Single Crystal Neutron Diffraction Study of the Complex [Ru(H…H)(C₅Me₅)(dppm)]BF₄ Which Contains an Elongated Dihydrogen Ligand

The H-H distance of the dihydrogen ligand in [Ru(H…H)(C₅Me₅)(dppm)]BF₄ is 1.08(3) A as determined by neutron diffraction on a crystal of volume 3.2 mm³ at 15 K: monoclinic, space group P2₁, a = 10.833(3), b = 15.045(4), c = 10.781(2) Å, β = 114.47(2)°, V= 1599.3(7) ų, and D_c = 1.473 g cm^-3 for Z = 2; R(F) = 0.071, R(F²) = 0.151, R_w(F²) = 0.099 for 4198 reflections. The H-H distance, when corrected for the effects of thermal motion, lengthens slightly from 1.08 (3) to 1.10(3) Å. The complex contains an elongated dihydrogen ligand which is symmetrically, side-on bonded to the ruthenium as one leg of a three-legged piano stool structure. The H₂ ligand lies parallel to the C₅Me₅ ligand plane, an orientation where π-overlap with an a "d orbital on the metal is optimum. The T₁ NMR method gives an H-H distance of 1.10(1) Å for the case of restricted rotation. The present structure provides a model for coordinated H₂ at an intermediate stage of oxidative addition. The ¹J(H,D) coupling in the Ru(H-D) isotopomer increases with decreasing temperature; this is interpreted as a slight shortening of the H-D distance. The structural and ¹H NMR data are compared for dihydrogen complexes which have been studied by single crystal neutron diffraction.