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Vol. 29 No. 5
September-October 2007

Tools of the Trade |

See also

The Red Book—Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005*

by Richard Hartshorn

Nomenclature is a core area of IUPAC activity, and nomenclature may indeed be the context in which most chemists first become aware that IUPAC exists. High school pupils are introduced to basic concepts in systematic nomenclature, and many teachers and textbooks will rightly attribute these nomenclature recommendations to IUPAC, or perhaps, depending on student reaction, try to shuffle blame onto IUPAC!

Today, it is the Division of Chemical Nomenclature and Structure Representation that is responsible for devising, revising, and codifying nomenclature recommendations for existing and new compounds, or classes of compounds. Indeed, the establishment of the division in 2002 is a reflection of the ongoing commitment of IUPAC to nomenclature, and the importance attached to such activities. Previously, nomenclature commissions within other divisions carried the responsibility for nomenclature recommendations within their areas of interest and, more particularly, for the preparation of the “color books” in which these recommendations are collected. The Red Book is an example from Division II, Inorganic Chemistry.

Preparation of what became the 2005 Recommendations (RSC Publishing 2005, ISBN 0854044388) began in 1998, under the auspices of the Commission for Nomenclature of Inorganic Chemistry, as the field had moved on considerably since the 1990 Recommendations were prepared (which was significantly before the 1990 publication due to the extensive nature of the review process to which all such recommendations must be submitted). The task of preparing the new recommendations was transferred to the new Division of Chemical Nomenclature and Structure Representation during the 2001 restructuring of IUPAC activities, and regular meetings over the next several years allowed work to progress. The need to align the content of the new recommendations as closely as possible with those being developed in a parallel project to update the Blue Book (Nomenclature of Organic Compounds) led to additional meetings as part of an alignment project in 2001 and 2002.

Perhaps the most significant addition to the recommendations is the chapter on the nomenclature of organometallic compounds. Previously this had been treated in a short section of the chapter covering coordination compounds. The huge growth in the area of organometallic chemistry in the 1980s and 1990s and the different nomenclature that is required for p-bonded ligands was the justification for devoting a chapter to this kind of system. Another innovation is the introduction of flowcharts to assist in choosing between the different kinds of nomenclature that are available, and in the application of the additive nomenclature that is widely used and recommended for coordination and organometallic compounds.

Development of nomenclature recommendations can be a balancing act, as there are demands to reflect common practice while at the same time retaining the kind of consistency and logical structure that will both make it easy to learn and apply, and to extend to new systems without ambiguity. The problem is that nomenclature methods are often developed in an ad hoc way in order to describe new compounds or classes of compounds. The resulting rules are not always as easily generalized as may be desirable and, in some cases, may lead to ambiguous names (i.e., names for which more than one structure could be drawn). A complicating factor is that the existence of such ambiguities may only become obvious some time after the initial invention. At that point, nomenclaturists face a dilemma: Either make the nomenclature more complicated by introducing some new exception, rule, or convention that resolves the ambiguity, or adopt an entirely new approach and create a new kind of nomenclature. Of course adopting new nomenclature systems then creates complications at a different level, as it becomes possible to produce a number of quite different, but still systematic, names for the same compound. Addressing that kind of complication is the goal of a separate IUPAC initiative, introduction of Preferred IUPAC Names (PINs). Two projects are currently active in developing rules to select PINs for different classes of compounds.

The Red Book can help chemists create names from structures, or decipher names to give structures. Clearly, it is desirable to ensure that changes to the recommendations are minimized so that existing names can be deciphered consistently and without ambiguity in the future. However, we believe that there are occasions where recommendations should be changed as attempts are made to strike the balance between reflecting common usage, and making the recommendations as simple and straightforward as possible. There are particular examples of this kind of change made in the 2005 recommendations, and it is perhaps instructive to consider some of the issues that underlie the choices that have been made as these recommendations have been formulated. Some of the changes are less controversial than others, some may be widely adopted, and some may even be largely ignored! Here are a few examples:

Kappa and Eta Conventions
The kappa and eta conventions are used to describe the bonding of ligands to central atoms. The kappa convention (page 155) has its roots in coordination chemistry and allows the donor atom(s) in a ligand to be specified and, in the case of polynuclear systems, it is also used to indicate the central atom to which the donor atom is bound. The eta convention (page 216) is widely used in organometallic chemistry to describe ligand binding modes, particularly those where the bonding is through p-electron donation to the central atom from unsaturated hydrocarbons.

Clearly, there is an overlap in the way these conventions could be used, and with that comes the possibility for confusion regarding which convention should be used in a particular set of circumstances. The 2005 recommendations therefore provide an explicit comparison of the two conventions (page 161), in which the eta convention is used to describe bonding in which contiguous atoms in a ligand are bound to the same central atom, while the kappa convention is used for cases where an isolated donor atom is bound to one or more central atoms.

