35 No. 3
||A column about the Principles of Chemical Nomenclature—A Guide to IUPAC Recommendations, 2011 Edition
The nomenclature of organometallic chemistry often poses a challenge to the chemist. For example, when constructing a systematic name for such a compound should one employ the methods of organic chemical nomenclature, or should one try to adapt the methods of inorganic nomenclature? The answer will depend upon the compound under consideration, with the addition that neither may be directly applicable because the compound presents a problem of assigning a structure with conventional electron-pair bonds. Principles goes some way to dealing with this poser.
Some organometallic compounds, primarily of main-group elements in Groups 13–16, are clearly formally so similar to their carbon analogs that they may be named in a rather like fashion, using an organic-type nomenclature. Hence, we can derive names such as methylalumane and trimethylsilanamine for the compounds AlH2Me and SiMe3NH2 (Me = CH3). The first name is derived from the name alumane, assigned to AlH3, and the second from silane, assigned to SiH4. More complex compounds, such as chain compounds may often be named by applying the methods of substitutive nomenclature to the names of the formal hydrocarbon parents. For example, skeletal replacement nomenclature can be used to develop a name for substances such as MeSiH2OPHOCH2Me. This may be considered to be derived from heptane, which, of course, contains a seven-carbon chain. Hence, the suggested name would be 3,5-dioxa-4-phospha-2-silaheptane. Although it actually contains only three carbon atoms in the chain, the name is unequivocal.
Although organometallic derivatives of Main Groups 1 and 2 may often also be conveniently named by using established additive nomenclature, originally developed to name transition-metal complexes such as [Co(NH3)6]Cl3, hexaamminecobalt(III) trichloride, organometallic derivatives of transition elements may often not be dealt with so easily because they exhibit not only metal-carbon single bonds, but features such as metal-carbon multiple bonds, and also bonds between a metal ion and unsaturated molecules and groups. Nevertheless, additive nomenclature has been adapted to name them too, though additional strategies have had to be devised.
To define the names of non-organometallic complex compounds which exhibit formally different possibilities for the donor atoms, it has been found useful to employ the so-called κ (kappa) convention, whereby the actual donors are specifically indicated. An example is shown below, where two modes of designating the binding of a nitrite ion (NO2 ̶ ) to a metal ion, M, are exemplified.
M-NO2 designated nitrito-κN
M-ONO designated nitrito-κO
Compounds that contain a single metal-carbon bond present no new problems, but if such a bond is formally double or triple, the name of the carbon-donor must be altered by changing the termination –yl (as in methyl) to –ylidene or –ylidyne. However, the κ convention may also be applied to define which of more than one available carbon atom is a donor. Sometimes, two correct systematic names may be derived, as in the example below:
The carbon ligand is regarded as anionic in the first name and a radical in the second. Whether it is preferable to treat a specific carbon-donor ligand as an anion or as a radical is a matter still open for discussion, and Principles does not attempt to differentiate between the two approaches.
Sometimes contiguous atoms in a ligand act together as a donor group to a metal ion. This is particularly true of carbon-containing systems such as alkenes, alkynes, and various aromatic groups. To indicate which of the carbon atoms in a ligand is bonded to the metal ion, then the η (eta) or hapto system is applied, as in the examples below.
However, some compounds, notoriously those of boron, are even more difficult to name. They are sometimes termed electron-deficient and do not always obey the more usual rules governing compounds with two-atom localized electron-pair bonds. Researchers have developed a unique approach to naming these compounds, based upon the names for simple borane polyhedra and boron hydride anions. Often the numbers of hydrido-ligands combined with the polyboron skeleton need to be stated. The practitioners in this area also employ several specialized terms. For example, they may differentiate between heteraboranes and metallaboranes, and they employ the word “subrogation” where others might prefer to use the phrase “skeletal replacement.” A whole new chapter in the new Principles is devoted to these compounds alone.
The methodologies outlined here are described in the new version of Principles, together with guidance upon where to apply them and to which type of compound, with examples of such applications and, for those who require them, references to the more detailed literature.
last modified 20 May 2013.
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