Among the early (group 4 and 5) transition metal elements, few examples of arsenic activation has been reported to date. Carbon monoxide complexes of zirconium with derivatized cyclopentadienyl ligands were shown to react with yellow arsenic in boiling xylene to release CO and bind the As moiety in η-fashion. Trace amounts of a zirconium dimer bridged by a (μ,η-As)-moiety were also reported in this study, which described the complexes as possible reagents for As transfer. In group 5, arsenic activation has been more widely explored, with complexes of both niobium and tantalum known. Investigation of the electron density topology in a phosphorus/arsenic/niobium-containing system demonstrated the unique η-bonding configuration in these complexes, in which an arsenic-phosphorus double bond binds side-on to a niobium center.