Chemical Bonding of Main-Group Elements

Prior to any meaningful discussion of bonding in main-group chemistry, we have to provide a reasonably accurate definition of what a main-group element is. In general, we assume that main-group elements are those that essentially use only their valence sand p-orbitals for chemical bonding. This leads to a number of borderline cases that require closer inspection. Assuming that the outer d-orbitals, that is, those with principal quantum number n equal to the period in question, are not true valence orbitals (see discussion of outer d-orbital participation in bonding later in the text), we may safely define groups 13–18 as main-group elements. Group 1 is also reasonably assigned to the main groups, albeit under extreme hydrostatic pressures it appears that the elements K, Rb, and Cs turn from ns1 metals into transition metals and use predominantly their inner (n−1) d-orbitals for bonding [1]. However, sufficiently high pressures may change fundamental bonding in many elements and compounds [2]. We disregard here such extreme pressure conditions and count group 1 in the main groups. Matters are less straightforward for group 2: whereas Be andMg utilize only their sand p-orbitals, Ca, Sr, Ba, andRa use their inner (n−1) d-orbitals predominantly in covalent bonding contributions when sufficiently positively charged, as we discuss in the following text [3]. This leads to a number of peculiar structural features that bring these elements into the realm of ‘‘non-VSEPR d0 systems’’ that encompass early transition elements and even lanthanides, and they have also been termed ‘‘honorary d-elements’’ [4]. The heavy group 2 elements are nevertheless usually placed with the main groups, and it seems appropriate to include the discussion of these interesting features in the last section of this chapter. It thus remains to discuss the inclusion of groups 11 and 12. The group 11 elements Cu, Ag, Au, and Rg clearly have a too pronounced involvement of their (n −1) d-orbitals in bonding, even in their lower oxidation states, to be safely considered main-group elements. The group 12 elements Zn, Cd, Hg, and Cn are usually considered to be main-group or ‘‘post-transition’’ elements. Yet recently quantum-chemical predictions [5] of oxidation-state Hg(+IV) in the form of the