The Two-Electron Bond

Note that if instead of using purely localized AOs for a and b, we use semilocalized Coulson-Fischer orbitals, Equation 3.37 will no more be the 

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Jorgensen, Ch. K. The Problems for the Two-electron Bond in Inorganic Compounds, 124, 1-31... 

The problems of the two-electron bond in inorganic compounds analysis of the coordination number, N. C. K1 Jorgensen, Top. Curr. Chem., 1984,124,1 (125). 

The sharing of electrons between carbon and hydrogen is an example of atom to-atom bonding known as covalency and the two-electron bond is called a covalent bond. 

VB description. Just as R—X is a representation of the two-electron bond, (R—X) is the representation we will use for the three-electron bond between R and X. In VB terms such a bond may be described by a linear combination of the configurations, R- X and R -X. Both of these configurations are repulsive with respect to R---X approach due to exchange repulsion. The exchange repulsion comes about from the overlap of two electrons, one on R and the other on X, possessing the same spin (Pauling and Wilson, 1935). The linear combination of the two VB forms may, however, lead to the formation of a stable three-electron bond. Let us see how this comes about. 

P.R. Surjin. The Two-Electron Bond as a Molecular Building Block. In Z.B. Maksic (ed), Theoretical Models of Chemical Bonding, Part 2, Springer, Heidelberg (Topics Curr. Chem.) pp. 205, 1989. 

Although our discussion in the next seven chapters will center on simple tetrahedral structures, in which all electrons form simple two-electron bonds, it is desirable to introduce two other types of structures in these, some of the electrons form two-electron bonds (and are understandable in the same terms used for the tetrahedral solids) and other electrons are accommodated in pure p slates, similar to the 71 states discussed in Chapter 1 for diatomic molecules. For a discussion of the stability of these structures, see Friedel (1978). The two-electron bonds are simpler geometrically in these systems than in the tetrahedral solids and will provide very good problems. (Many of the problems at the ends of the following chapters will explore the theory of the bonds for these simpler systems.)... 

The whole story of the two-electron bond, with the S Bond as a paradigm ... 

Chemical bonds in inorganic chemistry are not limited to covalency. Very important are the dative donor-acceptor) bonds. Unlike normal covalent bonds, which are formed by pairing of electrons with a one-electron contribution from each atom, dative bonds are two-electron bonds formed by donation of an electron pair from one atom to another. There is a general tendency to assume that the two-electron bonds between a certain pair of atoms e.g. boron and nitrogen in H2B-NH2 and HsB- NH3) are identical, regardless of the origin of electrons (i.e. covalent and dative), but it has been pointed out that a distinction between the two types should be made. ... 

All of the atoms are drawn in, but the two-electron bond lines are generally omitted. 

Not only does the addition of an electron to the n MO of ethene inherently greatly decrease the extent of bonding relative to the two electron bond present in ethene, but it also appears highly likely that because of the additional electronic... 

I think the problem was that nobody understood the two-electron bond until Heitler and London s research in 1926. Then Hund and Mulliken developed the molecular orbital method. It s true that Lewis lived much beyond 1926, but Lewis himself did not understand the nature of the electron-pair bond in his early research. Failing that, I think his time sort of passed. It may have been a good idea to give him the prize but I can understand why they didn t, because Lewis himself certainly didn t formulate the physical basis of the bond. 

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