Dealing with dinuclear and polynuclear entities

The additive names are easy to construct for mononuclear entities, that is, when we have to describe an entity with just one central atom. What if we have two?

(a)

[ClO2] OClO• dioxidochlorine(•) μ-chlorido-dioxygen(•)

If we consider the structure (a) a mononuclear entity, [ClO₂], we would call it dioxidochlorine(•). Alternatively, we may think of two oxygen atoms bridged by one chlorine atom, OClO, and call it μ-chlorido-dioxygen(•). Here, a bridging ligand is indicated by the Greek letter μ.

(b)

[Au2Cl6] di-μ-chlorido-tetrachloridodigold di-μ-chlorido-bis(dichloridogold)

This method could be extended to the entities with more than one bridging ligand, e.g. for structure (b) the name will be di-μ-chlorido-tetrachloridodigold or, to stress the symmetry, di-μ-chlorido-bis(dichloridogold).

As the structures become more complex, the names may need to incorporate geometrical and structural descriptors such as catena-, cyclo-, triangulo-, quadro-, tetrahedro-, octahedro-, closo-, nido-, arachno-, etc. (Red Book, p. 259).

(c)

W3O9 hexaoxidotri-μ-oxido-triangulo-tritungsten cyclo-tri-μ-oxido-tris(dioxidotungsten)

The cyclical structure (c) could be systematically named hexaoxidotri-μ-oxido-triangulo-tritungsten or, to emphasise the symmetry, cyclo-tri-μ-oxido-tris(dioxidotungsten).

The bridging index, i.e. the number of central atoms connected by a bridging ligand, can be more than two, as exemplified by the structures (d) and (e):

(d)	(e)

In these cases the bridging index is placed as a subscript to the Greek letter μ (μ₂ is rarely used):

[Fe4(μ3-S)4] tetra-μ3-sulfido-tetrairon [Cu4(μ4-S)] μ4-sulfido-quadro-tetracopper(4Cu—Cu)

Let’s have a look now at asymmetrical entities. Consider the structure (f), an isomer of (a):

(f)

ClOO• chlorido-1κCl-dioxygen(O—O)(•) chloridodioxygen(O—O)(•)

The Red Book suggests chlorido-1κCl-dioxygen(O—O)(•) as the name for (f). This name, compared with the IUPAC name for (a), is rather cumbersome, and, in my view, for no good reason. To start with, it is unpronounceable. Then, it appears to be highly redundant: it includes both names of the elements (chlorido, dioxygen), their symbols (Cl, O) and even two-thirds of its structural formula (O—O). Slightly simplified chloridodioxygen(O—O)(•) is, well, only slightly better. I’d call the structure (f) peroxidochlorine(•) or superoxidochlorine.

(g)

ONNO2 trioxido-1κ2O,2κO-dinitrogen(N—N)

For marginally more complex dinitrogen trioxide, ONNO₂ (g), the IUPAC name is trioxido-1κ²O,2κO-dinitrogen(N—N).

The naming rules for di- and polynuclear entities are explained in detail in the Red Book (IR-7.3.2, IR-7.3.3) and, I’m sure, they make a lot of sense — shame that the names generated according to them are ghastly.

To sum up: the additive nomenclature can deal pretty well with mononuclear entities and, with help of appropriate geometrical/structural descriptors, with symmetrical polynuclear entities. The asymmetry brings about unwieldy additive names. We need a different approach.