Metal-alkyl bridge structures

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). 

The synthesis, structures, and reactions of silicon-substituted alkyl derivatives of metals from groups 1, 2, and 3 have been recently reviewed. A diverse range of structures has been discovered with the alkali metals. More recently, some additional structures and an important discussion of structural trends observed among the silicon-substituted alkyl derivatives of the alkali metals have also been published. " Tris(trimethylsilyl)methyl lithium forms a solvent-free dimer, as well as several ate complexes in the presence of bases (THF or TMEDA). In the alkyl-bridged dimer of tris(trimethylsilyl)methyl lithium (2), there are interactions between the C-H bonds of the silyl ligand and the lithium atoms (3) (Li to C distances of 254.1(7) and 246.6(6) pm). ... 

In Group III we find a third type of structure for the metal alkyl, namely, the bridged dimer (b) of Al2(CH3)g which is structurally similar to the AI2X6 molecules. There are also less symmetrical molecules with the same type of structure, for example, (CH3)sAl2N(C6Hs)2, where the bridge consists of one CH3... 

OR) bridges (VIII and IX) rather than doubly bridged structures (see below). It is also apparent in the structures of MX+ complexes (X), where M is a metal or metalloid and X = Cl or an alkyl/alkoxo group (236). Such compact structures are favored by the ability of the metal atoms to utilize more atomic orbitals for bonding purposes (237). 

For most of the systems discussed so far, hypercoordinated carbon atoms have featured in the most stable forms of the compounds in question. For example, the bridged metal alkyl structures found by X-ray studies on crystalline samples of such substances as (AlMe3)2 ° or (l. iMc)/ persist in solutions of... 

In order for it to function as a Lewis acid catalyst, the dimer AbCU can undergo the less energetic opening of only one of the two chlorine bridges to form the half-bridged structure 3 [Eq. (2.2)] in which the three-coordinate metal atom can function as a strong Lewis acid. " Similar half-bridged structures in dimeric aluminum alkyls and other associated metal alkyls can be expected to play roles in their reactions with bases. 

Interestingly, the compound that was once thought to provide the first example of a bridged metal alkyl to be structurally characterized by X-ray diffraction turned out to have been wrongly identified. This was the tetrameric... 

The most prominent suggestions as to the structure of the active site can be illustrated by reference to a bimetallic complex where polymer propagation could conceivably occur from (1) the transition metal center which is really a transition metal alkyl, (2) the reducing agent center, or (3) a bridge position between the two metals... 

Gomberg s and Paneth s 43) studies of free organic radicals, the bridge structure of metal alkyls 103), the organic sandwich complexes of the transition metals 71, 86), all have in their turn spurred the theory of the chemical bond on to fresh viewpoints. 

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