Page 64 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

IR-4.2 F O R M U L A E

IR-4.2.3 Structural formulae a nd the use o f e nclosing marks in formulae
A s tructural f ormula g ives partial or complete information about the way in which the atoms
in a m olecule are connected and arranged in space. In simple cases, a l ine f ormula t hat is just
a s equence of atomic s ymbols gives s tructural i nformation provided the reader knows t hat
the formula represents the order of the atoms in the linear structure.
Examples:

1. HOCN (empirical formula CHNO)
2. HNCO (empirical formula also CHNO)
3. HOOH (empirical formula HO)
As soon as the compound has even a s lightly more complex structure, it becomes necessary
to use enclosing marks in line f ormulae to separate s ubgroups of atoms. Different enclosing
marks must b e u sed for repeating units and sidechains in order to avoid ambiguity.
The basic rules for applying enclosing marks in structural formulae are as follows:

(i) Repeating u nits in chain compounds a re enclosed in square brackets.
(ii) Side groups to a m ain chain and groups (ligands) attached to a c entral atom are
enclosed in parentheses (except single atoms when t here is no ambiguity regarding
their attachment in the structure, e.g. hydrogen in hydrides with a c hain structure).
(iii) A f ormula or part of a f ormula w hich represents a m olecular entity may be placed in
enclosing marks. If an entire formula is enclosed, square brackets m ust b e u sed,
except if rule (v) applies.
(iv) A p art of a f ormula which is to be multiplied b y a subscript may also be enclosed in
parentheses or braces, except i n t he case of repeating units in chain compounds,
cf. r ule (i).
(v) In the case of polymers, if the bonds between r epeating units are to be shown, the
repeating unit is enclosed in strike-through parentheses, with the dash superimposed
on the parentheses representing the bond. (If this is typographically inconvenient,
dashes can be placed before a nd after the parentheses.)
(vi) Inside square b rackets, enclosing marks are nested as follows:
( ) , { ( ) }, ({( ) }), {({( ) })}, etc.
(vii) Atoms or groups of atoms which are represented together with a p refixed s ymbol,
e.g. a s tructural m odifier such as ‘m’, are placed within e nclosing marks, using the
nesting order of (vi).
The use of enclosing marks for the specification of isotopic modification is described in
Section IR-4.5.
Compared to line formulae, displayed formulae (Examples 12 and 13 below) give more (or
full) information about the structure.
(The rules needed f or ordering the symbols in some o f t he example f ormulae below are
given in Section IR-4.4.3.)

Examples:
[rule (i)]
4. SiH 3 [SiH 2 ] 8 SiH 3
[rules (i) and (ii)]
5. SiH 3 [SiH 2 ] 5 SiH(SiH 3 )SiH 2 SiH 3

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