Quantum chemical calculations of the CpMoE6MoCp (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 CpMoE6MoCp (E = P, As, Sb) sandwich, with the E-6 ring becoming planar (D-6h) upon complex formation. The AdNDP analysis revealed that bonding between C5H5- units and Mo atoms has a significant ionic contribution, while bonding between Mo atoms and E-6 fragment becomes appreciably covalent through the delta-type M -> L back-donation mechanism.