How to use parentheses in non-polymer molecule

I would llike to draw the following molecule in tex using chemfig:

I tried in the following way:

\chemleft[ \chemfig{HOH_2C-[:-60]P^{+}(-[:-120]HOH_2C)(-[:-60]CH_2OH)-[:60]CH_2OH }\chemright]${}_2$\chemfig{SO_4^{2-}}


But it is not working properly. I get this result:

Does anyone could help me to get the subscripted number and the sulphate ion (SO42-) in the right place ?

Here is an idea:

\documentclass{article}
\usepackage{chemfig}

\begin{document}

%\schemedebug{true}

\schemestart
\subscheme{
\chemleft[
\chemfig{P^{+}(-[:120]HOH_2C)(-[:-120]HOH_2C)(-[:-60]CH_2OH)-[:60]CH_2OH}
\chemright]
}
\chemfig{SO_4^{2-}}
\schemestop
\chemmove{\node at (c2.south east) {$\scriptstyle2$};}

\end{document}