Stock nomenclature

Stock nomenclature, sometimes call the Stock system, is a system of chemical nomenclature used for some inorganic compounds, particularly ionic compounds of the transition metals. It was first proposed by German chemist Alfred Stock in 1919, and is one of the systems of inorganic nomenclature recommended for use by the International Union of Pure and Applied Chemistry (IUPAC).

The principle of Stock nomenclature is the the indication of the oxidation state of one or more of the elements in the compound. This is important for elements which have more than one common oxidation states, e.g. the majority of the transition metals. Copper, for example, forms two chlorides, CuCl and CuCl₂: these are named copper(I) chloride and copper(II) chloride respectively. The oxidation number is placed in parentheses immediately after the name of the element it refers to (there is no space between the end of the element name and the opening parenthesis): roman numerals are used for oxidation numbers, which are assumed to be positive unless a minus sign is present, e.g. sodium pentacarbonylmanganate(−I).

Cations

For simple monoatomic cations in ionic compounds, the oxidation number is the same as the charge number of the ion. Some elements form polyatomic cations, for which the oxidation state must be calculated by the normal rules, for example oxidovanadium(IV), VO²⁺; dioxidouranium(VI), UO2+2.

Anions

It is not usually necessary to give the oxidation state for simple monoatomic anions: hence, chloride is assumed to refer to the species Cl⁻, and it is unnecessary to write chloride(−I).

Usage

Stock nomenclature was originally developed in German. Stock's original proposal was to use normal (Arabic) numerals with hyphens separating them from the rest of the name: hence (in English) "copper-2-chloride"; German Kupfer-2-chlorid. However the use of Roman numerals and parentheses quickly became established (although the second hyphen is retained in German names: Kupfer(II)-chlorid).

The use of Stock nomenclature in English became established after the Second World War, especially in introductory chemistry teaching. It has the advantage that students do not have to learn sets of adjectives for each element (e.g. "cuprous", "cupric", for copper(I) and copper(II) respectively) and the construction of names becomes an additional exercise in calculating oxidation numbers. The system also proved convenient to adapt to different languages.

Stock nomenclature is particularly useful for preparing alphabetical lists of inorganic compounds, for example in chemical catalogues.

Limitations

The Chemical Abstracts Service (CAS) used Stock nomenclature in its 7th Collective Index ("7CI", 1966), but has since moved to a system based on additive nomenclature and charge numbers, which is more general. Stock nomenclature is poor at dealing with mixed valence species or species with fractional oxidation numbers: for example, the superoxide anion, O−2, can be easily an systematically named as "dioxide(1−)" using charge numbers but cannot be named in Stock nomenclature. The use of charge numbers also avoids a number of problems in coordination chemistry, where ligands such as nitrosyl may have different formal charges in different species.

IUPAC specifically recommends against using Stock nomenclature for homonuclear polyatomic ions: in some cases, Stock nomenclature is possible, such as "dimercury(I)" for dimercury(2+) and "dioxide(−I)" for dioxide(2−) (peroxide), but it is still confusing.