Molecular orbital tetrahedral case

For instance let us consider the case of the tetrahedral carbonyl cluster [Rh4(CO)i2], together with the theoretical analysis of its molecular orbitals, Figure 1. 

Many such examples are known. In most cases where the stereochemistry has been investigated, retention of configuration is observed,225 but stereoconvergence (the same product mixture from an E or Z substrate) has also been observed,226 especially where the carbanionic carbon bears two electron-withdrawing groups. It is not immediately apparent why the tetrahedral mechanism should lead to retention, but this behavior has been ascribed, on the basis of molecular orbital calculations, to hyperconjugation involving the carbanionic electron pair and the substituents on the adjacent carbon.227... 

MOLECULAR ORBITALS FOR a BONDING IN ABW MOLECULES THE TETRAHEDRAL AB4 CASE... 

Such a transformation can be used for relocalizing a given set of delocalized molecular orbitals in conformity with the chemical formula. For instance, the occupied orbitals of methane can be transformed into orbitals very close to simple two-center MO s constructed from tetrahedral sp3 hybrid orbitals and Is hydrogen orbitals 24,25,26) a. unitary transformation can hardly modify the wave function, except for an immaterial phase factor therefore, it leads to a description which is as valid as that in terms of the canonical delocalized Hartree-Fock orbitals. Of course, the localization obtained in this way is not perfect, but it is usually much better than is often believed. In the case of methane, the best localized orbitals are uniquely determined by symmetry 27> for less symmetric molecules one needs a criterion for best localization 28 29>, a problem on which we shall not insist here. A careful inspection reveals that there are three classes of compounds ... 

Covalent random networks conform to the (8- ) rule. In these structures, a number of bonds are strained, either bent or over-stretched. Therefore, occasionally they get broken . These broken bonds correspond to single electron carrying orbitals and are described as dangling bonds. In the case of a tetrahedral semiconductor, the nature of such states can be understood with the help of simple molecular orbital diagrams as shown in Figure 8.14. The bonding orbitals are the sp ... 

All examples shown in this chapter were for high-symmetric, either octahedral or tetrahedral, complexes. One may therefore wonder if the present considerations still remain valid in cases without symmetry. For instance, will the metal d contributions still be confined to a limited set of molecular orbitals (i.e., the basic ten) in cases where such limitations are not enforced by symmetry That this is indeed the case was already illustrated by the tetrahedral examples (Table 4), where the M M orbitals can in principle be delocaUzed over two bonding ti shells, but in practice significantly contribute only to one of these shells. 

For molecular species with other than linear, trigonal planar or tetrahedral-based structures, it is usual to involve d orbitals within valence bond theory. We shall see later that this is not necessarily the case within molecular orbital theory. We shall also see in Chapters 14 and 15 that the bonding in so-called hypervalent compounds such as PF5 and SFg, can be described without invoking the use of J-orbitals. One should therefore be cautious about using sp"d hybridization schemes in compounds of />-block elements with apparently expanded octets around the central atom. Real molecules do not have to conform to simple theories of valence, nor must they conform to the sp"d" schemes that we consider in this book. Nevertheless, it is convenient to visualize the bonding in molecules in terms of a range of simple hybridization schemes. 

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