Electronic charge direct sharing

Chemisorption represents the formation of a surface chemical bond, which is either covalent (sharing of electrons) or ionic (electron transfer). The understanding of chemisorption phenomena is rather complex and requires knowledge on the geometrical structure of the system, adsorbate binding and charge transfer, the electronic structure of adsorbate and substrate, as well as vibrational frequencies [63]. Three dominant types of interactions may occur between chemisorbed species dipole-dipole (direct and screened by the electrolyte), electron-electron (indirectvia substrate electrons or direct at short distances), and elastic (via substrate ions) [31, 32, 65, 66]. 

The electric dipole moment of ethane is zero by symmetry. The gas-phase electric dipole moment of amine borane is 5.22 Debye. The negative pole is at the boron atom. If we assume that the two N-B bonding electrons are equally shared between the two atoms, the charge on the B atom would be -e and that on the N atom +e. Multiplication with the experimental bond distance yields an estimated dipole moment of 8.0 Debye. The bonding electron pair is obviously not equally shared, but remains closer to the N atom. We shall nevertheless indicate the direction of negative charge transfer by an arrow pointing from the donor to the acceptor atom N B. 

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. 

The forces involved in chemistry are essentially electrostatic. They are variants on the Coulomb force. We can distinguish two orders primary forces and secondary forces. Primary forces are those which hold the atoms together in molecules, and the oppositely charged ions in crystalline salts. Respectively, they are known as covalency and electrovalency (or, sometimes, the ionic force). The latter is directly electrostatic, the mutual attraction between Na+ and Cl" in common salt, for example. The former is usually figured as the sharing of an electron-pair between two atoms— Cl-Cl in the chlorine molecule, where the bond stands for a shared pair of electrons. We need quantum mechanics to understand why, in certain circumstances, electron density builds up in the region between the two chlorine atoms. Granted that it does so, we can explain the covalent bond as due to a resultant electrostatic effect. 

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