An Acyclic Arsenium Cation Stabilised by a Single P–As π‐Interaction and a Cyclic Diphosphinophosphonium Salt

Stable acyclic arsenium cations R 2 As + , isoelectronic analogues of germylenes, are rare in comparison to the corresponding phosphenium cations. The first example of a diphosphaarsenium salt, [{(Dipp) 2 P} 2 As][Al{OC(CF 3 ) 3 } 4 ]⋅1 1 / 2  PhMe, is described. This salt exhibits remarkable stability due to the delocalisation of a lone pair from a planar phosphorus centre into the vacant p‐orbital at arsenic; the bonding in 2 has been probed by DFT calculations. An attempt to synthesise an analogous diphosphaphosphenium salt unexpectedly generated the cyclic phosphonium salt [ cyclo ‐{(Mes)P} 2 P(Mes) 2 ][BAr F 4 ]⋅CyMe through the cyclisation of a putative phosphine‐substituted diphosphene cation intermediate.