Covalent bonds Lewis acid-base interactions

By far the dominant adhesion mechanism, particularly in the absence of covalent linkages, is the electrostatic attraction of the polar groups of the adhesive to polar groups of the adherends. These are mainly forces arising from the interaction of permanent dipoles, including the special cases of hydrogen bonding (10-25 kJ/mol) and Lewis acid-base interactions (<80 kJ/mole).25 26 These forces provide much of the attraction between the... 

Thus, the bonding in metal compounds and complexes has traditionally been viewed as ionic, with a positive metal center interacting with negative ions, such as HO , 0 , Cl , AcO , and coordinate donor, Lewis acid-base interactions with a positive metal center interacting with negative ions and electron-pair Lewis bases, such as iNHs, iPPhs, HOH. Examples of ionic versus covalent bonding illustrate the tradition H+CH versus H-Cl (ls-3p), C +(C1 )4 versus C(-C1)4 [2sp -(3p)4], Fe +(C1 )3 versus Fe(-Cl)3 [3d sp2-(3p)3], H+-OH versus H-OH (ls-2p), C +(Q2-)2 versus 0=C=0 [2sp -(2p )2], and Fe +(0 ) versus Fe=0 [3d sp-(2p )]. Such ionic formulations for these molecules in an inert matrix are not consistent with their physical... 

Ion Pairing Upon dissolution, the metal ion can coordinate with a variety of counterions in the aqueous layer. This ion-pairing process can be rationalized by means of the Lewis acid-base concept. When the metal ion (the Lewis acid, that is, a species that accepts an electron pair) and the counterion (the Lewis base, that is, a species that donates an electron pair) interact, the electrons involved form a covalent bond. Lewis acids and bases are more likely to coordinate if their electrons possess similar properties. So-called soft acids or bases have valence... 

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

Figure 24.28 Representation of the metal-ligand bond in a complex as a Lewis acid-base interaction. The ligand, which acts as a Lewis bas donates charge to the metal via a metal hybrid orbital. The bond that results is strongly polar, with some covalent character. It is often sufficient to assume that the metal-ligand interaction is entirely electrostatic in character, as is done in the crystal-field model. 

Note that in organomercury compounds, Hg forms two linear (or very near linear) non-polar a bonds. In addition, the presence of two empty p orbitals conveys strong Lewis acidity to the plane perpendicular to the essentially covalent a bonds, which can be involved in Lewis acid-base interaction. In mercuracarborands the... 

If we apply this theory, automatically need arises to modify the term neutralization. It can no more be used in the sense in which it has been hitherto used. Since Lewis acid base interaction invariably results in the formation of a covalent bond, the word co-ordination is more appropriate than neutralization. However, one might still use the term neutralization. 

In the term Lewis acid-base interaction virtually all other types of specific chemical interactions are lumped together. In the most general imder-standing a Lewis acid-base interaction between two participants is present whenever an electron pair is shared by the two. This can range from a covalent bonding in one extreme to an ionic interaction in the other. This type of interaction is related to certain functional groups and does not need to be present permanently. Thus, it could be taken as an additional chemical contribution to ST, SFE and IFT in addition to the always present Lifshitz-van der Waals interactions. 

Each arrow represents a directed two-electron ai donor—acceptor interaction from the filled lone pair of Cl into the unfilled ai hole of Al. Such a Lewis acid-base interaction is called a coordinate covalent or dative bond, and is often symbolized by a double-dot (A1 C1) or directed arrow (Al<—Cl) in the Lewis structure diagram, to distinguish it from an ordinary covalent (Al—Cl) bond-stoke. The distinctive coordinative A1< C1 bond will be written as cfAtcr to distinguish it from the covalent caici bond of Fig. 8.1a. 

Acid-base interactions in the most general Lewis sense occur whenever an electron pair from one of the participants is shared in the formation of a complex, or an adduct . They include hydrogen bonding as one type of such a bond. The bond may vary from an ionic interaction in one extreme to a covalent bond in the other. Acid-base interactions and their importance in interfacial phenomena have been reviewed extensively elsewhere [35,78] and will be described only briefly here. 

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. 

Basic Mechanisms of Adhesion Acid-Base Interactions. The understanding of polymer adhesion has been greatly advanced in recent years by the recognition of the central role of acid-base interactions. The concept of an acid was broadened by G. N. Lewis to include those atoms, molecules, or ions in which at least one atom has a vacant orbital into which a pair of electrons can be accepted. Similarly, a base is regarded as an entity which possesses a pair of electrons which are not already Involved in a covalent bond. The products of acid-base interactions have been called coordination compounds, adducts, acid-base complexes, and other such names. The concept that... 

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

The bonding for oxygen atoms in heteratomic molecules is viewed as essentially covalent (see Covalent Bonds) (e.g. MeOH, Mc2C=0, MeCHO, and MeC(O)OOH) and similar to that for carbon, nitrogen, and chlorine atoms. In contrast, a Lewis acid base formahsm often is used for metal oxygen compounds with ionic interactions by dianionic 0x0 groups (e.g. [Ba2+ q2-], [Fe + (0 -) ] -, [Mn + (Q2-)4]-, and [(Cu+)20 ]). This results from the thermodynamic relations for ionic solution equihbria, and the inference that the combination of ions results in molecules and complexes held together by electrostatic interactions (equations 33 35). 

The interaction between a metal ion and a ligand can be viewed as a Lewis acid-base reaction, with the ligand donating a lone pair of electrons to an empty orbital on the metal ion to form a coordinate covalent bond ... 

The bond formed between the hydrogen ion and the water, in this example, is a coordinate covalent bond. A coordinate covalent bond is formed between two atoms, where one atom provides both of the electrons that become shared. In the Lewis Acid-Base Theory, all interactions between acids and bases will involve coordinate covalent bonding. 

---