Brief Guide to the Nomenclature of Inorganic Chemistry

Members of Working Party (1998-2002): Richard M. Hartshorn* (New Zealand), Karl-Heinz Hellwich (Germany), Andrey Yerin (Russia), Ture Damhus (Denmark) and Alan T. Hutton (South Africa)

This version is as close as possible to the published version [see Pure Appl. Chem. 87(9-10), 1039-1049 (2015). Copyright IUPAC; reproduced with the permission of IUPAC]. If you need to cite these rules please quote this reference as their source.

A PDF of the printed version is available. An A4 four sided version 1.3 is available.

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Chem.Listy 2021, 115, 673-678. (in Czech)
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The French version was published in Actual. Chim., 2019, 437, 12-17.
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The Spanish version was also published in Annales to Química, 2015, 111(4), after p. 268.
German, Portugese and Thai translations are in preparation.
Catherine E. Housecroft and Alan G. Sharpe, Inorganic Chemistry, 5th edition, 2018, 1248-1251.

Contents

• Preamble
• 1. Stoichiometric or compositional names
• 2. Complexes and additive nomenclature
• 2.1. Overall approach
• 2.2. Central atom(s) and ligands
• 2.3. Assembling additive names
• 2.4. Specifying connectivity
• 2.5. Bridging ligands
• 2.6. Organometallic compounds
• 2.7. Formulae of coordination compounds
• 2.8. Inorganic oxoacids and related compounds
• 3. Stereodescriptors
• 4. Summary
• References

Preamble

The boundaries between ‘organic’ and ‘inorganic’ compounds are blurred. The nomenclature types described in this document are applicable to compounds, molecules and ions that do not contain carbon, but also to many structures that do contain carbon (Section 2), notably those containing elements of Groups 1–12. Most boron-containing compounds are treated using a special nomenclature [8].

1. Stoichiometric or compositional names

For homoatomic entities, where only one element is present, the name is formed (Table 1) by combining the element name with the appropriate multiplicative prefix (Table 2). Ions are named by adding charge numbers in parentheses, e.g., (1+), (3+), (2–), and for (most) homoatomic anion names ‘ide’ is added in place of the ‘en’, ‘ese’, ‘ic’, ‘ine’, ‘ium’, ‘ogen’, ‘on’, ‘orus’, ‘um’, ‘ur’, ‘y’ or ‘ygen’ endings of element names [9]. Exceptions include Zn and group 18 elements ending in ‘on’, where the ‘ide’ ending is added to the element names. For some elements (e.g., Fe, Ag, Au) a Latin stem is used before the ‘ide’ ending (cf. Section 2.3) [9]. Certain ions may have acceptable traditional names (used without charge numbers).

Binary compounds (those containing atoms of two elements) are named stoichiometrically by combining the element names and treating, by convention, the element reached first when following the arrow in the element sequence (Figure 1) as if it were an anion. Thus the name of this formally ‘electronegative’ element is given an ‘ide’ ending and is placed after the name of the formally ‘electropositive’ element followed by a space (Table 3).

Figure 1: Element sequence

Again, multiplicative prefixes (Table 2) are applied as needed, and certain acceptable alternative names [10] may be used. Stoichiometry may be implied in some cases by the use of oxidation numbers, but is often omitted for common cases, such as in calcium fluoride.

Heteropolyatomic entities in general can be named similarly using compositional nomenclature, but often either substitutive [11] or additive nomenclature (Section 2) is used. In the latter case information is also provided about the way atoms are connected. For example, POCl₃ (or PCl₃O, compositional name phosphorus trichloride oxide) is given an additive name in Table 10.

Certain ions have traditional short names, which are commonly used and are still acceptable (e.g., ammonium, NH₄⁺; hydroxide, OH^–; nitrite, NO₂^–; phosphate, PO₄^3–; diphosphate, P₂O^4–).

Inorganic compounds in general can be combinations of cations, anions and neutral entities. By convention, the name of a compound is made up of the names of its component entities: cations before anions and neutral components last (see examples in Table 4).

