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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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