Striking Borane Planarization in the Thermal Rearrangement (η 5 -C 5 H 5 )Fe(η 3 -B 5 H 10 )→(η 5 -C 5 H 5 )Fe(η 5 -B 5 H 10 ).

In 1977 Weiss and Grimes, by means of mass spectrometry and 1 H and 11 B NMR spectroscopy, proposed two structures (I and II) for the ferraborane (η 5 -C 5 H 5 )Fe(B 5 H 10 ), 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 1 H and 11 B NMR chemical shifts. A striking result from this study is the planarization (3D→2D) of the B 5 H 10 - ligand - an unknown isolated anion, isoelectronic with aromatic cyclopentadienyl anion C 5 H 5 - - when attached to the (η 5 -C 5 H 5 )Fe + moiety, thus resulting in a more stable ferraborane isomer II.