Donor-acceptor interactions bases, Lewis

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

The enhanced tendency of the heavier chalcogens (S< coordination spheres, either by donor-acceptor interactions with Lewis acids or bases, or by intermolecular association, allowed to characterise crystallographically in recent years a considerable amount of supramolecular structures. 

In summary, we can say that, because of the unique absence of angular and radial nodes in the H-atom valence shell, the hydride oah orbital is uniquely suited to strong n-a donor-acceptor interactions with Lewis bases. In turn, the unique energetic and angular features of nB-aAH interactions (or equivalently, of B H—A <—> B—H+ A covalent-ionic resonance) can be directly associated with the distinctive structural and spectroscopic properties of B - H—A hydrogen bonding. 

Group (1) Cations and anions which are incapable of donor-acceptor interactions. These are the large univalent ions. Bonding is purely by Coulomb and Madelung electrostatic interactions. From the Lewis point of view these are not acids or bases. They have no cement-forming potential. 

The general Lewis-acid-base reaction (3.95) exemplifies the two-electron stabilizing donor-acceptor interaction of Fig. 1.3 (namely the nN->-nB interaction for (3.94)), which may be distinguished from the complementary bi-directional donor-acceptor interactions of covalent-bond formation (Section 3.2.1). However, this leaves open the question of whether (or how) the equilibrium bond reflects the formal difference between heterolytic (3.95) and homolytic (3.96) bond formation. 

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

Table 5.8 summarizes the NBO descriptors of the net charge transfer from Lewis base to Lewis acid (Qcf), change in covalent-bond polarization (A/Ah), and (P)NBO overlap of n0 with bond (Sna) and antibond (Sna>) orbitals of the Lewis acid. The entries in Table 5.8 show the unfavorable diminution of l/w /5nal and reduced charge transfer as the Lewis acid changes from polar HF to apo-lar CH4. These NBO descriptors can also be closely correlated with quantities in Table 5.7, showing their mutual dependence on the strength of n-a donor-acceptor interaction. 

Figure 5.38 displays the optimized structure and primary npt— ooh donor-acceptor interaction of the complex (5.70a) in which PtH2 serves as the Lewis-base donor. The qualitative similarity to the water dimer structure (Fig. 5.5(b)) is immediately apparent, including the short Pt H distance (2.47 A, more than 0.5 A inside van der Waals contact), the roughly linear Pt H—O angle (161°), and the characteristic elongation of the H-bonded versus free O—H bond of the water monomer (by 0.01 A). The leading nPt a0H donor-acceptor interaction in Fig. 5.38(b) is... 

The nature of the donor-acceptor interactions in main group complexes has also been studied theoretically [88-91], The calculated BDEs of main-group complexes predicted at the MP2/II level of theory are in very good agreement with experimental results [88], This is an important difference from the performance of the MP2/II level of theory for TM complexes, where the computed BDEs are always too high. Table 7.18 lists the calculated BDEs for complexes of group-13 Lewis acids EX3 with various Lewis bases. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

The first step is the nucleophilic attack of the catalyst at the silicon atom, caused by the donor-acceptor interactions between the Lewis base and the silicon compound. The components may be associated very weakly. Here no interaction between SiCl4 and PPh3 was found by means of P NMR. In some cases the interactions lead to solid adducts. In solution they dissociate into the original components. Complexes are formed in which the coordination at the silicon atom is increased to five or six. 

---