Cationic dihydrogen/dihydride complexes of osmium: Structure and dynamics

Reaction of Cp*Os(CO)₂Cl with [Et₃Si][BAr ₄^F] under hydrogen gas affords the cationic hydrogen complex [Cp*Os(CO)₂(H₂)][BAr₄ ^F] (1) (Cp* = C₅Me₅; Ar^F = C₆F₅). When this reaction is carried out with HD gas, complex 1-d₁ results, with J_HD -24.5 Hz. When solutions of complex 1 are monitored by ¹H NMR spectroscopy over several days, the gradual formation of a trans dihydride species is observed. Similarly, reaction of CpOs(dppm)Br with Na[BAr^F*₄] (Ar ^F* = 3,5-(CF₃)₂C₆H ₃) under hydrogen affords the cationic dihydride complex [CpOs(dppm)H₂][BAr^F*₄] (2). At 295 K, complex 2 exists as a 10:1 mixture of cis and trans isomers. The ¹H NMR spectrum of the cis form in the hydride region exhibits a triplet with JHP = 6.5 Hz, due to rapid exchange of the hydrogen atoms. At low temperature, static spectra of the HH′PP′ spin system can be obtained, revealing quantum mechanical exchange coupling between the two hydride ligands. The observed J_HH′ is temperature dependent, varying from 133 Hz at 141 K to 176 Hz at 198 K. This is the first report of detectable exchange coupling between pairs of chemically equivalent hydrogen atoms.