Complex ligands: classification, naming and searching

Contribution from Axel's Collection of Notes and Communications
Subject: Coordination Chemistry
Edited and published by Axel Drefahl • www.axeleratio.com

Virtually any molecule can take on the role of a ligand, assuming that an interaction between the molecule and a metal atom occurs, resulting in a coordination structure or at least a short-lived binding of the molecule to the metal atom. A typical ligand is a non-metal atom, an inorganic molecule or an organic molecule containing heteroatoms and/or unsaturated bonds.

The following distinction of ligands is useful while documenting or searching ligand information:

This is a formal classification meant to apply to the uncoordinated ligand. In a coordination molecule (metal-ligand complex), a ligand is often classified according to the number of electrons that it supplies to the coordination sphere of the metal atom. Sometimes, the symbol X is applied to denote a ligand that supplies one electron and the symbol L is applied for a ligand supplying one pair of electrons. Ligands can supply more than two electrons, often involving many donor atoms to form chelate complexes. Ligands may also bind to more than one metal atom, function as bridging molecules and building frameworks. Classification criteria may then include the number of donor atoms or the number of rings in the ligand molecule. For more information on metal-ligand interactions and complex architectures a text book should be consulted.

Keeping ligand nomenclature simple

If the name for a molecular structure is known or has been constructed, deriving the ligand name is straight forward:
  • For a neutral-molecule ligand take the molecule name without modification.
  • For a functional-group ligand take the functional group name, typically ending in -yl, without modification.
  • For an anionic ligand change the anion-specifying affix (in most cases a suffix) -ate, -ide, or -ite to -ato, -ido, or -ito, respectively.
According to IUPAC recommendations, short forms and abbreviations of ligand names should be written in lower-case letters. A few exceptions apply, including functional-group names that start with an upper-case letter and formula-like notations that contain element symbols or stereochemical descriptors.

Selected references on ligand and complex nomenclature

[Beck1970] Margot Becke-Goehring und Harald Hoffmann: Komplexchemie. Springer-Verlag, Berlin · Heidelberg · New York, 1970.
Section 2.2: Nomenklatur der Komplexe (pages 12 to 20)
[Gisp2008] Joan Ribas Gispert: Coordination Chemistry. Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim, Germany, 2008. ISBN: 978-3-527-31802-5
Introduction: Definitions, History, Nomenclature (pages XXIX to XL)
[Hell2001] D. Hellwinkel: Systematic Nomenclature of Organic Chemistry. Springer-Verlag, Berlin · Heidelberg · New York, 2001. ISBN: 3-540-41138-0
Chapter 4: Metallorganic and Metalloidorganic Compounds (pages 148 to 154)
[IUPAC2005] Nomenclature of Inorganic Chemistry. IUPAC Recommendations 2005 Prepared for publication by Neil G. Connelly, Ture Damhus, Richard M. Hartshorn, and Alan T. Hutton. RCS Publishing, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK, 2005.
Section IR-4.4.4: Ligand abbreviations (also: Table VII)
Chapter IR-9: Coordination Compounds
Chapter IR-10: Organometallic Compounds


Last update was made on May 5 in 2009 by Axel Drefahl
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