Main group organometallic chemistry
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Main group organometallic chemistry concerns the preparation and properties of main-group elements directly bonded to carbon. The inventory is large. The compounds exhibit a wide range of properties, including ones that are water-stable and others that are pyrophoric.[1] Many are very useful themselves, as chemical reagents, or as catalysts.[2]
Main group organometallic chemistry are typically categorized according to their position in the periodic table. Another possible classification scheme organizes these compounds according to the nature of the organic substituent: alkyls, aryls, vinyls, etc.
Most homoleptic organo-main group compounds adopt a characteristic oxidation state: RLi, R2Be, R3B/R3Al, R4Si, R3P, R2S. Members where the simplest stoichiometry violates the octet rule often aggregate by formation of bridging alkyl groups. When the alkyl group bridges two main group elements, the bonding is called three-center two-electron bonds. This pattern is seen for dimethyl beryllium and trimethylaluminium. In the case of methyl lithium, the methyl group can be shared (bonded to) three Li centers. These bonding aspects influence the structures: Trimethylaluminium, dimethyl beryllium, and methyl lithium are dimers, polymers, and clusters, respectively.
Academic research often seeks exceptions to conventional stoichiometries and oxidation states, often by use of bulky ligands. Examples include (pentamethylcyclopentadienyl)aluminium(I) (Al(I)), stannylenes (Sn(II)), and diphosphenes (P(I)).
Reactivity
Thus organic derivatives of the electropositive alkali metals and alkaline earth metals tend to be highly reactive toward electrophiles, e.g. oxygen and water. Organic derivatives of the less electropositive main group elements are often robust.
Like their derivatives lacking organic substituents, halides and alkoxide ligands for the later organomain group compounds tend to hydrolyze. Organophosphorus and silanes exhibit this pattern:
- R3SiCl + H2O → R3SiOH + HCl
- R2PCl + H2O → R2POH + HCl
