  We study the interaction of H$_2$, O$_2$, CO, H$_2$O and OH molecules with
the vacancy defects of graphene and silicene. Atoms around the bare vacancy
reconstruct and specific chemically active sites are created. While H$_2$,
O$_2$ and CO remain intact on both pristine graphene and silicene, these
molecules can dissociate when they are placed at the close proximity of these
chemically active sites and nucleate centers for the hydrogenation and
oxygenation. Saturation of the dangling bonds at the defect sites by
constituent atoms of dissociated molecules gives rise to significant
modification of electronic and magnetic properties. We analyzed the mechanism
of the dissociation and revealed a concerted action of surrounding host atoms
together with dissociated molecules to lower the energy barrier needed for
dissociation. The dissociations of H$_2$O and OH are hindered by high energy
barriers. Our study suggests that graphene and silicene can be functionalized
by creating meshes of single vacancy, where specific molecules can dissociate,
while some other molecules can be pinned.
