Submonolayer coverages of Al were evaporated onto hydrogen-terminated Si(001)−(2×1) surfaces and studied by scanning tunneling microscopy (STM). Nanoscale patterns of bare Si were created by STM desorption of hydrogen, and the size and number density of Al islands on bare and monohydride areas of the surface were compared. Dramatic differences in island nucleation are observed which suggest that the diffusion length of Al adatoms on the monohydride region is much longer than on bare Si. With lowered deposition rates or faster diffusing species, the effects studied here may provide a basis for selective metal patterning at atomic dimensions.