Addi(c)tive names

Let’s have a look at uranium hexafluoride, UF₆. As you remember, in binary-type nomenclature both formulae (e.g. UF₆) and names (e.g. uranium hexafluoride) are divided in two parts, electropositive (or less electronegative) always followed by (more) electronegative. Grammatically, the binary name is a noun phrase consisting of a head noun such as hexafluoride preceded by an attributive noun such as uranium^*.

Neither the compositional name nor formula tell us anything about its structure. For all we know, it could be an ionic compound composed of ions U⁶⁺ and F⁻.

Now let’s say we learn from Wikipedia that this compound consists of discrete molecules which could be represented like this:

(a)

Can we give a name that reflects the structure (a)?

Yes we can do it using additive nomenclature, also known as coordination nomenclature. Here, the order is determined not by what is positive or negative, but by what is in the centre and what is on periphery of the coordination entity defined as

an assembly consisting of a central atom (usually metallic) to which is attached a surrounding array of other groups of atoms (ligands).

In case of our compound, the central atom is uranium and the six fluorine atoms are the ligands. The molecular formula of the coordination entity, with the central atom listed first, is enclosed in square brackets: [UF₆].

The name is now a single word, hexafluoridouranium, and the ligands are listed first, followed by the central atom. It is as if the formula is being read aloud back to front: ‘6FU’. The ligand name, ‘fluorido’, a modification of the free anion name ‘fluoride’, is a prefix to the root ‘uranium’.

In general, for anionic ligands, the endings -ide, -ate and -ite change to -ido, -ato and -ito, respectively:

Anion			Ligand
Formula	Name		Formula	Name
Br−	bromide	→	–Br	bromido
CN−	cyanide	→	–CN	cyanido
Cl−	chloride	→	–Cl	chlorido
F−	fluoride	→	–F	fluorido
H−	hydride	→	–H	hydrido
I−	iodide	→	–I	iodido
NO2−	nitrite	→	–N(O)2	nitrito
NO3−	nitrate	→	–ON(O)2	nitrato
O2−	oxide	→	=O or –O−	oxido
OCN−	cyanate	→	–OCN	cyanato
			–NCO	isocyanato
OH−	hydroxide	→	–OH	hydroxido
S2−	sulfide	→	=S or –S−	sulfido
SCN−	thiocyanate	→	–SCN	thiocyanato
			–NCS	isothiocyanato
SO32−	sulfite	→	–S(O)2O−	sulfito
SO42−	sulfate	→	–OS(O)2O−	sulfato

While the names of neutral and cationic ligands remain unchanged, with a few exceptions:

Molecule			Ligand
Formula	Name		Formula	Name
CO	carbon monoxide	→	–CO	carbonyl
NH3	ammonia	→	–NH3	ammino
NO	nitrogen monoxide	→	–NO	nitrosyl
OH2	water	→	–OH2	aqua

Unlike binary-type nomenclature, which implies that the substance being named is neutral, the additive nomenclature can be used to name both neutral and charged molecular entities. In the case of ions, the name is followed by overall charge of the entity in parentheses, while the anions additionally acquire the ending -ate:

(b)	(c)	(d)

[Ni(CO)4] tetracarbonylnickel [Fe(NO)(OH2)5]2+ pentaaquanitrosyliron(2+) [Ag(SCN)4]3− tetrakis(thiocyanato)argentate(3−)

Although additive nomenclature was originally developed for coordination compounds, it could be applied to molecular entities not usually thought of as coordination ones:

(e)	(f)	(g)

[NO]• oxidonitrogen(•) [OCl2] dichloridooxygen [SH3]+ trihydridosulfur(1+)

The entity (e) is a radical, which is indicated by the radical dot • in both formula and name. Note that in the entities (f) and (g) the central atom is more electronegative than the ligands.

Additive names can be used within binary-type names:

(h)	(i)	(j)

[NH4]+ ammonium [Fe(CN)6]4− hexacyanidoferrate(4−) (NH4)4[Fe(CN)6] ammonium hexacyanidoferrate(4−)

Here, the molecular entities with known structure, (h) and (i), are stoichiometrically combined into a compound (j) without specified structure. Accordingly, the trivial name ‘ammonium’ [one could have named it additively ‘tetrahydridonitrogen(1+)’], and the additive name ‘hexacyanidoferrate(4−)’ are combined into a binary-like name, ammonium hexacyanidoferrate(4−) [or ammonium hexacyanidoferrate(II), or tetraammonium hexacyanidoferrate, depending on your preferences].

I like additive names. They are easy to construct — almost from the “first principles” — and interpret. I have to add that, if not for my acquaintance with coordination nomenclature in early 2000s, I would probably still think that chemical nomenclature is boring. Just a couple of observations.

First, can you see a difference between fluoro group –F and fluorido ligand –F? Me neither. According to the Red Book (IR-7.1.3),

it is a matter of convention whether a ligand is considered to be anionic, neutral or cationic.

But what’s the point of considering a ligand anionic? Historically, the “anionic” ligands that now end with -ido were named bromo, chloro, cyano, fluoro, hydro, hydroxo, iodo, oxo, etc. Since -ido names are not used in either compositional or substitutive nomenclature, they serve as markers of additive names. However, this does not seem to be necessary as we can recognise the additive name anyway. For example, hexafluorouranium or hexacyanoferrate(4−) are unmistakably additive as well as unambiguous names for the entities (a) and (i), respectively. Moreover, they are one syllable shorter than IUPAC-recommended additive names, and shorter is always better in my book.

Second, you may have noticed that the name of the central atom in anionic coordination entities is modified, sometimes dramatically, to end with -ate (iron → ferrate, silver → argentate) while in the cationic ones it stays unchanged (iron → iron). Why do we need that if the names of both cations and anions are explicitly followed by the charge (in parentheses)? Once again, the special treatment of anions appears to be redundant. The explanation along the lines “it’s historical” is not good enough: many historical names are no longer recommended. And if we continue to use acceptable non-systematic names such as sulfate for [SO₄]²⁻, I find it confusing to use ‘sulfate’ also as a part of any anion with sulfur as the central atom, as in hydridosulfate(1−) for [HS]⁻ or trioxidosulfate(2−) for [SO₃]²⁻.

*	In other languages the order of words could be different. Cf. Russian гексафторид урана or Spanish hexafluoruro de uranio. In German, the corresponding compositional name is just one word: Uranhexafluorid.