Reactivity and d orbitals

First- and Second-Row Anomalies 858 The Use of p Orbitals in Pi Bonding 861 The Use (or Not) of d Orbitals by Nonmetals 86ft Reactivity and d Orbital Participation 875... 

The periodic table is arranged more or less by chemical reactivity, using the number of electrons in the outermost shell of the element and the energy of those outermost (valence) electrons. In effect, elements are arranged according to their valence orbitals. The periodic table currently lists 109 elements. The first attempt to categorize elements in this manner was by Dmitri Ivanovich Mendeleev (Russia 1834-1907), in the nineteenth century. The first row of elements (H, He) have only the spherical s-orbitals, but the second row (Li, Be, B, C, N, O, F, He) has the Is-orbital and the 2s- and 2p-orbitals are in the outermost shell. The third row introduces 3s- and 3p-orbitals, and d-orbitals appear in the fourth row. Each shell will have one s-, three p-, five d-, and seven f-orbitals (1, 2, 3, 4), and the d- and f-orbitals accept more electrons or give up more electrons than a p-orbital. Indeed, elements with d- and f-orbit-als are characterized by multiple valences. This stands in sharp contrast to... 

This greater reactivity of the silanes may be due to several factors, for example, the easier approach of an oxygen molecule (which may attach initially to the silane by use of the vacant silicon d orbitals) and the formation of strong Si—O bonds (stronger than C—O). 

Both the reactivity data in Tables 11.3 and 11.4 and the regiochemical relationships in Scheme 11.3 ean be understood on the basis of frontier orbital theory. In reactions of types A and B illustrated in Seheme 11.3, the frontier orbitals will be the diene HOMO and the dienophile LUMO. This is illustrated in Fig. 11.12. This will be the strongest interaction because the donor substituent on the diene will raise the diene orbitals in energy whereas the acceptor substituent will lower the dienophile orbitals. The strongest interaction will be between j/2 and jc. In reactions of types C and D, the pairing of diene LUMO and dienophile HOMO will be expected to be the strongest interaction because of the substituent effects, as illustrated in Fig. 11.12. 

The elements in Groups 3 through 11 are called the transition metals because they represent a transition from the highly reactive metals of the s block to the much less reactive metals of Group 12 and the p block (Fig. 16.1). Note that the transition metals do not extend all the way across the d block the Group 12 elements (zinc, cadmium, and mercury) are not normally considered to be transition elements. Because their d-orbitals are full, the Group 12 elements have properties that are more like those of main-group metals than those of transition metals. Just after... 

Aluminum is unique among the main group metals. All other p block metals have filled valence d orbitals. As a consequence, these metals have much in common with their transition metal neighbors. They tend to be soft Lewis bases. Aluminum, on the other hand, lacks a filled d orbital set and is a hard Lewis acid that has more in common with its nearest neighbor, magnesium. Highly reactive, aluminum is found naturally in the +3 oxidation state and is difficult to reduce to the pure metal. Thus, although tin and lead have been known since antiquity, aluminum was not discovered until 1825 and did not become a common commodity until more than 60 years later. 

A knowledge of the behavior of d orbitals is essential to understand the differences and trends in reactivity of the transition metals. The width of the d band decreases as the band is filled when going to the right in the periodic table since the molecular orbitals become ever more localized and the overlap decreases. Eventually, as in copper, the d band is completely filled, lying just below the Fermi level, while in zinc it lowers further in energy and becomes a so-called core level, localized on the individual atoms. If we look down through the transition metal series 3d, 4d, and 5d we see that the d band broadens since the orbitals get ever larger and therefore the overlap increases. 

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