New Ordering Rules in Names—Central Atoms
In additive nomenclature, the name for a coordination entity is produced by listing the ligand names (along with the appropriate multipliers) in alphabetical order, followed by the name of the central atom to which they are attached. In the case of polynuclear compounds, those with more than one central atom, the ligand names are again listed first in alphabetical order, and the central atom to which they are attached is identified using the kappa convention that incorporates a priority number for the central atom (pages 162–163). The central atoms are assigned their priority number based on their position in the periodic table, or, if they are of the same element, by the numbers and kinds of ligands attached (page 168). The central atom names are listed together after the ligand names, according to their priority number, but in the 1990 recommendations they were listed alphabetically. The following example is taken from the Red Book.

The recommended name for the complex [(OC)5ReCo(CO)4] is nonacarbonyl-1k5C,2k4C-rheniumcobalt, where the first kappa term, 1k5C, indicates that five of the carbonyl ligands are coordinated to central atom 1 through their carbon atoms. Rhenium is assigned the central atom number 1 because it is reached last on following the arrow shown in Figure 1 (below). Since the listing of the central atoms is based on this same sequence, the reader can interpret the 1 as meaning that the kappa term relates to the first central atom in the list of central atoms at the end of the name. The reader can decipher the name without needing to know or use the complicated sequence of rules that may be required to assign central atom numbers (particularly if the central atoms are of the same element but have different ligand sets). The 1990 recommendations, on the other hand, listed the central atoms alphabetically. This meant that the reader could only work out which central atom each kappa term referred to by applying the rules that the author also used.

Figure 1. The element sequence depicted in this table is also adhered to when ordering central atoms in polynuclear compounds for the purpose of constructing additive names.

Chloro vs. Chlorido
In the names of coordination entities, anionic ligand names are generally modified so that ~ide ligands become ~ido and ~ate ligands become ~ato. However, common practice and past recommendations provided for a number of exceptions to this rule, so that, for example, chloride ligands were represented by chloro in the name, nitrite ligands by nitro (if bonded through the nitrogen atom), and cyanide ligands by cyano. The 2005 recommendations remove these exceptions, so that the general rule is applied to all ligands (page 151). A principal purpose of this change is to make the nomenclature more systematic and easier for novices to learn. An additional benefit is that there is a clearer differentiation between parts of a name that are produced using substitutive nomenclature (commonly used in the nomenclature of organic chemistry, and therefore for organic ligands) and those that relate to the additive nomenclature used when describing a ligand bound to a central atom. Thus, under the new recommendations, chloro is used to indicate a chlorine atom substituent within a ligand that is named substitutively, while chlorido is used to indicate a chloride ligand that is bound directly to a central atom. Under the new recommendations, the phrase chloromethyl in a name would refer to one ligand, but chloridomethyl would refer to two. Under the previous recommendations the differentiation would be critically dependent on the appropriate placement of enclosing marks, as chloromethyl would be used in both cases. This recommendation therefore reduces the potential for names to be ambiguous.

New Recommendations for Formulae—Ordering, Donor Atoms
The recommended order of symbols in formulae of coordination entities is to place the central atom symbol first, followed by the ligand symbols (which may be line formulae, abbreviations, or acronyms) arranged in alphabetical order (noting that element symbols are treated as a single character—so CO precedes Cl, because C alphabetically precedes Cl). There is no longer any requirement to list the anionic ligands before neutral ligands, which should make producing formulae more straightforward, as there is now only one ordering principle being applied, and there is no need to decide if a ligand is anionic (page 153).

It should be noted that there is an explicit statement that these ordering recommendations should be violated in order to provide additional structural information. Thus, for example, bridging ligand symbols should be placed between the relevant central atom symbols wherever possible. In addition, some ligand symbols might be placed first in a formula, as a way of grouping ligands bound to one central atom separately from those that are bound to another (see the formula given for the rhenium-cobalt complex, above).
In addition, when line formulae are used for ligand symbols, the line formula should be altered so that the donor atom is closest to the central atom symbol wherever possible. The rationale for these recommendations is that formulae are rather limited in the way in which they can describe structural information and identify donor atoms, particularly in more complicated cases such as polynuclear systems. This is because the kappa convention is not generally applied to formulae. Careful location of ligand/element symbols in formulae can go some way to replacing that convention, at least in the case of systems with two central atoms.

These are just some examples of the issues that are faced by those revising nomenclature. Hopefully the resulting decisions make a useful tool for those who need to use nomenclature for inorganic compounds.

The author wishes to thank his coeditors—Neil G. Connelly, Ture Damhus, and Alan T. Hutton—for their useful comments on this article and for the fun times spent working on the Red Book revision.

Richard Hartshorn <[email protected]> is an associate professor of chemistry and biochemistry at the University of Canterbury in Christchurch, New Zealand. In IUPAC, he is a member of the Chemical Nomenclature and Structure Representation Division (VIII) and he is the division representative on the Committee on Chemistry Education.

*The Royal Society of Chemistry, 2005 [ISBN 0 85404 438 8]

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