Synthesis of triple-decker sandwich compounds featuring a M–M bond through cyclo-Bi5 and cyclo-Sb5 rings

Abstract The cyclopentadienyl anion is a π -aromatic five-membered ring ligand that is widely used in organometallic chemistry. By replacing the CH groups in cyclopentadiene with isoelectronic group-15 elements, an inorganic analogue can be obtained. In this line, Pn 5 (Pn = P, Sb) rings have been stabilized in a triple-decker sandwich structure, prepared via high-temperature reactions, and an example of a Bi 5 − ring stabilized in a cobalt-based inverse-sandwich-type complex has been reported. Here we report the synthesis and structural characterization of two complexes, [Cp–V(cyclo-Sb 5 )V–Cp] 2− and [Cp–Nb(cyclo-Bi 5 )Nb–Cp] 2− , which are stabilized by [K(18-crown-6)] + or [K(2.2.2-crypt)] + cations at room temperature under mild conditions. Our bonding analysis through various quantum-chemical methods reveals that V‒V and Nb‒Nb bonds pass through the centre of the E 5 rings (E = Sb, Nb). In contrast to free cyclo-E 5 (E = Sb, Bi) the cyclo-E 5 moieties between Cp–E units do not possess any aromatic character because the M‒M (M = V, Nb) bond passes through the centre of the ring.