Striking Borane Planarization in the Thermal Rearrangement (η5-C5H5)Fe(η3-B5H10) → (η5-C5H5)Fe(η5-B5H10) §.

In 1977 Weiss and Grimes, by means of mass spectrometry and 1H and 11B NMR spectroscopy, proposed two structures (I and II) for the ferraborane (η5-C5H5)Fe(B5H10), isoelectronic with ferrocene. In this work, by means of high-level quantum-chemical computations, we confirm the experimental structures of the two isomers with their corresponding energies, and assign the reported 1H and 11B NMR chemical shifts. A striking result from this study is the planarization (3D → 2D) of the B5H10- ligand - an unknown isolated anion, isoelectronic with aromatic cyclopentadienyl anion C5H5- - when attached to the (η5-C5H5)Fe+ moiety, thus resulting in a more stable ferraborane isomer II.