Is there a convention for breaking chemical formulae across lines? I found a chat where a user asked for chemical formulae to be broken across lines (with LaTeX/mhchem). I could imagine offering a switch to allow this. But only, if line-breaking within formulae is not completely uncommon.

According to him/her, possible breaking points in $\ce{[nBu4N]2[(UO2)2(\mu-\eta^2:\eta^2-O2)(NO3)2(\mu-Au(CN)2)2]}$ would be
$\ce{[nBu4N]2~\color{red}{|}~[(UO2)2~\color{red}{|}~(\mu-\eta^2:\eta^2-O2)~\color{red}{|}~(NO3)2~\color{red}{|}~(\mu-Au(CN)2)2]}$.
So a rule could be "allow a line-break before (, [ or \{ (which would introduce another possible breakpoint into the example).

Is it common to break long formulae across lines and is doing so sensible?
I remember a couple of books from 1970-80s, where long formulas were split with what would be a $\rm \LaTeX$ \cdot "$\cdot$" notation, I believe. This is an example from Pope, M. T.; Jeannin, Y.; Fournier, M. "Heteropoly and isopoly oxometalates" in Inorganic Chemistry Concepts; Springer: Berlin, 1983; Vol. 8.:



The reason for that, it seems, is that the book was printed in A5 format. Personally, I find an option for mhchem to be able to split formulas across pages very useful.