Organometallics, aggregate structures compounds

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

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. 

Many organometallic compounds of groups 1 and 2 exist in associated molecular form (as aggregates) or contain structural solvent, or both. However, their names are often based solely on the stoichiometric compositions of the compounds, unless it is specifically desired to draw attention to the extent of aggregation or the nature of any structural solvent, or both (see Example 3 below). In the examples below, note how the different types of name reflect the different structural content implied by the formulae shown. As usual, the formulae enclosed in square brackets designate coordination entities. 

The intra- and intermolecular rearrangements of C2h- and D2-74 clearly indicate the importance of rearrangements hitherto not normally covered in a chapter on carbanion rearrangements. Organometallic chemists, however, are learning more and more about the complex structure(s) of such compounds in solution and in the solid state, as well as their rearrangements, e.g. within aggregates. It is thus predictable that the near future will provide us with more examples of this sort. 

It turns out that they generally form complex aggregates with two, four, six, or more molecules bonded together, often with solvent molecules, one reason why apparently polar compounds such as BuLi dissolve in hydrocarbons. In this book we shall not be concerned with these details and we shall represent organometallic compounds as simple monomeric structures. 

The actual structures of organolithium and organomagnesium compounds are rarely monomeric as shown here dimers are common, as well as higher aggregates, depending on the structure of the organometallic and the solvent. 

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