On the suppression mechanism of the pseudo-Jahn-Teller effect in middle E6 (E = P, As, Sb) rings of triple-decker sandwich complexes.

Quantum chemical calculations of the CpMoE(6)MoCp (E = P, As, Sb) triple-decker sandwich complexes showed that E(6) fragments in the central decks of the complexes are planar. Analysis of molecular orbitals involved in vibrational coupling demonstrated that filling the unoccupied molecular orbitals involved in vibronic coupling with electron pairs of Mo atoms suppresses the PJT effect in the CpMoE(6)MoCp (E = P, As, Sb) sandwich, with the E(6) ring becoming planar (D(6h)) upon complex formation. The AdNDP analysis revealed that bonding between C(5)H(5)(-) units and Mo atoms has a significant ionic contribution, while bonding between Mo atoms and E(6) fragment becomes appreciably covalent through the δ-type M → L back-donation mechanism.