Synthesis and NMR studies of [(C₅Me₅)Os(L)H ₂(H₂)⁺] complexes. Evidence of the adoption of different structures by a dihydrogen complex in solution and the solid state

Protonation of the osmium(IV) trihydrides (C₅Me ₅)OsH₃(L) with HBF₄ in diethyl ether affords the molecular dihydrogen complexes [(C₅Me₅)Os(H ₂)H₂(L)][BF₄], where L is PPh₃ (1), AsPh₃ (2), or PCy₃ (3). Ruthenium analogues of these species are not stable and instead lose H₂ readily. These compounds adopt four-legged piano-stool geometries in which the phosphine ligand is "trans" to an elongated dihydrogen ligand. For 1 and 2, the coordinated H₂ ligand is oriented with its H-H vector nearly parallel with the Os-Ct vector, where Ct is the centroid of the C₅Me ₅ ring; in contrast, in 3 the H₂ ligand is oriented with its H-H vector perpendicular to the Os-Ct vector. In the ¹H NMR spectra, exchange between the Os-H and Os-H₂ environments can be slowed at low temperatures for the arsine complex 2 (but not for 1 or 3), and separate resonances could be observed for the hydride and dihydrogen sites; the barrier for exchange is approximately 6.0 kcal/mol. Partially deuterated samples were prepared, and H-H distances within the bound H₂ ligands were deduced from the observed ¹J_HD(av) coupling constants. In addition, H-H distances were deduced from the T₁(min) values for the osmium-bound hydrogen atoms, after correction for exchange and ligand-induced dipolar relaxation effects. In all cases, the two solution measurements were in agreement but differed from that deduced from neutron diffraction data. Specifically, for 1 the solution data gave a distance of ca. 1.07 vs 1.01 Å in the solid state; similarly, for 2 the solution value of ca. 1.15 Å was longer than the 1.08 Å value seen in the solid state. In both cases, the ∼0.06 Å lengthening in solution, if real, is the result most likely of one or both of two factors: the effect of removing the BF ₄ counterion from the vicinity of the cation and the effect of librational motion that tends to shorten artificially H-H distances deduced from neutron diffraction data. In contrast, for 3 the solution H-H distance of ca. 1.12 Å is significantly shorter than the 1.31 Å distance determined from the neutron diffraction data. DFT calculations support the hypothesis that different structures are adopted by 3 in solution and in the solid state and that in solution an equilibrium is established between two dihydrogen-dihydride structures, one with a considerably shorter H-H bond than is seen in the solid state.