Synthesis, structure, and properties of the stable and highly acidic dihydrogen complex trans-[Os(eta(2)-H-2)(CH3CN)(dppe)(2)](BF4)(2). Perspectives on the influence of the trans ligand on the chemistry of the dihydrogen ligand

The complex trans-[Os(H)(CH3CN)(dppe)(2)]BF4 (2) is prepared from the known complex trans-[Os(eta(2)-H-2)(H)(dppe)(2)]BF4 (1), dppe = Ph(2)PCH(2)CH(2)PPh(2), by substitution of the eta(2)-H-2 ligand with CH3CN. 2 was identified as a potential precursor for a highly acidic and stable eta(2)-H-2 complex on the basis of its electrochemical potential E(1/2)(Os-III/Os-II). The stable complex trans-[Os(eta(2)-H-2)(CH3CN)(dppe)(2)](BF4)(2) (3) is formed when 2 is protonated with excess HBF4 . Et(2)O in anhydrous CH2Cl2. The pK(a) of 3 is estimated to be -2 because it is partially deprotonated by Et(2)O. This makes it the most acidic, stable dihydrogen complex to be fully characterized. Its properties are compared to the known complex trans-[Os(eta(2)-H-2)(CH3CN)(en)(2)](CF3SO3)(2) (5), en = H2NCH2CH2NH2, to illustrate the influence of the chelating ligand (pi-acidic dppe versus sigma-basic en), and to the complexes trans-[Os(eta(2)-H-2)(X)(dppe)(2)]PF6, X = H (1), Cl (4), and Br (6), to illustrate the influence of the trans ligand on the characteristic properties of the eta(2)-H-2 ligand. The ligand field strength of X is an important factor. The structures of 2 and 3a,b (3 crystallizes in 2 forms) were determined by X-ray diffraction. In 3a the hydrogen atoms of the eta(2)-H-2 ligand were isotropically refined, resulting in an H-H distance of 0.9(1) Angstrom. In 3b there is residual electron density associated with the eta(2)-H-2 ligand, but the hydrogen atoms were not located. There is a close dihydrogen-fluorine contact of approximately 2.4 Angstrom in 3a.