Absolute scattering probabilities of nearly monoenergetic D-2 and He beams are compared for the highly reactive clean Ni(110) surface at a surface temperature T-s=700 K along the more corrugated [001] direction. At incident energies between 20 and 110 meV the total reflectivity of D-2 is about a factor 200 smaller than for He, whereas the first order diffraction intensities relative to the specular peak are a factor 7 larger. The D-2 angular distributions also show clear evidence of rotationally inelastic diffraction peaks. The diffraction intensities of both He and D-2 can be accounted for by a conventional hard wall model with reasonable values of the corrugation amplitudes of 0.060 Angstrom for He and 0.091 Angstrom for D-2 without including a lateral variation in the probability for chemisorption. The reflectivity results when extrapolated to T-s=0 indicate that for He only 33% of the incident atoms are coherently reflected. For D-2 only 9% are coherently scattered and approximately 24% are chemisorbed. The coherently scattered fraction is attributed to D-2 molecules with orientations not sufficiently parallel to the surface plane to permit chemisorption to occur.