Nomenclature of Inorganic Chemistry

The purpose of nomenclature in chemistry is to convey information about the material being described. The designation chosen should be unequivocal, at least within the limitations of the type of nomenclature adopted. The type adopted will depend in part on the total amount of information to be conveyed, the kind of compound to be described, and the whim of the person describing the compound.

Nomenclaturists use the terms "trivial" and "systematic" to describe two major divisions of nomenclature. Systematic nomenclature is based on established principles so that it can be extended in a logical way to describe known, new, and hypothetical compounds. A trivial nomenclature is one established by rule of thumb and includes many of the older names (spirit of salt, aqua regia, etc.) and lab nomenclatures (the green chelate, etc.).

Actual usage is often a mixture of the two types, and the fundamental bases of all chemical names, those of the elements, are essentially trivial. Note that the name methane is trivial, but that the name pentane is not. For this discussion, a formula representing a compound can be regarded simply as a kind of name. The principal general (but by no means the only) types of nomenclature used in inorganic chemistry are substitutive and additive (coordination).

Substitutive Nomenclature

Substitutive nomenclature is essentially an organic invention and follows the historical development of organic chemistry. It starts with the designation of an appropriate parent compound from which the compound under discussion can be developed formally by substitution or replacement processes. In organic chemistry these parents can be the paraffins, and in inorganic chemistry they are generally (and arbitrarily) taken to be the hydrides of the elements of Periodic Groups 14, 15, and 16, plus boron, which also has an additional rather specific nomenclature of its own. Thus the formula SiH₃Cl can be named chlorosilane, as a substituted derivative of the saturated parent SiH₄, silane (compare chloromethane). The generation of a radical by the loss of a hydrogen atom from the parent is indicated by modification of the termination, silane becoming silyl, SiH₃^· (the superscript dot indicates an unpaired electron). The name silyl can be used to represent a substituent group in another parent hydride (compare methyl) or for the unbound radical, and the procedure is quite general for all parent hydrides to which the methodology is applied. Silane can also be modified formally by the removal of a proton, yielding the anion SiH₃⁻. The name then takes the characteristic anion ending -ide: silanide. The formal addition of a proton is indicated by another termination (-ium), giving SiH₅⁺, silanium. These terminations are used generally in inorganic nomenclature, as in chloride for Cl⁻ and ammonium for NH₄⁺. Other formal operations recognized in substitutive nomenclature include addition or removal of a hydride from the parent. This can be indicated by the termination -ylium, giving the name silylium for SiH₃⁺.

Other Modifications of Names

The terminations cited above can be used generally in inorganic nomenclature. However, they are sometimes not applicable, especially where parent hydrides are not reasonably definable. Inorganic chemists have tended to assign electropositive and electronegative character to elements, though numerical values are not necessarily easy to define. Metals are generally assigned electropositive character and nonmetals electronegative character. The names developed on this basis may imply formally a saltlike nature even in compounds that are not really salts at all. Thus common salt is called sodium chloride, which is ionic, but phosphorus trichloride is certainly not saltlike. It is not wise to infer the detailed physical nature of a compound from the name alone. In this system the name of the (electropositive) metal is not modified from that of the element, but the name of the electronegative element is, and in the way described in the Substitutive Nomenclature section, above. Similarly we derive oxide and sulfide, for example, from oxygen and sulfur. The same division between electronegative and electropositive parts is evident in the covalent nonionic compound SiCl₄, which can be named silicon tetrachloride , though an equally valid substitutive name is tetrachlorosilane.

Inorganic chemists also use a further termination to indicate the name of a cation. This is the ending -ate, and it is used as a modification of the name of an oxoacid. Thus sulfuric acid, H₂SO₄, gives rise to sulfate, SO₄²⁻, phosphoric acid to phosphate, PO₄³⁻, and nitric acid to nitrate, NO₃⁻. The partially deprotonated anions such as HSO₄⁻ and H₂PO₄²⁻ are rather more complicated to deal with, and are discussed in Nomenclature of Inorganic Chemistry, often referred to as the Red Book.

In an older procedure that is no longer recommended, the name of an electropositive element displaying more than one oxidation state in its compounds was sometimes modified to indicate the particular oxidation state involved. Thus iron chlorides were often named ferrous chloride and ferric chloride to convey the two oxidation states of II and III (note that, like normal arabic numbers, these Roman numerals are positive unless otherwise shown by a negative sign). However, the use was not consistent. Cuprous and cupric chlorides indicated oxidation states I and II, and phosphorous and phosphoric chlorides indicated oxidation states III and V. Modern nomenclature specifies the oxidation state of the electropositive partner in these compounds directly: iron(II) chloride, iron(III) chloride, copper(I) chloride, copper(II) chloride, phosphorus(III) chloride, and phosphorus(V) chloride. These designations are unequivocal. The number of counter anions, 1, 2, 3, or 5, should immediately be evident. Examples of negative oxidation states include oxide(−II) or oxide(2−), and dioxide(−I) or dioxide(1−). Note that in a multi-atom group, of which PO₄³⁻ may be taken as an example, the charge on any given atom may not be evident, even if the overall charge is known. In contrast, the oxidation states phosphorus(V) and oxide(−II) are much more readily defined. The use of such charges in names and formulae in these circumstances is not recommended.

