# What does fractional notation represent in molecular formulas such as SiO4/2 or H2SiO2/2?

I'm going through some papers about HSQ (hydrogen silsesquioxane) and keep seeing this fractional notation being used: HSiO3/2 , SiO4/2 , H2SiO2/2 , H3SiO1/2.

I understand that the proper formula for HSQ is [HSiO3/2]2n. Because we can't have fractional atoms, it's always multiplied by an even number and as such the fraction is never a consideration. But then why are the fractions maintained in the other compounds, and not reduced to SiO2 and H2SiO ?

Does keeping the fraction confer some structural information ? I've tried to find out what this notation means, but could not find a proper explanation.

Edit: What do the numerator and denominators mean, formally, in those formulas ? If it's that the oxigen atoms share outside bonds, why not write the compound as SiO4−4 ? If the numerator is the number of atoms, what does the denominator represent ? And is it always /2 ? Can we have /x , where x is a different integer ? I've never seen this type of notation before. Does the denominator mean that the valence electrons are halved ?

• Well, yes, it does confer certain structural information. You don't have an entity SiO2, for example. What you have is an entity SiO4 in which all oxygen atoms are shared with others. May 13 at 6:34
• So then what does the /2 in $SiO_{4/2}$ mean ? I've never seen the fractional notation before. What is it telling us ? That all of the oxigen atoms are shared ? Why "4/2" then if they all share outside bonds ? Why not write it as $SiO_{4}^{−4}$ ? If the numerator is the number of atoms, what does the denominator represent ? Right until now I've only seen whole numbers in the subscripts, not fractions. Just trying to understand what it represents formally. And is it always /2 ? Can we have /x , where x is a different integer ? May 13 at 12:01
• 4 stands for four oxygens, 2 means that each oxygen is shared between two SiO4 entities, and we don't write $\ce{SiO4^4-}$ because that's a thing we don't have here either. (We may have it elsewhere, but that's another story.) May 13 at 12:16
• Indeed, you'd better accept it in a more hand-wavy and less formal way. Even if there is a formal definition out there, this notation is far from being used universally. OK, SiO4/2 means that each oxygen atom is shared between two entities which do not have to be similar. Say, between SiO4/2 and HSiO3/2. May 13 at 13:34
• The terminology of metal clusters and ligands... well, I guess it can technically be used here, but then it doesn't sit well with writing "clusters" by parts. When you call your "ligand" $\mu_2$, you imply that it is $\mu_2$ with respect to the whole "cluster". May 14 at 7:36