Nomenclature of organometallic compounds of the transition elements
Concepts and conventions
The (additive) nomenclature of coordination complexes, the general deﬁnitions and rules of which are given in Sections , provides the basis for the system presented here for naming organometallic compounds of the transition elements. The general concepts of coordination chemistry can be applied to organometallic compounds but need to be expanded to deal with the additional modes of connectivity afforded by the interaction of metals with, for example, organic ligands containing unsaturated groupings, such as alkenes, alkynes and aromatic compounds. This section examines relevant concepts and conventions from coordination chemistry as they are applied to organometallic compounds, and indicates what new conventions need to be introduced in order to designate unambiguously the special bonding modes of organometallic compounds.
The deﬁnition of coordination number as being equal to the number of s-bonds between the ligands and the central atom also applies to ligands such as CN-, CO, N2 and PPh3, where the bonding of a single ligating atom to a metal may involve a combination of s- and p-components. The p-bond components are not considered in determining the coordination number, and so [Ir(CO)Cl(PPh3)2], [RhI2(Me)(PPh3)2] and [W(CO)6] have coordination numbers of four, ﬁve and six, respectively.
However, this deﬁnition cannot be applied to the many organometallic compounds in which two or more adjacent atoms of a ligand interact with the central metal atom through what is often a combination of s, p and d bonding (the labels s, p or d referring to the symmetry of the orbital interactions between ligand and central atom).
For example, a ligand such as ethene, consisting of two ligating carbon atoms, nevertheless brings only one pair of electrons to the central atom. Likewise, ethyne, coordinating via both carbon atoms, can be thought to bring either one or two pairs of electrons to a single metal atom, depending on the type of coordination involved. Both ligands are normally regarded as monodentate. This changes when ethene or ethyne is considered to add oxidatively to a central metal atom; they are then considered to be bidentate chelating ligands which, on electron counting and dissection of the coordination entity to determine oxidation numbers, are assumed to take two pairs of electrons with them. This different view can be expressed by referring to compounds of such ligands as metallacyclopropanes or metallacyclopropenes rather than ethene or ethyne complexes.
The concept of chelation can again be applied strictly only to those organometallic complexes in which the donor atoms of a ligand are attached to the central metal atom through s-bonds alone. Otherwise, ambiguities will result, as outlined above, even with a simple ligand such as ethene. Butadiene and benzene supply two and three pairs of electrons upon coordination and are therefore regarded as bi- and tridentate ligands, respectively. In stereochemistry, however, such ligands are often treated as if they were monodentate.
Compounds with one metal–carbon single bond
In naming organometallic compounds the usual rules for naming ligands in coordination entities are applied if the ligand coordinates via an atom other than carbon . Thus, the ligand MeCOO- is named acetato, Me2As- is named dimethylarsanido, and PPh3 is named triphenylphosphane.
If an organic ligand coordinating via one carbon atom is regarded as an anion formed by the removal of one hydron from that atom, the ligand name is formed by replacing the ending ‘ide’ of the anion name by ‘ido’.
1. 3- methanido
2. 3CH2- ethanido
3. 2¼CHCH2)- prop-2-en-1-ido
4. H5- benzenido
5. 5H5)- cyclopentadienido