How to typeset indices before a symbol?

You have to supply a "fake" object for TeX to put indices to:

${}^1_2X^3_4$

However, for chemistry typesetting you should use one of the specialized packages, such as mhchem. This example is taken from mhchem documentation:

\ce{^{227}_{90}Th+}

It will typeset the symbol for a positive Thorium ion with a charge of 90 and atomic mass of 227.

mathtools provides among other useful things the prescript command for this. Verbose, but useful if you don't want to fiddle with negative spaces and fake objects.

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[
{}^{4}_{12}\mathbf{C}^{5+}_{2}
\quad
\prescript{14}{2}{\mathbf{C}}^{5+}_{2}
\quad
\prescript{4}{12}{\mathbf{C}}^{5+}_{2}
\quad
\prescript{14}{}{\mathbf{C}}^{5+}_{2}
\quad
\prescript{}{2}{\mathbf{C}}^{5+}_{2}
\]
\end{document}

The easiest in this case would be to use the leftidx package. Taken directly from the package documentation:

\usepackage{leftidx}% http://ctan.org/pkg/leftidx
...
$\leftidx{_1^2}{\left(\frac{1}{b}\right)}{_3^4}$

The package offers the command \leftidx{<left indices>}{<stuff>}{<right indices>}.