NATURE OF DONOR-ACCEPTOR BOND IN ACETONITRILE-BORON TRIHALIDES . STRUCTURES OF 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 D2h16-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. © 1969, American Chemical Society. All rights reserved.