Metal-alkyl complexes Bridging

Bochmann M, Lancaster SJ (1995) Cationic group IV metal alkyl complexes and their role as olefin polymerization catalysts The formation of ethyl-bridged dinuclear and heterodinuclear zirconium and hafnium complex. J Org Chem 494 55-59... 

Characteristic of most reactive RM compounds is their existence as two or more R M units closely associated with each other. The nature of these metal alkyl complexes has been extensively studied via X-ray crystal analysis by Rundle and co-workers, who showed that the a-carbon atom of the alkyl groups forms a bridge between metal atoms in different RM units. Thus, the dimer of trimethyialuminum may be viewed as... 

Half-sandwich rare earth metal alkyl complexes can act as catalyst precursors for the cross-coupling of various terminal alkynes with isocyanides selectively affording the (Z)-l-aza-l,3-enyne products (Scheme 20). The unprecedented Z selectivity could arise from the formation of an alkynide-bridged binuclear catalyst species, in which the crosscoupling reaction took place at the two metal centers in an intermolecular fashion. 

The bond between tetrahedral carbon and transition metals mainly exists in the form of terminal metal-alkyl complexes such as [WCCHs) ]. - Metal-alkyl complexes are now numerous. It has been understood that they are stable if the p-elimination path is blocked either because the complexes have 18 valence electrons, - or have less valence electrons, but no p hydrogen. - Complexes of other tetrahedral carbon are also known in which two, three or four metal substituents are metal fragments. They are respectively bridging alkylidene, alkylidyne and carbide complexes, below ... 

Finally, an alkyl group can also occasionally bridge two or three metals in various modes. Metal-alkyl complexes are examined in this chapter, terminal and bridging alkylidene and alkylidyne complexes being studied in the following chapter. 

The mixed metal alkyl-amido base [BuNa(TMEDA)][TMP2Mg] (TMP = 2,2,6,6-tetramethylpiperidine) is capable of deprotonating furan selectively at its 2-position in THF as a solvent. The product obtained is a complex tris-furylmagnesiate, with the empirical formula [Na2(THF)3][2-furyl6Mg2(TMEDA)] (15) . An X-ray crystal-structure determination showed that in the solid state this compound exists as a coordination polymer of [Na2(THF)3][2-furyl6Mg2] units linked by bridging TMEDA molecules (Figure 16). 

In this reaction [Mn(CO)5], which is quite nucleophilic, increases its metal coordination number by one. The reaction may be viewed as an electrophilic attack by R+ on the metal. Similarly, it is possible to prepare bridging alkyl complexes by this method/10... 

Sulfur dioxide is capable of reacting with metal alkyl, aryl or u-allyl complexes in an insertion-type reaction to yield S-sulfinate (4), O-sulfinate (5) or 0,< -sulfinate complexes (6).13 It can also insert into the metal-metal bond in the cobalt complex (7) to give the S02-bridged complex (8).38... 

Also, alkyls can bridge two metal centers, for example like methyl groups in (CFT3)2Al(pL-CFT3 2A 1(C E 1 )2 [184e], Coordinately saturated complexes are more... 

The electron counting" in structure X deserves a brief comment. In the absence of a M-M bond, the Mo in CpMo(C0)2(n3-ally1) has an 18-electron count. The Mo in X corresponds to a 16-elec-tron metal in a CpMo(C0)2(a-alkyl) complex. We therefore propose that the metal-metal bond is formed by a Mo— Mo dative bond which renders Mo negative with respect to Mo in structure X. The semi-bridging carbonyl then removes some of the excess negative charge on Mo by accepting electrons into the CO ir -orbitals. 

The analogous uranium(III) compounds also show unusual reactivity patterns. For example, addition of COT to (Cp )3U yields a mixed metallocene dimer (18) bridged by a COT ligand in this reaction, the (Cp )3U complex has effectively acted as a formal three-electron reductant (equation 3)." A variety of substituted (Cp )3U complexes form adducts with CO and CNR (isocyanides) - these are rare examples of actinide metals with r-acidic ligands." Paramagnetic uranium alkyl complexes are active in a range of catalytic processes." ... 

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