Carbon-free sandwich compounds based on arsenic and antimony with icosahedral metal cores

Traditionally, a metallocene complex comprises a metal centre sandwiched between two aromatic organic ligands and such complexes have been extensively investigated. Carbon-free analogues of metallocene with homoleptic As 5 or Sb 5 ligands, however, have remained experimentally elusive, especially analogues of higher nuclearity. Here we report the synthesis and characterization of sandwich-type clusters [Pn@M 12 Pn 10 (Pn 5 ) 2 ] 4–/5– (Pn = As, Sb; M = Co, Fe), where the endohedral icosahedral cluster [Pn@M 12 ] can be regarded as a spherical-like three-dimensional coordination centre surrounded by a Pn 10 ring and two Pn 5 pentagonal caps. Quantum chemical calculations reveal that the [Pn@M 12 ] has orbitals that mimic 1S, 1P and 1D electronic shells, which can interact with the outer (Pn 5 ) 2 ring layer. These interactions occur because the frontier orbitals are dominated by the ( n  − 1) d block orbitals of the metal atoms. The π 1 -As 5 orbitals interact with 1S and part of the 1P shell of the central core, while the π 2 - and π 3 -As 5 orbitals interact with the symmetry-allowed part of the 1P and 1D shells. The characteristics of this orbital interaction are analogous to those of mononuclear metallocenes.