# kann an vielen Stellen nicht verwendet werden. Die Fehlermeldung, die das Minimalbeispiel
\documentclass{minimal}
\usepackage[version=3]{mhchem}
\begin{document}
\textbf{\ce{H-C#C-H}}
\end{document}
erzeugt, ist eigentlich recht aussagekräftig:
ERROR: Illegal parameter number in definition of \reserved@a.
--- TeX said ---
<to be read again>
C
l.4 \textbf{\ce{H-C#C-H}}
--- HELP ---
This is probably caused by a \newcommand, \renewcommand,
\newenvironment, or \renewenvironment command in which a # is used
incorrectly. A # character, except as part of the command name \#,
can be used only to indicate an argument parameter, as in #2, which
denotes the second argument. This error is also caused by nesting one
of the above four commands inside another, or by putting a parameter
like #2 in the last argument of a \newenvironment or \renewenvironment
command.
Verwende stattdessen \tbond:
\documentclass{minimal}
\usepackage[version=3]{mhchem}
\begin{document}
\textbf{\ce{H-C\tbond C-H}}
\end{document}