Page 25 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

O R G A N O M E T A L L I C C O M P O U N D S IR-10.2

However, this definition cannot be applied to the many organometallic compounds in
which two or more adjacent atoms of a l igand interact with the central metal atom through
what is often a c ombination of s, p and d bonding (the labels s, p or d referring to the
symmetry of the orbital i nteractions between ligand and central atom).
For example, a l igand such as ethene, consisting of two l igating 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 s ingle metal
atom, depending on the type of coordination involved. B oth ligands are normally regarded as
monodentate. This changes when e thene or ethyne is considered to add oxidatively to a c entral
metal atom; they are then considered to be bidentate chelating ligands which, on electron
counting and dissection o f t he coordination entity to determine o xidation numbers, are
assumed to take two pairs of electrons with them. This different view can be expressed b y
referring to compounds of such ligands as metallacyclopropanes o r m etallacyclopropenes
rather than ethene or ethyne complexes.

IR-10.2.1.2 Chelation
The concept of chelation (Section IR-9.1.2.7) can again b e a pplied strictly only to those
organometalliccomplexes in which the donor atoms of a l igand 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 t wo and three pairs of electrons
upon coordination and are therefore regarded as bi- and tridentate ligands, r espectively.
In stereochemistry, however, such ligands are often treated as if they were monodentate.

IR-10.2.1.3 Specifying connectivity

In the event of a l igand containing several different donor atoms, particularly w hen not all
are used, the point or points of attachment to the metal are specified using the kappa (k)
convention (see Sections IR-9.2.4.1 and IR-9.2.4.2). In organometallic nomenclature the
ligating carbon atoms are often sufficiently specified within the ligand name. However, use
of the kappa notation becomes necessary to indicate the attachment of heteroatoms, and also
to specify the particular p oints of attachment of a s ingle ligand when bridging different metal
centres in a p olynuclear complex. The s trength of the kappa convention is that its use
completely avoids any ambiguities in describing the connectivities between a ligand and one
or more m etal centres Its use in organometallic nomenclature is discussed further in Section
IR-10.2.3.3.
A c omplementary notation, the eta (Z) c onvention, is used to specify the number
(‘hapticity’) of contiguous ligating atoms that are involved in bonding to one or more metals.
The need for this convention arises from the special nature of the bonding of unsaturated
hydrocarbons to metals via their p-electrons, and it is used only when there are several
contiguous atoms involved in the bond to the metal. The contiguous atoms of the
p-coordinated ligand are often the same element, but they need not be, and they may also be
atoms other than carbon. The eta convention is defined in Section IR-10.2.5.1, where its use
is extensively i llustrated. Even though all connectivity can be expressed b y t he kappa
convention alone, the practice in organometallic nomenclature is that the eta convention
should be used wherever there a re contiguous ligating atoms. Complicated structures may
require the use of both conventions (see Section IR-9.2.4.3).

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