Molecular Polyarsenides of the Rare‐Earth Elements

Reduction of [Cp*Fe(η 5 ‐As 5 )] with [Cp′′ 2 Sm(thf)] (Cp′′=η 5 ‐1,3‐( t Bu) 2 C 5 H 3 ) under various conditions led to [(Cp′′ 2 Sm)(μ,η 4 :η 4 ‐As 4 )(Cp*Fe)] and [(Cp′′ 2 Sm) 2 As 7 (Cp*Fe)]. Both compounds are the first polyarsenides of the rare‐earth metals. [(Cp′′ 2 Sm)(μ,η 4 :η 4 ‐As 4 )(Cp*Fe)] is also the first d/f‐triple decker sandwich complex with a purely inorganic planar middle deck. The central As 4 2− unit is isolobal with the 6π‐aromatic cyclobutadiene dianion (CH) 4 2− . [(Cp′′ 2 Sm) 2 As 7 (Cp*Fe)] contains an As 7 3− cage, which has a norbornadiene‐like structure with two short As−As bonds in the scaffold. DFT calculations confirm all the structural observations. The As−As bond order inside the cyclo As 4 ligand in [(Cp′′ 2 Sm)(μ,η 4 :η 4 ‐As 4 )(Cp*Fe)] was estimated to be in between an As−As single bond and a formally aromatic As 4 2− system.