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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
groups that coordinate via the p-electrons o f t heir multiple bonds, such as alkenes, alkynes and
aromatic compounds, but they may also be carbon-free entities c ontaining bonds between
heteroelements; the complexes are then generally referred to as ‘p-complexes’. However, the
exact nature of the bonding (s, p, d) i s o ften uncertain. The atoms bonded to the metal atom
are therefore indicated in a m anner independent of theoretical implications. Thus, the use of
the prefixes s and p is not recommended in nomenclature; these symbols refer to the
symmetry of orbitals and their interactions, which are irrelevant f or nomenclature purposes.
Ligands such as alkenes, alkynes, nitriles and diazenes, and others such as allyl (C 3 H 5 ),
butadiene (C 4 H 6 ), cyclopentadienyl (C 5 H 5 ), cycloheptatrienyl (C 7 H 7 ) a nd cyclooctatetraene
(C 8 H 8 ), may be formally r egarded as anionic, neutral (or sometimes cationic). T he structures
of, and bonding in, their complexes may also be complicated or ill-defined. N ames for such
ligands are therefore chosen that indicate stoichiometric composition and are derived in a
similar way to those f or the ligands discussed in preceding Sections.
Ligands considered as neutral molecules are given a n ame according to the rules of
Ref. 3, including the special nomenclature and numbering applied to fused p olycyclic or
unsaturated heterocyclic ligands (see Section P-25 of Ref. 3).
Ligands regarded as substituent groups derived by removing hydrogen atoms from
(substituted) parent hydrides are given the substituent n ames ending in ‘yl’, ‘diyl’, ‘ylidene’,
etc., depending on the number of hydrogen atoms removed, a gain following Ref. 3 ( in
particular Section P-29). L igands regarded as anions obtained b y r emoving hydrons f rom
(substituted) parent hydrides are given the endings ‘ido’, ‘diido’, etc., depending on the
number of hydrons r emoved.
IR-10.2.5.1 The eta (Z) c onvention
The special nature of the bonding of unsaturated hydrocarbons t o m etals via their p-electrons
has led to the development of the ‘hapto’ nomenclature to designate u nambiguously t he
4
unique b onding modes of the compounds so formed. (See also S ection IR-9.2.4.3.) The G reek
symbol Z (eta) provides a t opological description by indicating the connectivity between t he
ligand and the central atom. The n umber of contiguous atoms in the ligand coordinated to the
3
4
metal is indicated by a r ight superscript numeral, e.g. Z (‘eta three’ or ‘trihapto’), Z (‘eta
5
four’ or ‘tetrahapto’), Z (‘eta five’ or ‘pentahapto’), etc. T he symbol Z is added as a p refix to
the ligand name, or to that portion of the ligand name most appropriate to indicate the
2
5
connectivity, as in cyclopenta-2,4-dien-1-yl-Z -ethene versus vinyl-Z -cyclopentadienyl:
M
M
5
2
cyclopenta-2,4-dien-1-yl-η -ethene vinyl-η -cyclopentadienyl
5
The ligand name Z -cyclopentadienyl, although strictly speaking ambiguous, is
5
acceptable as a s hort form of Z -cyclopenta-2,4-dien-1-yl, due to common usage.
These ligand names are enclosed in parentheses in the full name of a c omplex. Note the
importance o f m aking rigorous use of enclosing marks, etc. to distinguish the above bonding
modes from the other four cases b elow. Note also that when c yclopenta-2,4-dien-1-yl
coordinates at the carbon with the free valence, a k term is added for explicit indication of that
bonding. In general, t his is necessary with names of unsaturated ligands which may participate
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