Synthesis and NMR studies of [(C5Me5)Os(L)H2(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 (C5Me5)OsH3(L) with HBF4 in diethyl ether affords the molecular dihydrogen complexes [(C5Me5)Os(H-2)H-2(L)][BF4], where L is PPh3 (1), AsPh3 (2), or PCy3 (3). Ruthenium analogues of these species are not stable and instead lose H-2 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 H-2 ligand is oriented with its H-H vector nearly parallel with the Os-Ct vector, where Ct is the centroid of the C5Me5 ring; in contrast, in 3 the H-2 ligand is oriented with its H-H vector perpendicular to the Os-Ct vector. In the H-1 NMR spectra, exchange between the Os-H and Os-H-2 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 H-2 ligands were deduced from the observed (1)J(HD)(av) coupling constants. In addition, H-H distances were deduced from the T-1(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 angstrom in the solid state; similarly, for 2 the solution value of ca. 1.15 angstrom was longer than the 1.08 angstrom value seen in the solid state. In both cases, the similar to 0.06 angstrom lengthening in solution, if real, is the result most likely of one or both of two factors: the effect of removing the BF4 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 angstrom is significantly shorter than the 1.31 angstrom 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.