  Using {\em ab initio} density functional theory calculations, we characterize
changes in the electronic structure of MoS$_{2}$ monolayers introduced by
missing or additional adsorbed sulfur atoms. We furthermore identify the
chemical and electronic function of substances that have been reported to
reduce the adverse effect of sulfur vacancies in quenching photoluminescence
and reducing electronic conductance. We find that thiol-group containing
molecules adsorbed at vacancy sites may re-insert missing sulfur atoms. In
presence of additional adsorbed sulfur atoms, thiols may form disulfides on the
MoS$_{2}$ surface to mitigate the adverse effect of defects.
