We present results of calculations on dissociative and rotationally (in)elastic diffractive scattering of H-2 from Pt(111), treating all six molecular degrees of freedom quantum mechanically. The six-dimensional (6D) potential energy surface was taken from density functional theory calculations using the generalized gradient approximation and a slab representation of the metal surface. The 6D calculations show that out-of-plane diffraction is very efficient, at the cost of in-plane diffraction, as was the case in previous four-dimensional (4D) calculations. This could explain why so little in-plane diffraction was found in scattering experiments, suggesting the surface to be flat, whereas experiments on reaction suggested a corrugated surface. Results of calculations for off-normal incidence of (v=0,j=0) H-2 show that initial parallel momentum inhibits dissociation at low normal translational energies, in agreement with experiment, but has little effect for higher energies. Reaction of initial (v=1,j=0) H-2 is predicted to be vibrationally enhanced with respect to (v=0,j=0) H-2, as was also found in three-dimensional (3D) and 4D calculations, even though H-2+Pt(111) is an early barrier system.