Synthesis and onwards coordination of an As(I) centered zwitterion.

The bis(phosphino)borate ligand class is used as an anionic anchor to stabilize reactive, low coordinate arsenic centers. The neutral, zwitterionic As(I) species, 2, is formed very cleanly, and isolated in good yields using cyclohexene as a halogen scavenger. The uniqueness of this heterocyclic As(I) compound is on display with the coordination to Group 6 metal centers, (2 M(CO)5; M = Cr, Mo, W). The arsenic-metal bond lengths are longer than the related AsPh3 complexes suggesting that compound 2 is a weak sigma donor. The metal complexes reveal a trigonal pyramidal arsenic atom, which provides the first experimental evidence for the presence of two "lone pairs" of electrons on the As(I) center. When more flexible and more electron-donating isopropyl substituents were used, an intermediate (compound 5) in the formation of low coordinate pnictogen compounds was crystallographically characterized. This structure, formally a base-stabilized dichloroarsenium cation, provides an alternative mechanistic proposal to the one described in the literature.