Organometallics, aggregate structures organometallic

Reaction conditions can be modified to accelerate the rate of lithiation when necessary. Addition of tertiary amines, especially TMEDA, facilitates lithiation53 by coordination at the lithium and promoting dissociation of aggregated structures. Kinetic and spectroscopic evidence indicates that in the presence of TMEDA lithiation of methoxybenzene involves the solvated dimeric species (BuLi)2(TMEDA)2.54 The reaction shows an isotope effect for the o-hydrogcn, establishing that proton abstraction is rate determining.55 It is likely that there is a precomplexation between the methoxybenzene and organometallic dimer. 

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. 

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

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. 

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