Delocalized 7i-electron system

The dication of 5-indacene (19), however, provided a 10 7i-electron periphery free optimization gave Z>2h symmetry and bond orders and charges as shown. A quite different electronic structure has been obtained in this case from that in the dianion. Whereas the latter has a peripheral delocalized 7i-electron system, the cation appears to have a 6 n-electron system in the central ring with allyllic 2 7i-electron systems in the outer rings. Charge is largely concentrated on the end atoms of the allyl systems however, there is little interaction between the three delocalized systems <84T4455>. 

With their delocalized 7i-electron system, allyl ligands can bond to metals in a manner similar to olefins. Allyl complexes have been detected as intermediates in catalytic processes involving propene or higher olefins and dienes. Examples include the cyclooligomerization of butadiene and the codimerization of butadiene with ethylene. 

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