Ammonia-boron trifluoride complex

Figure 1.16 The ammonia-boron trifluoride donor-acceptor complex (a) donor Lewis base, (b) acceptor Lewis acid, (c) the donor-acceptor or Lewis acid-base complex.

Being an electron deficient compound, boron trifluoride forms complexes with Lewis bases and compounds that have unshared pair(s) of electrons. With ammonia, it forms boron trifluoride ammonia. Similar coordination compounds are formed with monoethylamine, BF3-NH2C2H5 diethyl ether, CH3CH20(BF3)CH2CH3 and methanol, BF3—OHCH3. It forms a sohd complex HNO3-2BF3 with concentrated nitric acid. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

BF3 + NH3 —> BF3NH3. In the ammonia molecule there is a nonbonding pair of electrons that can react in a variety of ways. Later in the book, you will learn about complexes that can form with ammonia. In this reaction, boron trifluoride is a molecule that is usually represented with only 6 electrons around the central boron, making it a perfect mate for the ammonia molecule. As tempted as you might be to try to apply a double-replacement strategy here, remember that they are both molecular substances. This suggests another approach a combination reaction. 

Lewis acids and bases, because of their complexity, shall be examined briefly. Consider the structure of ammonia with its free pair of electrons. If the free pair of electrons were to make a bond with boron trifluoride, which substance is labeled as an acid and which one is the base Because the boron accepted a pair of electrons it is considered to be the Lewis acid. Ammonia is the substance that donated the electron pair and is classified as the Lewis base. (See Figure 9.3.)... 

Lewis acids like boron trifluoride form Lewis acid Lewis base complexes such as H3N BF3 with ammonia. Ammonia also forms metal complexes, the aminocomplexes (see Ammonia N-donor Ligands). Deprotonation leads to amido, imido, and nitridocomplexes (see Scheme 14). 

A definite improvement in the synthesis of A -methoxy aziridines was achieved by substituting boron trifluoride with trimethylsilyl triflate and diethyl ether with dichloromethane"9. In this way, the A -methoxy aziridines were obtained in good yields from a variety of linear and cyclic alkenes, e.g., 6-8. For comparison, the aziridine 8 was obtained in 50% yield by using boron trifluoride- diethyl ether complex in dichloromethane. Complex product mixtures were obtained with allyl and crotyl alcohols and with cyclohexenone. Further transformation of the A -methoxy aziridines into the N-H aziridines was possible using sodium/ ammonia reduction, e.g., 9. 

Theoretical Predictions of the Infrared Spectra of Hydrogen-Bonded and Charge Transfer Complexes Ammonia and Boron Trifluoride Dimers. ... 

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