  We study spin-orbit torques in two dimensional hexagonal crystals such as
graphene, silicene, germanene and stanene. The torque possesses two components,
a field-like term due to inverse spin galvanic effect and an antidamping torque
originating from Berry curvature in mixed spin-$k$ space. In the presence of
staggered potential and exchange field, the valley degeneracy can be lifted and
we obtain a valley-dependent Berry curvature, leading to a tunable antidamping
torque by controlling the valley degree of freedom. The valley imbalance can be
as high as 100\% by tuning the bias voltage or magnetization angle. These
findings open new venues for the development of current-driven spin-orbit
torques by structural design.
