– the positioning and grouping of kappa terms in chemical names


During the review of the project on boron hydride nomenclature (project 2012-045-1-800) in the first half of 2016 inconsistencies in the use of kappa (κ) symbols were noticed. Further, during the meeting on the kappa document (Preferred names for inorganic compounds, project 2006-038-1-800) in August 2016 it became apparent that the approach followed until then, namely to use kappa terms at the end of a ligand name otherwise unchanged as compared to the non-ligated species is not feasible. This raised the question how to handle substituents in a ligand with different ligating atoms/kappa terms. The logical step was to look for precedent solutions in similar situations, e.g. isotopically modified substituents. However, when comparing the possible approaches it was further noticed that in the Blue Book inconsistencies exist concerning the treatment of substituents with different kinds of modification, specifically, when lambda symbols, or stereodescriptors, or isotopic modifications are added. The Blue Book task group (project 2015-052-1-800) had already decided that in the alphabetical ordering of substituents an unmodified substituent is always to be listed before the same, but modified substituent – the so-called “nothing before something” principle which was approved by the Division Committee. With this now again the question on the consequences for the numbering arose.

The task group will have to review the inconsistent sections in the Blue Book and sections on comparable situations in the Red Book and then find a consensus on the seniority order of differently modified substituents in a uniform manner.

First, all the different kinds of modifications to substituents will be collected before the meeting and the underlying principles will be reviewed during the meeting. Next, it will have to be discussed and decided which approach on alphabetical order and as a consequence on numbering of the differently modified substituents will be most uniform. A ranking of the different modifications: isotopical modification versus stereochemical modification, versus modification of bonding number, etc. will have to be defined.

The probably most challenging question to solve will certainly be the positioning of locants. While in substitutive nomenclature the locants are placed in front of the substituent names, in additive nomenclature for coordination compounds a kappa term is usually added at the end of the ligand name including locants for the ligating atom and for the atom to which the ligating atom is attached. With more and more compounds of mixed character, i.e. parts typically named by additive nomenclature and others typically named by substitutive nomenclature, e.g. metallacycles (project 2013-030-1-800) and boron hydrides (project 2012-045-1-800), there is a need of combining additive approaches and substitutive approaches within one name for a compound. It hence becomes essential to find a compromise which still unambiguously defines both the number of groups and their positions in the structure by the name of a compound.


Aug 2018 – Project announcement published in Chem Int July 2018, p. 30;

Page last updated 6 Aug 2018