# In what order should the compounds containing Ru and Os, or Mo and W be named?

According to IUPAC recommendations for naming of inorganic compounds, the more electropositive element is written first with a suitable prefix, and the more electronegative element is written at the last with a suffix -de and a prefix. For example, $\ce{H2O}$ (Water) can be called dihydrogen monooxide. [this is an modified statement extracted from the wiki encyclopedia-IUPAC nomenclature of inorganic chemistry]
In the above example, $\ce{H}$ (Pauling scale = 2.20 or Allen scale = 2.300) is less electronegative than $\ce{O}$ (Pauling scale = 3.44 or Allen scale = 3.610). So, $\ce{H}$ is placed first, followed by $\ce{O}$.

If suppose we were needed to name the compounds containing

• $\ce{Ru}$ (Pauling scale = 2.2) and $\ce{Os}$ (Pauling scale = 2.2)
• $\ce{Mo}$ (Allen scale = 1.47) and $\ce{W}$ (Allen scale = 1.47)

, in what order should we write the elements?

If we use pauling scale as referance, we would face difficulty in naming $\ce{Ru}$ and $\ce{Os}$, if we use allen scale, we would be in trouble with respect to the elements like $\ce{Mo}$ and $\ce{W}$, etc, as we can see that elements have same electronegativity value. I don't know whether the compounds with the above mentioned elements exist or not, but if we suppose, assume that, they exist, in what order should they be named?

• Electronegativity hasn't been very well defined for the $d$ and $f$ blocks, so I think it may not be a good idea to take its importance in nomenclature too strictly. Also substances with more than one metal element are usually alloys rather than molecules or salts, and only in rather rare occasions do they come without any other elements that have a clear electronegativity difference. – Nicolau Saker Neto Oct 30 '13 at 21:11
• @NicolauSakerNeto yes, but… don't underestimate the power of IUPAC :) even if it's not likely to happen, there is a rule to govern it… – F'x Oct 30 '13 at 22:03
• @F'x Haha, I knew there was something weird about IUPAC basing standards on such a tricky property, but didn't consider they could very well define their own scale! – Nicolau Saker Neto Oct 30 '13 at 22:26

When IUPAC rules refer to electronegativity (and electropositivity), they don't mean actual electronegativity measured by one or another scale. IUPAC decided, for nomenclature's sake, on a formal ordering of electronegativity, which goes like this:

This is Table VI, on page 260 of IUPAC's very own 2005 Red Book, aka Nomenclature of inorganic chemistry. The cases in which it is used are detailed in IR-2.15.3.1 (page 42).

The explanation of the changes from earlier IUPAC recommendations is in IR-1.6.3 of the Red Book:

The element sequence of Table VI
In Nomenclature of Inorganic Chemistry, IUPAC Recommendations 1990 (Ref. 11), the position of oxygen in certain element sequences was treated as an exception. Such exceptions have been removed and the element sequence of Table VI is now strictly adhered to. In particular, oxygen is treated as the electropositive component relative to any halogen for constructing compositional names (Section IR-5.2) and corresponding formulae (Section IR-4.4.3) for binary compounds. This results in, for example, the formula O2Cl and the name dioxygen chloride rather than the formula ClO2 and the name chlorine dioxide.

In Ref. 11, the formulae for intermetallic compounds were also subject to an exceptional rule although no guidance was given for naming such compounds, and the term ‘intermetallic compound’ was not defined. The problem is to define the term ‘metal’. Therefore, no attempt is now made to make a separate prescription for either the formulae or the names of intermetallic compounds. It is stressed, however, that the present recommendations allow some flexibility regarding formulae and compositional names of ternary, quaternary, etc. compounds. Several ordering principles are often equally acceptable (see Sections IR-4.4.2 and IR-4.4.3).

The element sequence of Table VI is also adhered to when ordering central atoms in polynuclear compounds for the purpose of constructing additive names (see Section IR-1.6.6).

For more on this topic, you've got to go read (and upvote) this excellent answer by Manishearth, which is where I first learnt that useful and intriguing nugget of IUPAC idiosyncrasy!