The number of each entity present has to be specified in order to reflect the composition of the com- pound. For this purpose multiplicative prefixes (Table 2) are added to the name of each entity. The prefixes are ‘di’, ‘tri’, ‘tetra’, etc., for use with names for simple entities, or ‘bis()’, ‘tris()’, ‘tetrakis()’, etc., for names for most entities which themselves contain multiplicative prefixes or locants. Care must also be taken in situations when use of a simple multiplicative prefix may be misinterpreted, e.g., tris(iodide) must be used for 3I^– rather than triiodide (which is used for I^3–), and bis(phosphate) rather than diphosphate (which is used for P₂O₇^4–). Examples are shown in Table 4. There is no elision of vowels (e.g., tetraaqua, pentaoxide), except in the special case of monoxide.

Names of neutral components are separated by ‘em’ dashes without spaces. Inorganic compounds may themselves be components in (formal) addition compounds (last four examples in Table 4). The ratios of component compounds can be indicated, in general, using a stoichiometric descriptor in parentheses after the name (see the last three examples in Table 4). In the special case of hydrates, multiplicative prefixes can be used with the term ‘hydrate’.

2. Complexes and additive nomenclature

2.1 Overall approach

Table 5: Producing names for complexes: simple ligands.

Structure to be named
Central atom(s)	cobalt(III)	2 × rhenium
Identify and name ligands	ammonia → ammine water → aqua	chloride → chlorido
Assemble name	pentaammineaquacobalt(III) chloride	caesium bis(tetrachloridorhenate)(Re—Re)(2–)

2.2 Central atom(s) and ligands

Each ligand is named as a separate entity using appropriate nomenclature [4] – usually substitutive nomenclature for organic ligands [2, 4, 6] and additive nomenclature for inorganic ligands. A small number of common molecules and ions are given special names when present in complexes. For example, a water ligand is represented in the full name by the term ‘aqua’. An ammonia ligand is represented by ‘ammine’, while carbon monoxide bound to the central atom through the carbon atom is represented by the term ‘carbonyl’ and nitrogen monoxide bound through nitrogen is represented by ‘nitrosyl’. Names of anionic ligands that end in ‘ide’, ‘ate’, or ‘ite’ are modified within the full additive name for the complex to end in ‘ido’, ‘ato’, or ‘ito’, respectively. Note that the ‘ido’ ending is now used for halide and oxide ligands as well. By convention, a single coordinated hydrogen atom is always considered anionic and it is represented in the name by the term ‘hydrido’, whereas coordinated dihydrogen is usually treated as a neutral two-electron donor entity.

2.3 Assembling additive names

If there is more than one ligand of a particular kind bound to a central atom in the same way, the number of such identical ligands is indicated using the appropriate multiplicative prefix for simple or complicated ligands (Table 2), not changing the already established alphabetical order of ligands. The nesting order of enclosing marks, for use in names where more than one set of enclosing marks is required, is: (), [()], {[()]}, ({[()]}), etc.

Any metal-metal bonds are indicated by placing the central atom symbols in parentheses, in italics and connected by an ‘em’ dash, after the name of the complex (without spaces). The charge number of the complex or the oxidation number of the central atom is appended to the name of the complex. For anions that are named additively, the name of the central atom is given the ‘ate’ ending in a similar way to the ‘ide’ endings of homoatomic anions (Section 1). In some cases, by tradition, the Latin stem is used for the ‘ate’ names, such as in ferrate (for iron), cuprate (for copper), argentate (for silver), stannate (for tin), aurate (for gold), and plumbate (for lead) [12]. Finally, the rules of compositional nomenclature (Section 1) are used to combine the additive names of ionic or neutral coordination entities with the names of any other entities that are part of the compound.

2.4 Specifying connectivity

A κ-term is required for ligands where more than one coordination mode is possible. Typical cases are thiocyanate, which can be bound through either the sulfur atom (thiocyanato-κS) or the nitrogen atom (thiocyanato-κN), and nitrite, which can be bound through either the nitrogen atom (M–NO₂, nitrito-κN) or an oxygen atom (M–ONO, nitrito-κO). The names pentaammine(nitrito-κN)cobalt(2+) and pentaammine(nitrito-κO)cobalt(2+) are used for each of the isomeric nitrito complex cations. More examples of constructing names using κ-terms to specify the connectivity of ligands are shown in Table 6. A κ-term may also be used to indicate to which central atom a ligand is bound if there is more than one central atom (Section 2.5).

