documentclass{article}

usepackage{mathspec}
makeatletter % undo the wrong changes made by mathspec
letRequirePackageoriginal@RequirePackage
letusepackageRequirePackage
makeatother

setmainfont{Constantia}
setmathsfont(Digits){TeX Gyre Termes}
setmathsfont(Latin){Constantia}
setmathrm{Constantia}

usepackage[version=4]{mhchem}
%All uses of ce are in math mode:
letoldcece
renewcommand{ce}[1]{ensuremath{oldce{#1}}}

begin{document}
oldce{[Co^{II}(NCS)6]^4-}ce{[Co^{II}(NCS)6]^4-}
end{document}