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)(3)B . NCR (R = CH3 la, p-CH3-C6H4 Ib, p-NO2-C6H4 1c), isonitrile complexes (C6F5)(3)B . CNR (R = C(CH3)(3) 3a, C(CH3)(2)CH2C(CH3)(3) 3b, 2,6-(CH3)(2)-C6H3 3c), and the phosphine adduct (C6F5)(3)B . P(C6H5)(3) (6) could be prepared. The compounds were characterized by IR and NMR spectroscopy and by X-ray structure analyses (la, Ic, 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 equivalent to N bond strength. This is evident from a marked shift; of the (C=NIR)-I-(v) over tilde band to higher wavenumbers, and this interpretation is supported by the small but experimentally significant decrease of the C equivalent to N bond length observed by X-ray diffraction. The experimental work is complemented by a density functional study on the model complexes (C6F5)(3)B . L, L = CNCH3, NCCH3, PH3, CO. A detailed analysis revealed that the bonding in (C6F5)(3)B . 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 d(0)-Cp3M+ system (M = Zr, I-If). 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)(3)B . CO, is addressed.