  Previous work by various authors has suggested that the detection by
Herschel/HIFI of nitrogen hydrides along the low density lines of sight towards
G10.6-0.4 (W31C) cannot be accounted for by gas-phase chemical models. In this
paper we investigate the role of surface reactions on dust grains in diffuse
regions, and we find that formation of the hydrides by surface reactions on
dust grains with efficiency comparable to that for H$_2$ formation reconciles
models with observations of nitrogen hydrides. However, similar surface
reactions do not contribute significantly to the hydrides of O$^+$ ions
detected by Herschel/HIFI present along many sight lines in the Galaxy. The
O$^+$ hydrides can be accounted for by conventional gas-phase chemistry either
in diffuse clouds of very low density with normal cosmic ray fluxes or in
somewhat denser diffuse clouds with high cosmic ray fluxes. Hydride chemistry
in dense dark clouds appears to be dominated by gas-phase ion-molecule
reactions.
