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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.

Example paper.

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 ?

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    $\begingroup$ 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. $\endgroup$
    – Ivan Neretin
    May 13, 2021 at 6:34
  • $\begingroup$ 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 ? $\endgroup$
    – Victor Pantazi
    May 13, 2021 at 12:01
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    $\begingroup$ 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.) $\endgroup$
    – Ivan Neretin
    May 13, 2021 at 12:16
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    $\begingroup$ 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. $\endgroup$
    – Ivan Neretin
    May 13, 2021 at 13:34
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    $\begingroup$ 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". $\endgroup$
    – Ivan Neretin
    May 14, 2021 at 7:36

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