2.5 Bridging ligands

di-μ-chlorido-bis[dichloridoaluminium(III)] [Cl2Al(μ-Cl)2AlCl2]	μ-peroxido-1κO1,2κO2-bis(trioxidosulfate)(2–) [O3S(μ-O2)SO3]2–

2.6 Organometallic compounds

The name for an organic ligand binding through one carbon atom may be derived either by treating the ligand as an anion or as a neutral substituent group. The compound [Ti(CH₂CH₂CH₃)Cl₃] is thus named as trichlorido(propan-1-ido)titanium or as trichlorido(propyl)titanium. Similarly, ‘methanido’ or ‘methyl’ may be used for the ligand –CH₃.

When an organic ligand forms two or three metal-carbon single bonds (to one or more metal centres), the ligand may be treated as a di- or tri-anion, with the endings ‘diido’ or ‘triido’ being used, with no removal of the terminal ‘e’ of the name of the parent hydrocarbon. Again, names derived by regarding such ligands as substituent groups and using the suffixes ‘diyl’ and ‘triyl’ are still commonly encountered. Thus, the bidentate ligand –CH₂CH₂CH₂– would be named propane-1,3-diido (or propane-1,3-diyl) when chelating a metal centre, and μ-propane-1,3-diido (or μ-propane-1,3-diyl) when bridging two metal atoms.

Organometallic compounds containing a metal-carbon multiple bond are given substituent prefix names derived from the parent hydrides which end with the suffix ‘ylidene’ for a metal-carbon double bond and with ‘ylidyne’ for a triple bond. These suffixes either replace the ending ‘ane’ of the parent hydride, or, more generally, are added to the name of the parent hydride with the insertion of a locant and the elision of the terminal ‘e’, if present. Thus, the entity CH₃CH₂CH= as a ligand is named propylidene and (CH₃)₂C= is called propan-2-ylidene. The ‘diido’/‘triido’ approach, outlined above, can also be used in this situation. The terms ‘carbene’ and ‘carbyne’ are not used in systematic nomenclature.

dichlorido(phenylmethylidene)bis(tricyclohexylphosphane-κP)ruthenium,
dichlorido(phenylmethanediido)bis(tricyclohexylphosphane-κP)ruthenium,
or (benzylidene)dichloridobis(tricyclohexylphosphane-κP)ruthenium

The special nature of the bonding to metals of unsaturated hydrocarbons in a ‘side-on’ fashion via their π-electrons requires the eta (η) convention. In this ‘hapto’ nomenclature, the number of contiguous atoms in the ligand coordinated to the metal (the hapticity of the ligand) is indicated by a right superscript on the eta symbol, e.g., η³ (‘eta three’ or ‘trihapto’). The η-term is added as a prefix to the ligand name, or to that portion of the ligand name most appropriate to indicate the connectivity, with locants if necessary.

(η6-benzene)[(1,2,5,6-η)-cycloocta-1,3,5,7-tetraene]cobalt(1+)	tris(η3-prop-2-en-1-ido)chromium, tris(η3-prop-2-en-1-yl)chromium, or tris(η3-allyl)chromium

A list of many π-bonding unsaturated ligands, neutral and anionic, can be found in the Red Book [14].

Note that the ubiquitous ligand η⁵-C₅H₅, strictly η⁵-cyclopenta-2,4-dien-1-ido, is also acceptably named η⁵-cyclopentadienido or η⁵-cyclopentadienyl. When cyclopenta-2,4-dien-1-ido coordinates through one carbon atom via a σ bond, a κ-term is added for explicit indication of that bonding. The symbol η¹ should not be used, as the eta convention applies only to the bonding of contiguous atoms in a ligand.

dicarbonyl(η⁵-cyclopentadienido)(cyclopenta-2,4-dien-1-ido-κC¹)iron
or dicarbonyl(η⁵-cyclopentadienyl)(cyclopenta-2,4-dien-1-yl-κC¹)iron

1-ferrocenylethan-1-one	1,1′-(osmocene-1,1′-diyl)di(ethan-1-one)

By convention, ‘organoelement’ compounds of the main group elements are named by substitutive nomenclature if derived from the elements of Groups 13–16, but by additive nomenclature if derived from the elements of Groups 1 and 2. In some cases compositional nomenclature is used if less structural information is to be conveyed. More detail is provided in the Red Book [15].

