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G R A M  M  A R                                                    I  R-2.15





                       (j) To indicate stereochemistry at particular atoms in structures where arabic numerals have
                       been used for numbering those  a toms.


                       Example:
                          12.
                                 1   2    3    4   5
                               ClSiH SiHClSiHClSiH SiH 3
                                                  2
                                    2
                               (2R,3S)-1,2,3-trichloropentasilane
           IR-2.14.3   Letter locants
                       Italicized upper case letters are used as locants in certain  s ubstitutive names. (See, for
                       example, Section IR-6.2.4.1.)
                       Lower c ase letters are used in polyoxometallate n omenclature to designate t he vertices of the
                       coordination polyhedra around the central atoms. They are attached to the number of the central
                       atom to which a p articular vertex refers. A  detailed treatment is given in Chapter II-1 of Ref. 2.


           IR-2.15     O R D E R I N G  P R I N C I P L E S


           IR-2.15.1   Introduction
                       Chemical  n omenclature deals with names of elements and their combinations. Whereas
                       writing the symbol or the name o f a n e lement is straightforward, a c hoice of which element to
                       write first in the formula and in the name h as to be made a s s oon as an element is associated
                       with one or more  o ther elements to form, for example, a  b inary compound. The  o rder of
                       citation  o f  e lements in formulae and names is based upon the methods outlined below.
                       Furthermore, groups of atoms, such as ions, ligands in coordination compounds and substituent
                       groups in derivatives of parent hydrides, a re ordered according to specified rules.


           IR-2.15.2   Alphabetical order
                       Alphabetical order is used in formulae as follows.

                       (a) Within  t he group of cations and within the group of anions, respectively, in formulae of
                       salts and double salts. Deviations from this rule  m  ay be acceptable if it is desired to convey
                       specific structural i nformation, as in Example 5  b elow.

                       Examples:


                           1. BiClO (anions Cl and O )
                                                   2

                           2. NaOCl (the anion is OCl , cf. S ection IR-4.4.3.1)
                           3. KNa 4 Cl(SO 4 ) 2
                           4. CaTiO 3 (perovskite type)
                           5. SrFeO 3 (perovskite type)

                       (b) In formulae of coordination compounds and species formally treated as coordination
                       compounds, ligands are cited in alphabetical order of their formulae or abbreviations


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