Stabilization of Unstable Nonbenzenoids

Probably the most academically dramatic use of metal Tc-complex formation was demonstrated by the trapping of the cyclobutadiene-silver nitrite complex. Many subsequent cyclobutadiene derivatives were isolated and proved to be rather stable complexes. For example, tetraphenylcyclo-butadieneiron tricarbonyl melts without decomposition at 234° and tetra-phenylcyclobutadiene 7r-cyclopentadienyl cobalt melts at 256°C under nitrogen. Upon examination of the electronic configuration the stability of these complexes often can be predicted. 

Unsubstituted cyclobutadiene and benzocyclobutadiene also were trapped as the iron tricarbonyl 71-complexes. 

Attempts to liberate stable cyclobutadiene from its Tt-complexes have been unsuccessful. However, good evidence have been obtained for the 

Stabilization of cumulenes by 7i-complex formation with an iron tricarbonyl group serve as further examples  

Unsubstituted butatriene is unstable at 0°, but the complex decomposes only above 230° under nitrogen. Allyl and tropylium ions were also trapped as metal complexes ([6-50], and [6-51]). 0-Quinodimethene iron carbonyl complex also serves as the metal-stabilized active intermediate [6-52]. 

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