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 As₅ or Sb₅ ligands, however, have remained experimentally elusive, especially analogues of higher nuclearity. Here we report the synthesis and characterization of sandwich-type clusters [Pn@M₁₂Pn₁₀(Pn₅)₂]^4–/5– (Pn = As, Sb; M = Co, Fe), where the endohedral icosahedral cluster [Pn@M₁₂] can be regarded as a spherical-like three-dimensional coordination centre surrounded by a Pn₁₀ ring and two Pn₅ pentagonal caps. Quantum chemical calculations reveal that the [Pn@M₁₂] has orbitals that mimic 1S, 1P and 1D electronic shells, which can interact with the outer (Pn₅)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-As₅ orbitals interact with 1S and part of the 1P shell of the central core, while the π ₂- and π ₃-As₅ 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. [Figure not available: see fulltext.]