Higher Ammoniates of BF3 and SiF4: Syntheses, Crystal Structures, and Theoretical Calculations

We report on the synthesis and crystal Structure of a higher ammoniate of BF3NH3, the monammine trifluorido boron(III) ammonia (1/3), [BF3NH3]center dot 3NH(3). The neutral compound crystallizes in the orthorhombic space group Pna2(1) with a = 11.5062(2), b = 9.9888(2), c = 6.5846(1) angstrom, V = 756,79(2) angstrom(3), and Z = 4, forming a complex three-dimensional network through N-H center dot center dot center dot F and N-H center dot center dot center dot N hydrogen bonding, including the rare case of a trifurcatcd N-H center dot center dot center dot F hydrogen bond. A higher ammoniate of SiF4(NH3)(2) was obtained and characterized for the first time. Diammine tetrafluorido silicon(IV) ammonia (1/2), [SiF4(NH3)(2)]center dot 2NH(3), was found to crystallize in the orthorhombic space group Cmca with a = 7.0595(1), h = 7.9814(1), c = 13.4595(2) angstrom, V = 758.37(2) angstrom(3), and Z = 4. Its structure displays discrete SiF4(NH3)(2) molecules, which are connected to crystal ammonia molecules via N-H center dot center dot center dot N hydrogen bonds and interconnected by N-H center dot center dot center dot F hydrogen bonds to (2)(infinity)[SiF4(NH3)(2)center dot 2NH(3)] sheets. These layers are further interconnected by N-H center dot center dot center dot F hydrogen bonds from the crystal ammonia molecules leading to a three-dimensional network. ELF and AIM calculations and population analyses of the ELF basins were carried out for the BF3NH3 and SiF4(NH3)(2) units of the two Compounds. The calculations reflect the partially covalent bonding Of fluorine to boron and silicon and confirm the expected trend of Lewis acidity. It is shown that ELF charges are a better descriptor for the bond covalency in these compounds than AIM charges.