2.7 Formulae of coordination compounds

2.8 Inorganic oxoacids and related compounds

The traditional oxoacid names may be modified according to established rules for naming derivatives formed by functional replacement [16]: thus ‘thio’ denotes replacement of =O by =S; prefixes ‘fluoro’, ‘chloro’, etc., and infixes ‘fluorid’, ‘chlorid’, etc., denote replacement of –OH by –F, –Cl, etc.; ‘peroxy’/‘peroxo’ denote replacement of –O– by –OO–; and so forth (Table 9).

If all hydroxy groups in an oxoacid are replaced, the compound is no longer an acid and is not named as such, but will have a traditional functional class name [16] as, e.g., an acid halide or amide. Such compounds may again be systematically named using additive nomenclature (Table 10).

A special construction is used in hydrogen names, which allows the indication of hydrons bound to an anion without specifying exactly where. In such names, the word ‘hydrogen’ is placed at the front of the name with a multiplicative prefix (if applicable) and with no space between it and the rest of the name, which is placed in parentheses. For example, dihydrogen(diphosphate)(2–) denotes H₂P₂O₇^2–, a diphosphate ion to which two hydrons have been added, with the positions not known or at least not being specified.

One may view the common names for partially dehydronated oxoacids, such as hydrogenphosphate, HPO₄^2–, and dihydrogenphosphate, H₂PO₄^–, as special cases of such hydrogen names. In these simplified names, the charge number and the parentheses around the main part of the name are left out. Again, these particular anions may be named systematically by additive nomenclature. The word ‘hydrogen’ is placed separately in forming analogous names in organic nomenclature, for example, dodecyl hydrogen sulfate, C₁₂H₂₅OS(O)₂OH. This difference between the two systems has the consequence that the important carbon-containing ion HCO₃^– can be named equally correctly as ‘hydrogen carbonate’ and as ‘hydrogencarbonate’ (but not as bicarbonate).

3. Stereodescriptors

The relative positions of ligating groups around a central atom can be described using a configuration index that is determined in a particular way for each geometry [18], based on the Cahn-Ingold-Prelog priorities of the ligating groups [19, 20], and it may change if the ligands change, even if the geometry remains the same. The absolute configuration can also be described. Generally configuration indices are used only if there is more than one possibility and a particular stereoisomer is to be identified. The full stereodescriptors for the particular square-planar platinum complexes shown below are (SP-4-2) and (SP-4-1), for the cis and trans isomers, respectively. Alternatively, a range of traditional stereodescriptors may be used in particular situations. Thus the isomers that are possible when a square-planar centre is coordinated by two ligating groups of one type and two of another are referred to as cis- (when the identical ligands are coordinated next to each other) or trans- (when they are coordinated opposite to each other).

cis-diamminedichloridoplatinum(II)	trans-diamminedichloridoplatinum(II)

Octahedral centres with four ligands of one kind and two of another can also be referred to as cis- (when the two identical ligands are coordinated next to each other) or trans- (when they are coordinated opposite each other). Octahedral centres with three of each of two kinds of ligand can be described as fac- (facial), when the three ligands of a particular kind are located at the corners of a face of the octahedron, or mer- (meridional), when they are not.

4. Summary

Names and formulae have only served half their role when they are created and used to describe or identify compounds, for example, in publications. Achieving their full role requires that the reader of a name or formula is able to interpret it successfully, for example, to produce a structural diagram. The present document is also intended to assist in the interpretation of names and formulae.

Finally, we note that IUPAC has produced recommendations on the graphical representation of chemical structures and their stereochemical configurations [21, 22].

References

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