Molecular bonding in the d-block elements

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.1 For normal and hypovalent molecules, Pauling and Slater proposed that sp 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. 

Pauling further extended the sp dm hybridization approach to the d-block compounds.3 By varying the relative importance of p and d orbitals, Pauling was able to construct hybrid orbitals that rationalized the geometries and magnetic properties of many transition-metal coordination complexes. For example, the square-planar 

Because the low-energy electronic configurations of d-block elements and their +1 ions are invariably of sdm form (see Table 2.2, Section 2.8), it is clear that both s and d orbitals will be involved in bond formation at transition-metal centers. What is less clear, a priori, is what role the valence p orbitals will play in bonding of the d-block elements. 

---