Ammonia dative bond with

The nitrogen-boron bond in the solid compound is a dative bond because both shared electrons come from the nitrogen atom of the ammonia. The bond can therefore be written H3N Bp3. An alternative, but precisely equivalent way of writing it is H3 N— BF3. The latter provides a more accurate description. Consider Equation 4.1 in which NH3 forms a dative bond with H+. The first representation gives Structure 4.3, which suggests that one of the H atoms is different from the other three. However, in the new representation (Structure 4.4), the four N—H bonds are equivalent, consistent with the fact that they are of equal length. 

Figure 3.33 displays the Obn dative bond NBO ofF3B NH3, comparing it with the nitrogen NHO (basically, the ammonia lone-pair hybrid) of which it is principally composed. Consistently with criterion DB-1, the atomic charges in the equilibrium geometry (Q (Re) = +1.2843, Qn(R ) = —0.9175) differ strongly from the values in the separated BF3(0b(oo) = +1.4071) and NH3(0n(oo) = -1.0535). 

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. 

One of the most remarkable molecules is thiazyl trifluoride, NSF (Fig. 18.7). This compound is very stable. It does not react with ammonia at room lemperaiure. with hydrogen chloride even when hented. or with metallic sodium at temperatures below 400 C. The S—N bond. 141.6 pm, is the shortest known between these two elements. The F.SF bond nngle.s of 94° are compatible with approximate j/ bonding and the presence of an rp hybrid cr bond. ind iwo p d n bonds between ihe sulfur and the niirogen. The contraccicr of ihc J orbiuils by the inductive effect of the fluorine atoms presumably permits effective overLip and -jr-bond formation. The alternative explanation would require a double dative bond from the sulfur atom, extremely unlikely in view of the positive character of ihe sulfur atom... 

Lithium iodide forms a solid complex with ammonia, Li(NH3)4l, but the related hydrate, alcoholate and amine complexes are less stable. These complexes presumably involve ion-dipole bonds (p. 115), the nitrogen lone pairs surrounding the Li+ some covalent character (dative bonding) is also permissible if s and p orbitals on the Li are invoked. The chloride, bromide and iodide of lithium are much more soluble in alcohol and ether than those of the other alkali metals, but this is not always a reliable indication of covalent character. The property is employed in separating lithium from sodium. 

Complexes with one and two amines are common. For example, bis-(trimethylamine) alane, A1H3(N(CH3)3)2, is a white crystalline solid with a low vapor pressure. Using ammonia as a model for trimethylamine, apply orbital interaction analysis to describe the bonding in the 1 1 and 1 2 complexes. Theoretical studies on the ammonia complexes of AIH3 have led to the conclusion that there is little dative bonding (as judged by the amount of charge transfer) (Marsh, C. M, B. Schaefer III, H. F., J. Phys. Chem., 1995, 99, 14309-14315). Comment on the theoretical results. 

There is one important distinction. When a base such as "NH2 donates electrons to the electron-deficient proton of HCl, for example, a conjugate acid is formed H-NH2, with the new covalent N-H bond. This reaction breaks the H-Cl bond, generating Cl. There are two products because the acid-base reaction breaks the bond between the proton and the other atom and forms the conjugate acid. When a Lewis base such as ammonia reacts with a Lewis acid such as BFg, however, a dative bond is formed to give the Lewis acid-Lewis base complexes (an ate complex), so there is one product rather than two. This statement is an overgeneralization, but it offers a useful starting point to allow the two definitions to be distinguished from one another. These points will be emphasized in this section. 

We can think of the amines as substituted ammonia (NHj) molecules. For example, a primary amine is an ammonia molecule with one of its H atoms replaced by an alkyl or aryl group. Ammonia and the amines act as bases because of the lone pair of electrons on the nitrogen atom. Remember that a base is a proton (H+ ion) acceptor. The nitrogen atom donates its lone pair to an ion, forming a co-ordinate (dative) bond. 

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