Introduction

Given the power of the concept of covalency and the deeper electronic implications that were realized by G. N. Lewis's octet-rule and shared-electron-pair concepts, it is natural to wonder whether these advances are limited to s/p-block elements or apply to the entire periodic table.

Soon after the quantum revolution of the mid 1920s, Linus Pauling and John C. Slater expanded Lewis's localized electronic-structural concepts with the introduction of directed covalency in which bond directionality was achieved by the hybridization of atomic orbitals. For normal and hypovalent molecules, Pauling and Slater proposed that spn hybrid orbitals are involved in forming shared-electron-pair bonds. Time has proven this proposal to be remarkably robust, as has been demonstrated by many examples in Chapter 3.

Hypervalent main-group compounds and most transition-metal complexes do not conform to Lewis's octet rule, because they exceed the ideal eight-electron count. Pauling first addressed this issue for main-group compounds by expanding the available valency of second- and higher-row elements by the addition of valence d orbitals to the set of orbitals that are active in bond formation. By using spn dm hybrid orbitals to form directed covalent bonds and by considering the strong role of resonance among ionic normal-valent structures, Pauling was able to rationalize a large number of hypervalent structures from the p block of the periodic table. However, as we have seen in Section 3.5, modern quantum-mechanical analyses of electron-density distributions downplay the importance of spn dm hybridization in hypervalent p-block compounds, emphasizing instead the importance of ionic resonance and 3c/4e bonding interactions.