\documentclass{book}
\usepackage[version=3]{mhchem}
\usepackage{xepersian}
\settextfont{XB Niloofar}
\setlatintextfont{Times New Roman}
\setdigitfont{XB Zar}

\begin{document}
\ce{[AgCl2]-} \qquad

\end{document}