Electron donor-acceptor bonds

The single bonds described in these examples are formed from two shared electrons, one furnished by each of the two bonded atoms. Bonds in which both of the shared electrons are furnished by one of the atoms can form also. Generally, such bonds involve a lone pair from the donor atom and an unfilled orbital in the acceptor atom, usually a positively charged ion. These bonds are called electron donor-acceptor bonds. 

Hydration is a consequence of two types of bonding (1) electron donor-acceptor bonding, and (2) hydrogen bonding. The primary t5rpe involved depends on the ion. 

The lone pair electrons of water (O atom), ammonia (N atom), and amino groups (N atom) influences the behavior and concentrations of hydrogen ions (H+) in water. Hydrogen ions, produced either by dissociation of water or by dissociation of acids, do not occur as free entities in aqueous solutions. They associate with the lone pair electrons of other water molecules to form hydronium ions, H3O+. This association involves the formation of an electron donor-acceptor bond. 

Most chemists would rationalize the existence of amine borane as due to the fact that the B atom in monomeric BH3 has only six electrons in the valence shell. The formation of the NB bond is described as due to donation of the electron lone pair on N to the electron poor boron atom. A species formed in this manner from two relatively stable chemical entities, is referred to as a complex or coordination compound. The chemical species providing the electron pair is referred to as the electron donor or the Lewis base. The bonding partner is referred to as the electron acceptor or the Lewis acid. The new bond that has been formed between the donor and acceptor atoms, has been referred to as an electron donor-acceptor bond or as a dative bond. 

It is also certain that, in many cases, there is the possibility of chemical interaction and bonding across the interface. Electron donor-acceptor bonds may be formed to add to the adhesive strength from the dispersion forces. Hydrogen bonding is a particular case but acid-base interactions may also be involved. 

Coordination Compounds and Electron Donor-Acceptor Bonds... 

Morokuma K 1977. Why Do Molecules Interact The Origin of Electron Donor-Acceptor Complexes, Hydrogen Bonding, and Proton Affinity. Accounts of Chemical Research 10 294-300. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

Let us now examine the consequences of the formation of a donor-acceptor bond in a little more detail. If the donor - acceptor bond is completely covalent, then we record net transfer of one unit of charge from the donor to the acceptor as a direct consequence of the equal sharing of the electron pair between the two centres. This result leaves a positive charge on the donor atom and a negative charge on the acceptor atom. The limiting ionic and covalent descriptions of a complex cation such as [Fe(H20)6] are shown in Fig. 1-1. 

When the reaction of two compounds results in a product that contains all the mass of the two compounds, the product is called an addition compound. There are several kinds. In the rest of this chapter, we will discuss addition compounds in which the molecules of the starting materials remain more or less intact and weak bonds hold two or more molecules together. We can divide them into four broad classes electron donor-acceptor complexes, complexes formed by crown ethers and similar compounds, inclusion compounds, and catenanes. 

In electron donor-acceptor (EDA) complexes, there is always a donor molecule and an acceptor. The donor may donate an unshared pair (an n donor) or a pair of electrons in a ti orbital of a double bond or aromatic system (a it donor). One test for the presence of an EDA complex is the electronic spectrum. These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Because the first excited state of the complex is relatively close in energy to the ground state, there is usually a... 

In addition to adsorbing at mineral-oil interfaces, asphaltene molecules also adsorb at oil-water interfaces. Strong intermolecular dipole-dipole, hydrogen bonding, electron donor-acceptor and acid-base interactions cause the surface-adsorbed asphaltene molecules to form rigid skins" at oil-water interfaces (41 43). When water droplets are dispersed in an oil which contains asphaltene molecules, molecularly thick, viscous asphaltene films form around the water droplets, inhibit the drainage of intervening oil and sterically stabilize the water-inoil emulsion. 

We should note that the formation of this bond confers formal charges on the B and N atoms. In this bond and many similar Lewis acid-base complexes both the electrons forming the bond come from the same atom rather than from different atoms, as in the formation of a bond between two chlorine atoms. This type of bond is often called a donor-acceptor bond, a dative bond, or a coordinate bond, and is sometimes given a special symbol—an arrow denoting the direction in which the electron pair is donated ... 

In a similar fashion the bonding in H2 might be formally regarded as a complementary pair of one-electron donor-acceptor interactions, one in the ot (spin up ) and the other in the 3 (spin down ) spin set.8 In the long-range diradical or spin-polarized portion of the potential-energy curve, the electrons of ot and (3 spin are localized on opposite atoms (say, at on HA and 3 on HB), in accordance with the asymptotic dissociation into neutral atoms. However as R diminishes, the ot electron begins to delocalize into the vacant lsB(a) spin-orbital on HB, while (3 simultaneously delocalizes into Isa on HA, until the ot and (3 occupancies on each atom become equalized near R = 1.4 A, as shown in Fig. 3.3. These one-electron delocalizations are formally very similar to the two-electron ( dative ) delocalizations discussed in Chapter 2, and they culminate as before (cf. Fig. 2.9) in an ionic-covalent transition to a completely delocalized two-center spin distribution at... 

---