Uranium allyl complexes

Allyl Complexes. Allyl complexes of uranium are known and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a uranium halide and an allyl grignard reagent. This synthetic method was utilized to obtain a rare example of a "naked" homoleptic allyl complex, U(T -C2H )4 [12701 -96-17, which decomposes at 0°C. Other examples, which are more stable than the homoleptic allyl complex have been synthesized, ie, U(allyl)2(OR)2 (R = alkyl), U(allyl)2X (X = halide), and U(allyl)(bipy)2. 

The tetraallylic complexes of thorium and uranium can be isolated as red sohds and are stable to 0° and — 20°C, respectively, whereas the uranium homoalkyls defy isolation at temperatures as low as — 80°C. The greater stability of the thorium complex has been attributed partially to the great instability of Th(III) as compared with U(III) (96). The tc mode of bonding and the resultant increase in coordination number to eight of the allyl complexes significantly increases their thermal stability relative to the uranium homoalkyls. 

Tetrakis(allyl) complexes of thorium and uranium have been synthesized via the addition of allyl Grignard to the metal tetrachloride (equation 43). Alkyl-substituted allyl... 

Temperature-dependent NMR studies have shown that the uranium allyl complex is fluxional. This fluxionality has been proposed as a o- ... 

The reaction of thorium and uranium triscyclopentadienide chloride with Grignard and lithium reagents has also led to a variety of thermally stable alkyls, aryls and allyls. These complexes will be discussed in a later part of this review. 

In transition metal complexes, where the 18-electron rule is essentially obeyed, the bonding mode of the allyl ligand is mainly determined by the electronic requirements of the metal. However, this does not appear to be the case in the rare earth complexes. Instead, the bonding mode may be largely influenced by steric requirements. In the case of uranium, the coordination site necessary for w-bonding is highly constrained relative to that in the lanthanide complexes. 

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