Pushed to the limit: Pushing CC π bonds to the breaking point by using a push–push substitution pattern forces allenes to bend (see structure; C light blue, N dark blue). An acyclic allene featuring a CCC bond angle of 134.8° has been isolated in which the typically sp-hybridized central carbon atom approaches a configuration that has two lone pairs of electrons, and acts as a very strong η1-donor ligand for transition metals.
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