Increased-Valence or Electronic Hypervalence via Pauling 3-Electron Bonds

In structure (7), we have indicated (cf. Sections 2-4 and 7-1) the presence of a long bond (or formal bond) between atoms Y and B by means of a pecked line (—). When Y and B are non-adjacent atoms, the y and b orbital overlap is very small, and therefore the Y-B bond is very weak (Section 2 ). 

Because structure (5) summarizes resonance between (6) and (7), not all of the unpaired electron charge on Y is used to form the Y-A bond of (5) some of it is used to form the long weak Y-B bond. We could indicate this extra bonding in (5) by a pecked line, as is shown in 10. However, since the Y-B bond will usually be very much weaker than the Y-A bond, in future we shall not indicate the Y-B bonding in stmctures of type (5). 

We note also that stracture (7) has no Y-A bond. Therefore, the Y-A bond-number (i.e. the number of electron-pair bonds) of structure (5) must be fractional and less than the value of unity that pertains to the Y-A single bond of the structure (6). This result is important, and as we shall find when we discuss some examples, it helps to provide a qualitative understanding of the properties of many bonds. To distinguish the fractional Y-A bond of structure (5) from that of structure (6), we have used a thin bond-line in structure (5). 

11-2 Increased-Valence or Electronic Hypervalence via Pauling 3-Electron Bonds  

In each of the Lewis structures (6) and (7), there are two bonding electrons, namely those that occupy the y and a, and y and b atomic orbitals, respectively. We shall now deduce that a maximum of three electrons can participate in bonding in structure (5). To do this, it is helpful to recall (Section 3-6) that the Pauling 3-electron bond configuration (a) ( tiab) (b) for the valence-bond 

---