Transition-Metal-Stabilized Heavy Tetraphospholide Anions

Phosphorus analogues of the ubiquitous cyclopentadienyl (Cp) are a rich and diverse family of compounds, which have found widespread use as ligands in organometallic complexes. By contrast, phospholes incorporating heavier group 14 elements (Si, Ge, Sn, and Pb) are hardly known. Here, we demonstrate the isolation of the first metal complexes featuring heavy cyclopentadienyl anions SnP₄^2- and PbP₄^2-. The complexes [(η⁴-tBu₂C₂P₂)₂Co₂(μ,η⁵:η⁵-P₄Tt)] [Tt = Sn (6), Pb (7)] are formed by reaction of white phosphorus (P₄) with cyclooctadiene cobalt complexes [Ar'TtCo(η⁴-P₂C₂tBu₂)(η⁴-COD)] [Tt = Sn (2), Pb (3), Ar' = C₆H₃-2,6{C₆H₃-2,6-iPr₂}₂, COD = cycloocta-1,5-diene] and Tt{Co(η⁴-P₂C₂tBu₂)(COD)}₂ [Tt = Sn (4), Pb (5)]. While the SnP₄^2- complex 6 was isolated as a pure and stable compound, compound 7 eliminated Pb(0) below room temperature to afford [(η⁴-tBu₂C₂P₂)₂Co₂(μ,η⁴:η⁴-P₄) (8), which is a rare example of a tripledecker complex with a P₄^2- middle deck. The electronic structures of 6-8 are analyzed using theoretical methods including an analysis of intrinsic bond orbitals and magnetic response theory. Thereby, the aromatic nature of P₅^- and SnP₄^2- was confirmed, while for P₄^2-, a specific type of symmetry-induced weak paramagnetism was found that is distinct from conventional antiaromatic species.