Chapter_21_Notes

Chapter_21_Notes - Organometallic chemistry Shriver,...

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1 Organometallic chemistry Shriver, Chapter 21 An organometallic complex is a coordination complex that contains at least one (covalent) metal carbon bond. Complexes that simply contain metal and carbon may not qualify. For example, NaCN and Ca 2 C do not contain covalent Na-C or Ca-C bonds, so are not considered organometallic. VO(acac) 2 contains carbon (in the acac ligand), but the vanadium is exclusively bonded to oxygen, not carbon (such molecules are sometimes called metalorganic complexes). The rules for naming organometallic complexes and for describing different modes of ligand binding (e.g., bridging, hapticity) are the same as for coordination complexes. Also, the rules for electron counting are the same. We have already encountered ligands based on carbon donors. Examples include methyl (a pure σ -donor) and CO (a π -acid). In organometallic chemistry, many carbon-based ligands involve π -donation to metal orbitals. Metal alkyls Alkyl ligands (methyl, ethyl, propyl etc.) bond to metals via σ -bonds. The most stable alkylmetal complexes tend to be those lack hydrogen atoms in the β -position (e.g., methyl, benzyl, neopentyl, trimethylsilylmethyl). This is because a facile decomposition pathway for metal alkyls involves β -H elimination to yield an olefin and a metal hydride: L n M C ! C " R H L n M H H CH 2 C R H + " -H elimination olefin insertion The reverse reaction is called olefin insertion, and is the key step in olefin polymerization. The relative rates of β -H elimination and olefin insertion determine the chain length (dimer, oligomer, polymer). Metal alkyls stabilized because they lack β -H’s: L n M CH 3 L n M CH 2 C CH 3 CH 3 CH 3 L n M CH 2 L n M CH 2 Si CH 3 CH 3 CH 3 methyl neopentyl trimethylsilylmethyl benzyl Me Np or neosilyl (Ns) Bn
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2 Replacing one methyl group by phenyl in the neopentyl ligand gives the neophyl ligand. A common method for making alkylmetal complexes is the reaction of a metal halide with a Grignard reagent: M–X + RMgX M-R + MgX 2 The driving force for the reaction is the precipitation of solid MgX 2 , when the reaction is conducted in a nonpolar solvent. The Grignard reagent RMgX is prepared by the reaction of an alkyl halide with Mg metal: RX + Mg RMgX It must be prepared with strict exclusion of O 2 and moisture, both of which destroy the Grignard. Almost all such air-sensitive organometallic compounds are synthesized using a Schlenk line or a glovebox filled with N 2 or Ar. Organolithium reagents RLi can also be used (prepared by the reaction of RX with Li); they tend to be more reactive for alkyl transfer than Grignards, and also more reducing. It is possible in this way to perform multiple alkylations on the same metal, for example:
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This note was uploaded on 08/06/2008 for the course CHEM 173B taught by Professor Scott during the Winter '08 term at UCSB.

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Chapter_21_Notes - Organometallic chemistry Shriver,...

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