X-ray structural and Mössbauer spectroscopic investigation of terminal ethynylmetallocenium salts

Oxidation of ethynyloctamethylferrocene (OMFA), [201472-67-5], with ferrocenium hexafluorophosphate in acetonitrile afforded previously unreported ethynyloctamethylferrocenium hexafluorophosphate salt 1 (OMFA+PF6−). Temperature-dependent 57Fe Mössbauer effect studies of 1 and neutral OMFA have shown a marked difference of behavior in the motion of the metal atom, which can be associated with the onset of ring rotation/libration in the latter at ∼248 K, but which is strongly inhibited in 1. Supplementary X-ray structure determinations of 1 and ethynylcobaltocenium hexafluorophosphate (CA+PF6−), [131276-87-4], a close structural model for the unstable, oxidized form of ferrocenylacetylene (FA+), have been performed. The most remarkable structural feature of this stable salt of oxidized cobaltocenylacetylene (CA) is its crystal constitution: alternating different cation layers are separated by identical layers of PF6− counter ions, thus exhibiting a unique type of lattice internal polymorphism.