Nature of the donor-acceptor bond in acetonitrile-boron trihalides. The structures of the boron trifluoride and boron trichloride complexes of acetonitrile

The crystal and molecular structure of the boron trichloride complex of acetonitrile has been determined and that of the boron trifluoride complex redetermined. Least-squares refinement of three-dimensional X-ray data led to a conventional R factor on F of 3.9% for the BCl₃ complex (441 nonzero reflections) and 5.6% for the BF₃ complex (382 nonzero reflections). Both complexes crystallize in the space group D_2h¹⁶-Pnma of the orthorhombic system with four molecules in unit cells of dimensions a = 8.67 (1), b = 7.32 (1), and c = 10.19 (1) Å for Cl₃BNCCH₃ and a = 7.784 (3), b = 7.177 (3), and c = 8.360 (3) Å for F₃BNCCH₃. The calculated densities of 1.65 g/cm³ for Cl₃BNCCH₃ and 1.55 g/cm³ for F₃BNCCH₃ are in reasonable agreement with previously measured densities of 1.60 and 1.59 g/cm₃, respectively. The molecules are required to have symmetry m. The configuration about the boron in each complex is nearly tetrahedral. The B-N bond length in the BCl₃ complex (1.562 (8) Å) is significantly shorter than the B-N bond length in the BF₃ complex (1.630 (4) Å). The X₁-B-N and X₂-B-N bond angles are, respectively, 105.3 (3) and 105.8 (2)° where X = F and 106.9 (3) and 106.7 (4)° where X = Cl. These results are rationalized in terms of a model for donor-boron halide interaction in which the acceptor strength of a boron halide increases with increasing distortion of the BX₃ group. This model also is consistent with thermochemical and spectroscopic data on boron halide complexes.