Weakly Bound but Strongly Interacting: The Structures, Stabilities, and Dynamics of Osmium(II) Ethane, Propane, and Butane Complexes

Low-temperature protonation of the osmium(II) alkyl compounds (C₅Me₅)Os(dfmpm)R, where dfmpm = (F₃C)₂PCH₂P(CF₃)₂ and R = ethyl, n-propyl, n-butyl, or i-butyl, generates σ-ethane, σ-propane, σ-n-butane, and σ-i-butane complexes. The alkane dissociation barriers are ∼13.2 kcal mol^-1 or about 0.5 kcal mol^-1 larger than that of the previously described methane complex [(C₅Me₅)Os(dfmpm)(CH₄)]⁺. The alkane ligands bind to osmium through one methyl group, which exchanges slowly with the unbound terminal methyl group(s). Within the bound methyl group, one bridging hydrogen atom interacts directly with osmium; it exchanges rapidly with the other two methyl C-H bonds at a rate consistent with a slightly hindered C-C bond rotation. The large difference in ¹J_CH between the bridging (75 Hz) and terminal (142 Hz) C-H sites is consistent with the view that the 16-electron [(C₅Me₅)Os(dfmpm)]⁺ fragment has partially abstracted a hydride group (H^-) from the alkane, which confers carbocation (sp²) character to the CH₂R portion of the Os-H-CH₂R unit. The extent of this distortion and the overall strength of the metal-alkane interaction are correlated with the alkane C-H orbital energies in a manner consistent with covalent metal-ligand bonding. Whereas most ligands can bind to metals with little structural reorganization, an alkane must undergo a significant structural change─weakening of a C-H bond─to become a sufficiently good donor and acceptor to bind to a metal. Collectively, these results show that the binding energies of alkane ligands are small not because the constituent metal-ligand interactions are weak but rather because the reorganization energy needed to form them is large.