Electronic and steric effects on molecular dihydrogen activation in [Cp*OsH₄(L)]⁺ (L = PPh₃, AsPh ₃, and PCy₃)

Single-crystal neutron diffraction, inelastic neutron scattering, and density functional calculations provide experimental and theoretical analyses of the nature of the osmium-bound, "elongated" dihydrogen ligands in [Cp*OsH₄(L)][BF₄] complexes (L = PPh₃, AsPh₃, or PCy₃). The PPh₃ and AsPh₃ complexes clearly contain one dihydrogen ligand and two terminal hydrides; the H₂ ligand is transoid to the Lewis base, and the H-H vector connecting the central two hydrogen atoms lies parallel to the Ct-Os-L plane (Ct = centroid of Cp* ring). In contrast, in the PCy₃ complex the H-H vector is perpendicular to the Ct-Os-L plane. Not only the orientation of the central two hydrogen atoms but also the H-H bond length between them depends significantly on the nature of L: the H⋯H distance determined from neutron diffraction is 1.01(1) and 1.08(1) Å for L = PPh₃ and AsPh₃, respectively, but 1.31(3) Å for L = PCy₃. Density functional calculations show that there is a delicate balance of electronic and steric influences created by the L ligand that change the molecular geometry (steric interactions between the Cp* and L groups most importantly change the Ct-Os-L angle), changing the relative energy of the Os 5d orbitals, which in turn govern the H-H distance, preferred H-H orientation, and rotational dynamics of the elongated dihydrogen ligand. The geometry of the dihydrogen ligand is further tuned by interactions with the BF₄ ^- counterion. The rotational barrier of the bound H₂ ligand in [Cp*OsH₄(PPh₃)]⁺, determined experimentally (3.1 kcal mol^-1) from inelastic neutron scattering experiments, is in reasonable agreement with the B3LYP calculated H₂ rotational barrier (2.5 kcal mol^-1).