Formulae enclosing marks

Enclosing Marks. Where it is necessary in an inorganic formula, enclosing marks (parentheses, braces, and brackets) are nested within square brackets as follows ... 

The dashes at each end of the formulae of entries 1, 2, 3, 4, 7, and 8 are drawn fully inside the enclosing marks, because the identities of the units at the ends of the chains are unknown. 

Rule 1.7 The formulae of end groups, if known, may be attached to the bonds at the ends of the constitutional units, but placed outside the enclosing marks. 

Where it is known that certain atoms in a molecular ion are bound together to form a group, as with S and O in S04 , these elements can be so grouped in the formula, with or without enclosing marks, depending upon the compound and upon the users requirements. 

Coordination compounds. [In the formula of a coordination entity.] the symbol of the central atom(s) is placed first, followed by the ionic and then the neutru] ligands. Square brackets are used to enclose the whole coordination entity whether charged or not. This practice need not be used for simple species such us the common oxoanions (NO, NOT, SO -, OH-, etc.). Enclosing marks are nested within the square brackets as follows [()]. U<) , [()]>]. [( [()] ]]. etc. 

In formulae, these enclosing marks are used in the following nesting order [],[()],[ () ], [( () )], [ ( () ) ], etc. Square brackets are normally used only to enclose entire formulae parentheses and braces are then used alternately (see also Sections IR-4.2.3 and IR-9.2.3.2). There are, however, some specific uses of square brackets in formulae, cf. Section IR-2.2.2.1. 

IR-4.2.3 Structural formulae and the use of enclosing marks in formulae IR-4.2.4 Formulae of (formal) addition compounds IR-4.2.5 Solid state structural information IR-4.3 Indication of ionic charge IR-4.4 Sequence of citation of symbols in formulae IR-4.4.1 Introduction IR-4.4.2 Ordering principles IR-4.4.2.1 Electronegativity IR-4.4.2.2 Alphanumerical order IR-4.4.3 Formulae for specific classes of compounds IR-4.4.3.1 Binary species... 

As soon as the compound has even a slightly more complex structure, it becomes necessary to use enclosing marks in line formulae to separate subgroups of atoms. Different enclosing marks must be used for repeating units and sidechains in order to avoid ambiguity. 

The basic rules for applying enclosing marks in structural formulae are as follows ... 

IR-9.2.3.1 Sequence of symbols within the coordination formula IR-9.2.3.2 Use of enclosing marks IR-9.2.3.3 Ionic charges and oxidation numbers IR-9.2.3.4 Use of abbreviations IR-9.2.4 Specifying donor atoms IR-9.2.4.1 General IR-9.2.4.2 The kappa convention... 

The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulae are enclosed in parentheses. Ligand abbreviations are also usually enclosed in parentheses. The nesting order of enclosing marks is as given in Sections IR-2.2 and IR-4.2.3. Square brackets are used only to enclose coordination entities, and parentheses and braces are nested alternately. 

Examples 1-11 in Section IR-9.2.2.4 illustrate the use of enclosing marks in formulae. Note also that in those examples there is no space between representations of ionic species within a formula. 

Note from the examples above that in order to stress the distinctions discussed, the square brackets are sometimes used in the Table to enclose formulae for molecular entities that are otherwise often written with no enclosing marks. When a formula with square brackets is shown, there will also be a coordination-type additive name. 

The chemical structure of the CRU is enclosed in parentheses or brackets. While dashes representing chemical bonds may be omitted within the formula unless necessary for clarity, at the ends of the CRU, dashes must be shown. They are drawn across the enclosing marks. 

The constitutional or nomenclatorial elements separated according to step 3 are individually named (or assigned to partial formulae) and then provided with appropriate locants (numerals, letters) and markers (enclosing marks, hyphens, primes, etc.). 

Blocks should be arranged in sequence as they are to appear in the text, with their location clearly indicated. Structures that are mentioned in the text should be numbered in sequence using arable numbers, marked for bold-face type, enclosed in parentheses, and centered beneath the formulae. In the text, structures should be referred to by these numbers. Formulae that are not mentioned in the text need not be numbered. 

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