Third main-Group elements

G. Third Main-Group Elements and Hydrogen Atoms as Ligands. 92... 

Let X be an element of the fourth to seventh main groups of the periodic table, i.e. an element that tends to attain the electronic configuration of the following noble gas by taking up electrons (the heavy elements of the third main group may also be included). An X atom has e(X) valence electrons. 

The elements may be divided into types (Fig. 17-10), according to the position of the last electron added to those present in the preceding element. In the first type, the last electron added enters the valence shell. These elements are called the main group elements. In the second type, the last electron enters a d subshell in the next to last shell. These elements are the transition elements. The third type... 

Bond energy variations over the periodic table will be subject to perturbations which reflect the underlying atomic configurations. Compounds derived from main-group elements of Period 4, for example, will show discontinuities in properties from those of Period 3 because of the extra d-electron shell. Conversely, the insertion of an f-electron shell brings together the properties of the second and third transition series, especially in the earlier groups. 

Homonuclear clusters of the heavier elements of the third main-group aluminum, gallium, indium and thallium having direct element-element interactions form a fascinating new class of compounds. As discussed in the previous Chapter 2.3, in some cases their structures resemble those known with the lightest element of that group, boron, while in other cases novel, metal-rich compounds were obtained which do not have any analogue in boron chemistry. 

Stuttgart-Dresden-Bonn relativistic ECPs [85]) for third- and fourth-row main group elements. 

Initially, the level of theory that provides accurate geometries and bond energies of TM compounds, yet allows calculations on medium-sized molecules to be performed with reasonable time and CPU resources, had to be determined. Systematic investigations of effective core potentials (ECPs) with different valence basis sets led us to propose a standard level of theory for calculations on TM elements, namely ECPs with valence basis sets of a DZP quality [9, 10]. The small-core ECPs by Hay and Wadt [11] has been chosen, where the original valence basis sets (55/5/N) were decontracted to (441/2111/N-11) withN = 5,4, and 3, for the first-, second-, and third-row TM elements, respectively. The ECPs of the second and third TM rows include scalar relativistic effects while the first-row ECPs are nonrelativistic [11], For main-group elements, either 6-31G(d) [12-16] all electron basis set or, for the heavier elements, ECPs with equivalent (31/31/1) valence basis sets [17] have been employed. This combination has become our standard basis set II, which is used in a majority of our calculations [18]. 

Comparative data for heavy-atom bond lengths and skeletal bond angles for molecules incorporating one or more third or fourth-row, main-group elements are provided in Appendix A5 Table A5-39 for Hartree-Fock models with STO-3G, 3-2IG and 6-3IG basis sets. Table A5-40 for the local density model, BP, BLYP, EDFl andB3LYP density functional models and the MP2 model, all with the 6-3IG basis set, and in Table A5-41 for MNDO, AMI and PM3 semi-empirical models. 6-31G, local density, density functional and MP2 calculations have been restricted to molecules with third-row elements only. Also, molecular mechanics models have been excluded from the comparison. A summary of errors in bond distances is provided in Table 5-8. 

Table 5-8 Mean Absolute Errors in Bond Distances for Molecules Incorporating Third and Fourth-Row, Main-Group Elements... 

Table A5-40 Heavy Atom Bond Distances and Skeletal Bond Angles in Molecules Incorporating Third-Row, Main-Group Elements. Correlated Models with the 6-31G Basis Set... 

Table 8.5 also indicates that the most popular functionals fail to be as accurate for molecules containing third-row main-group elements as they are for molecules made up of elements from the first two rows. The LYP correlation functional seems to perform particularly badly, while the PW91 functional is more robust. 

FIGURE 6.10 The octet rule occasionally fails for the shaded main-group elements. These elements, all of which are in the third row or lower, can use low-energy unfilled d orbitals to expand their valence shell beyond the normal octet. 

The reducing agent i.s an amalgam. Mercury is alloyed with an element from the third main group. 

Until recently only a handful of main group elements — carbon, nitrogen, oxygen and sulfur — were known to be able to form stable (p-p)rc bonds. The complete lack of such bonds in compounds of the heavier elements had been rationalized by theoretical treatments by Pitzer u and Mulliken 2). Indeed, many commonly used textbooks of inorganic chemistry contain statements such as,. . p-p-n bonding is of little importance in the third and higher period elements,3). 

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