The Proton Catalyzed Transformation of a Cobaltacyclopentadiene into a η4‐Cyclobutadiene Complex

H⊕ addition to the diene system of 1 gives the complex cation 2, isolated at − 30°C as its SbF or CF3CO salt. At − 20°C, 2 rearranges to give the unusual isomer 3, in which one phenyl ring is η2 coordinated. Apparently, intramolecular phosphane addition results in stabilization of the reactive cation 2.