High and low barriers to haptotropic shifts across polycyclic surfaces: the relevance of aromatic character during the migration process

Haptotropic shifts of ML n fragments, such as (C 5 H 5 )Fe, Mn(CO) 3 or Cr(CO) 3 , across polycyclic carbon frameworks generally follow a ‘non least-motion’ trajectory. In the absence of a definitive and unifying rationale for the preference of migratory pathways documented in previous experimental studies, a theoretical explanation has been sought. The potential energy surfaces (PES) for the migrations of organometallic fragments over the surfaces of syn - and anti -dibenzpentalene ( 19 and 20 , respectively) were initially surveyed at the extended Hückel level; subsequently, the migration barriers were refined by DFT calculations. The results suggest that the development of aromatic character during the migration process can play an important role in determining the activation energy barrier. The capacity of the organic substrate to retain π-electron delocalization during the rearrangement adds a new dimension to the topological picture of metal–ligand interacting orbitals first established for bicyclic systems. .