From mediawiki.org


the example string produces a minus which is not in superscript, while it works correctly with numbers before the minus (e.g. 2-)


you could use

Hmm, @Physikerwelt, Rjh, Leyo, and Mabschaaf: the ce is outdated and the chem-Author currently advises against the use of <chem> (Link), cheers, --Ghilt (talk) 13:48, 11 June 2017 (UTC)
Yes, I agree. Isn't it better to fix the bug instead of propose workarounds ? Otherwise that leads to bulk changes and a complicated and incomprehensible interface for the editors. Rjh (talk) 15:17, 11 June 2017 (UTC)