Lewis Acid Properties of Tris(pentafluorophenyl)borane. Structure and Bonding in L−B(C₆F₅)₃ Complexes^⊥

A variety of donor adducts of tris(pentafluorophenyl)borane were experimentally generated by reaction of a Lewis base with an excess of B(C₆F₅)₃ in pentane. In this way, nitrile complexes (C₆F₅)₃B·NCR (R = CH₃ 1a, p-CH₃−C₆H₄ 1b, p-NO₂−C₆H₄ 1c), isonitrile complexes (C₆F₅)₃B·CNR (R = C(CH₃)₃ 3a, C(CH₃)₂CH₂C(CH₃)₃ 3b, 2,6-(CH₃)₂−C₆H₃ 3c), and the phosphine adduct (C₆F₅)₃B·P(C₆H₅)₃ (6) could be prepared. The compounds were characterized by IR and NMR spectroscopy and by X-ray structure analyses (1a, 1c, 3a, 3b, and 6). Coordination of the nitriles as well as the isonitriles to the neutral Lewis acid leads to a substantial increase in the C⋮N bond strength. This is evident from a marked shift of the ν̃_C⋮N IR band to higher wavenumbers, and this interpretation is supported by the small but experimentally significant decrease of the C⋮N bond length observed by X-ray diffraction. The experimental work is complemented by a density functional study on the model complexes (C₆F₅)₃B·L, L = CNCH₃, NCCH₃, PH₃, CO. A detailed analysis revealed that the bonding in (C₆F₅)₃B·L complexes is mainly dominated by electrostatic interaction, which in turn is responsible for the observed structural and spectroscopic changes. In the context of this work, the bonding of the neutral B(C₆F₅)₃ Lewis acid is compared to the positively charged organometallic d₀-Cp₃M⁺ system (M = Zr, Hf). It was found that electrostatic effects are more pronounced for B(C₆F₅)₃ than for the transition metal fragments. The question as to the existence of a nonclassical main group carbonyl complex, (C₆F₅)₃B·CO, is addressed.