Formulae

The rules for formulae for the compounds discussed above are rather elastic. At its simplest, a formula is a list of element symbols accompanied by multiplying subscripts indicating the atomic proportions of each kind of atom. These formulae may be empirical, simply corresponding to the atom ratios, or stoichiometric, representing the totality of the atoms within a molecule. The latter can be used to calculate a molecular weight. Strictly speaking, for a compound that exists as discrete molecules, this latter can also be termed a molecular formula, but this is a misnomer for ionic compounds and for compounds of which the structure changes with temperature. The ordering of these symbols can be adjusted to suit the requirements of the user. At the simplest, an alphabetical order is used, since this is the same in most European languages. Many chemists emphasize the importance of carbon and hydrogen and adopt a sequence C, H, N, and then the remaining element symbols in alphabetical order. Such devices are often employed in indexes. Inorganic chemists often group the atomic symbols in a formula in electropositive and electronegative groups, designated as discussed above. This can be a somewhat arbitrary procedure, and the relative positions of atoms in an electronegativity sequence may be established using the Periodic Table. For simple cases, formulae such as NaCl or SiCl₄ are used. Anionic groups are assumed to be electronegative, hence Ca₃(PO₄)₂. The parentheses are used to define the associated groups of atoms within the formula.

Formulae can also be used to indicate two- or three-dimensional structures. This is particularly useful for coordination compounds, which are discussed next. However, this use is not restricted to classical coordination compounds, as the following examples show. Special devices are often adopted to indicate bonds or lines that are not in the plane of the paper. Their use is not consistent throughout chemistry, but the meaning in any given case is generally obvious.

The first example represents a tetrahedral arrangement, because the solid defined by the four chlorine atoms at its apices is a tetrahedron. The second is octahedral , and the third represents two edge-fused tetrahedra. The wedge bonds are pointing in front of or behind the plane of the paper; the thin lines designate bonds in the plane of the paper.

Inorganic chemists often represent tetrahedra, octahedra, and other shapes in their formulae, to help the reader identify molecular shapes. The broken lines designating these shapes are not intended to represent bonds between atoms.

Oxidation states may also be indicated in formulae where this is helpful, though the need to do so is not common in the simplest cases. The following examples show the formalism employed: Fe^IICl₂, Fe^IIICl₃, Cu^ICl, Cu^IICl₂, P^IIICl₃, P^VCl₅.

Coordination Nomenclature

This is an additive nomenclature, and just as organic chemists have developed substitutive nomenclature in parallel with the methodology of substitutive chemistry, inorganic chemists have developed a nomenclature for coordination compounds that arises from the formal assembly of a coordination entity from its components, a central metal ion (in the simplest cases) and its ligands . Such a coordination entity may be neutral or it may carry a charge, positive or negative. Any such charge may be shown in the usual way, using formalisms such as 2− and 3−. Clearly organometallic compounds , depending upon their type, may be named either from substituted parent hydrides or as coordination entities.

Formulae in Coordination Nomenclature

The general rule is that the formula of a coordination entity should always appear within square brackets, even when the entity itself is an infinite polymer. The use of enclosing marks (square brackets, curly brackets, and parentheses) is slightly different for the usage that is common in organic chemistry. The usual priority sequence is [( )], [{( )}], [{[( )]}], [{{[( )]}}], and so on. Brackets should always be used if they make the formula clearer. The order of symbol citation within the formula of a coordination entity should begin with the metal ion followed by the ligands, ideally with charged ligands cited in alphabetical order using the first symbol of the ligand formula, and these are then followed as a class by the neutral ligand formulae, similarly ordered. The division into neutral and charged ligands can be somewhat arbitrary. Since a ligand is generally assumed to present a lone pair of electrons to the central metal, groups such as CH₃ are formally regarded as anions rather than as radicals with unpaired electrons, even though they usually carry the names of radicals. Compounds that really do possess unpaired electrons in the free state can cause problems, especially when calculating oxidation states. For coordination nomenclature purposes, NO, nitrogen(II) oxide, is considered to be a neutral ligand. Complicated ligands may be represented by abbreviations rather than formulae, and lists of recommended abbreviations have been published in sources such as Nomenclature of Inorganic Chemistry. Some examples of these usages are shown in Table 1. The use of square brackets to indicate the coordination entity is fundamental and is a particularly useful device.

