Bonding considerations group 15 organometallic compounds

Main-group organometallic compounds are versatile tools in organic synthesis, but their structures are complicated by the involvement of the multicenter, two-electron bonds and ion-dipole interactions that are involved in aggregate formation (5). Electron deficiency or Lewis acidity of the metallic center and nucleophilicity or basicity of the substituents are important considerations in synthesis. The complexity of the structures and interactions is, however, the origin of much of the unique behavior of these organometallic compounds. 

The application of organometallic compounds or hydrides of the more electropositive main group metals lithium, magnesium or aluminium in organic synthesis, catalysis and polymerization is due to the polarity of the metal/carbon and metal/hydrogen bonds, i.e. to the considerably unsymmetrical electron distribution M —or —respectively. While there are... 

We begin, however, with a brief consideration of a group of phosphorus compounds of significance for their utility in the formation of carbon-phosphorus bonds via reaction with organometallic reagents in displacement reactions. 

Similarities and differences between Ge, Sn, Pb (M) atoms and their M—Y bonds (Y = inorganic, organic or organometallic substituents). In some cases the specific character of the M atom and the M—Y bonds is more clearly seen when compared with properties and bonds of carbon and especially silicon atoms — the lighter elements of the group 14 column of the periodic table. The peculiarities of M atoms and M—Y bonds determine to a considerable extent the similarity and difference of the physical properties and chemical behaviour of organic compounds of the elements in question. 

A common misconception is that carbon can, at most, be four-coordinate. Although carbon is bonded to four or fewer atoms in the vast majority of its compounds, many examples are now known in which carbon has coordination numbers of 5, 6, or higher. Five-coordinate carbon is actually rather common, with methyl and other groups frequently forming bridges between two metal atoms, as in Al2(CH3)g (see Figure 8-15). There is even considerable evidence for the five-coordinate ion Many organometallic cluster... 

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