Reactions Involving Rearrangement of Ligands

Cyclic ligands may sometimes undergo ring expansion or contraction leading to the formation of arene complexes. Accordingly, the cation tricarbonyltropylium-chromium(I) reacts with sodium cyclopentadienide to give benzenetricarbonyl-chromium  

The contraction of the ring occurs also in the case of the following seven-membered heterocyclic compound  

Dibromomethylboron as well as BBr2Ph and BBr3 add to the cyclopentadienyl ring of cobaltocene to give the complex containing the heterocyclic six-membered ring  

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