Note the negative oxidation state and the η (hapto) connectivity symbol in the last two examples. Where appropriate, stereochemical descriptors, such as cis -, trans -, mer -, and fac -, polyhedral descriptors, and chirality descriptors may be added to give structural information, but these are more often used in names, except for the simplest formulae. Polynuclear species may be described using the appropriate multiplicative suffixes, and bridging ligands can also be shown. The bridging symbol μ_n is useful for this purpose.

The subscript may be omitted if a ligand bridges only two groups. Polymeric materials can be indicated in an empirical formula using the indeterminate subscript n. When there are different central metal ions present in a polynuclear compound, the established priority sequence for metal ions should be used to determine the order of citation.

[{Cr(NH₃)₅}(OH){Cr(NH₃)₅]⁵⁺ or [{Cr(NH₃)₅}₂(µ‒OH)]⁵⁺

[Re₂Br₈]⁴⁻ or [(ReBr₄)₂]⁴⁻

[[IrCl₂(CO){P(C₆H₅)₃}₂](HgCl)]

[{PdCl₂}_n ] or [{Pd(µ‒Cl)₂}_n ]

Names in Coordination Nomenclature

The names of coordination entities are assembled using principles similar, but not identical, to those used for formulae. The central atom is always cited last. Its name may be modified by an oxidation state symbol. The ligands are presented in the alphabetical order of their initial letters, neglecting for this purpose any multiplicative prefixes. It is not necessary to divide the ligands into neutral and charged groups. However, the names of negatively charged ligands are generally modified by adding the postfix suffix -o in place of the final -e where it occurs, to indicate that they are indeed bound and not free. As an exception, this is not the case with hydrocarbon ligands such as methyl and ethyl, which retain the names of radicals. The names of neutral ligands are not modified. If the coordination entity itself is negatively charged (but not when it is neutral or positively charged), then the name of the central atom is modified by the ending -ate. These practices are illustrated below.

Note that in some cases it may be useful to introduce additional enclosing marks to ensure clarity: for example, to avoid possible confusion between chloromethylamine, ClCH₂NH₂, and (chloro)methylamine, which implies two separate ligands, Cl and CH₃NH₂. It is for the writer to decide whether such a strategy is useful, depending on the particular case under review. Ammonia as a ligand has the name ammine. Similarly, water has the coordination name aqua.

The symbol η is used above and also quite generally throughout organometallic coordination chemistry to indicate the number of carbon atoms in a ligand that are coordinated to the metal. Other devices to indicate connectivity are the italicized atomic symbols of the donor atoms (useful for indicating structure in complexes containing chelating and polydentate ligands), and for some complicated cases, the κ symbolism may be useful. Examples follow in Figure 3.

Further devices are used in coordination names to show polymeric structures, which may contain bridging groups and metal-metal bonds.

Where different metals are present, priority rules must be applied to assign metal locants.

For more information on this and other topics cited above, as well as for descriptions of the use of geometrical and stereo descriptors, polyhedral

symbols, and configuration indices in names, the reader is referred to the books cited in the bibliography. The international authority with the task of formalizing nomenclature rules, assigning the names of new elements, etc., is the International Union of Pure and Applied Chemistry (IUPAC). All the publications cited in the bibliography carry the authority of IUPAC. Some more specialized inorganic nomenclatures are described in Nomenclature of Inorganic Chemistry II. Principles of Chemical Nomenclature and A Guide to IUPAC Nomenclature of Organic Compounds offer more general treatments suitable for those not requiring the most detailed information.

see also Bonding.

G. J. Leigh

Bibliography

Leigh, G. J. (1990). Nomenclature of Inorganic Chemistry. Oxford, U.K.: Blackwell Science. (This title is often referred to as the Red Book, or since 2001, Red Book I.)

Leigh, G. J., ed. (1998). Principles of Chemical Nomenclature. Oxford, U.K.: Blackwell Science.

McCleverty, J. A., and Connelly, N. G., eds. (2000). Nomenclature of Inorganic Chemistry II. Cambridge, U.K.: The Royal Society of Chemistry. (This is sometimes referred to as Red Book II.)

Richer, J.-C., ed. (1993). A Guide to IUPAC Nomenclature of Organic Compounds. Oxford, U.K.: Blackwell Science.

Rigaudy, J., and Klesney, S. P., eds. (1979). Nomenclature of Organic Chemistry. Oxford, U.K.: Pergamon Press.