Geometry and Electronic Structure of Titanabenzene and Its Valence Isomers

Potentially aromatic metallocycles are of interest due to their possible use as experimental catalytic materials and to their assistance in developing theoretical understanding of metal-carbon bonds. Ab initio analysis of titanabenzene and its valence isomers reveals the lack of aromatic stability for the planar, heterosubstituted benzene molecule. The planar molecule is in fact a transition state on the potential energy surface. Due to a pseudo-Jahn-Teller effect, the planar titanabenzene molecule is predicted to be less stable than the Dewar-titanabenzene molecule by 25.9 kcal/mol. However, the most stable structural isomer of HTiC₅H₅ is found to be the open-sandwich (η⁵-cyclopentadienyl)titanium(II) hydride structure that is a ground-state triplet and 50.3 kcal/mol lower in energy than Dewar-titanabenzene. Electronic structures and bonding within these valence isomers are presented from analysis of the population bond order density matrices based on the novel oriented valence quasi-atomic molecular orbitals (QUAOs) obtained from TCSCF wave functions.