How do I force a fractional coefficient to exclude non-numeric symbols?

Question

Short version of question: Suppose I have an expression:

expression = a+3b;

When I divide by 2, I get:

expression/2

It is important for my application, however, to have this result displayed as:

If it helps at all, in context the quantities a and b are being displayed using a custom Notation from the Notation package and using a custom style that has been added to the stylesheet which already contains a custom DisplayFunction, so a solution that involves modifying either of those would be perfectly fine.

I recognize that this question is similar to this one. It is distinct, however, in that my situation is needing formatting done to the result of a calculation.

Application: My symbols are chemical formulas and the coefficients are stoichiometric coefficients, and standard notation in chemistry says that the coefficients are separately in front of the formulas.

Full Problem in Context: There are a number of pieces that have come together to create the full problem, but I think my minimal example above captures the essence. For context, however, here is what I am really working with.

First, I define string-based labels for my chemical species using my custom notations and styles through a function AssignSpecies, and then create a palette that allows me to enter those symbols easily.

Then I enter a couple of chemical reactions using those symbols and a custom-modified operator based on LongRightArrow (same precedence as ==).

I have set up some algebra rules for LongRightArrow to allow reactions to be added, subtracted, and multiplied and divided by a constant. So for example:

I have a function that will remove species on both sides of the reaction:

Very often, though, we want to have the reaction expressed such that one particular species (in this example, the main product) has a coefficient of 1. And this will lead to fractional coefficients in many cases. For example:

The H₂(g) product is the problem here. For the notation to be as chemistry-standard as possible, I need that species to have a 3/2 coefficient in front of the species.

Edited to Add: Answering @CarlWoll's question about the FullForm of this result:

LongRightArrow[Plus[complexsymbol13, complexsymbol14,
  Times[11, complexsymbol15], Times[2, complexsymbol18]],
  Plus[Times[Rational[3, 2], complexsymbol17], complexsymbol19]]

Second Edit: Some further information:

AssignSpecies calls AssignLabels, which looks like this:

AssignLabels[list_]:=Block[{symbollist={},tmpsymbol},
  Do[AppendTo[symbollist,tmpsymbol=constructnextsymbol];
     AssociateTo[runningsymbollist,tmpsymbol->list[[i]]];
     Notation[DoubleLongLeftRightArrow[
     ParsedBoxWrapper@TemplateBox[{"\""<>list[[i]]<>"\""},"complexSymbols",
                   Editable->False,Selectable->False],
     ParsedBoxWrapper@SymbolName[tmpsymbol]]],
  {i,Length[list]}];
  symbollist]

constructnextsymbol creates a symbol with the next index value. Other than the nature of the "complexSymbols" tag, everything else should be self-explanatory above. The complexSymbols style is:

Cell[StyleData["complexSymbols"],StyleMenuListing->None,
  TagBoxOptions->{SyntaxForm->"symbol"},ShowStringCharacters->False,
  ShowAutoStyles->False,ZeroWidthTimes->True,FontWeight->Plain,
  FontFamily->"Times", TemplateBoxOptions->
    {DisplayFunction->(RowBox[{#}]&),SyntaxForm->"symbol"}]

Answer 1

This is strictly a formatting issue, and unfortunately, the following natural approach doesn't work because the tag is buried too deep:

sym /: MakeBoxes[r_Rational sym, StandardForm] := MakeBoxes[Defer[r] sym]

TagSetDelayed::tagpos: Tag sym in MakeBoxes[r_Rational sym,StandardForm] is too deep for an assigned rule to be found.

$Failed

So, you will have to change the formatting of MakeBoxes instead. Something like:

SetAttributes[cQ,HoldAll]
cQ[complexsymbol17] = True;

MakeBoxes[r_Rational s_?cQ, StandardForm] := MakeBoxes[Defer[r] s]

Then:

3/2 complexsymbol17