Transition-Metal-Stabilised 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 heaviear group 14 elements (Si, Ge, Sn, and Pb) are hardly known. Here, we demonstrate the isolation of the first metal complexes featuring heavy cyclopen-tadienyl 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)] [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 eliminates Pb(0) below room temper-ature 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 analysed 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 which is distinct from conventional antiaromatic species.