Mellow yellow

Angew.Chem.Int.Ed.http://doi.org/fz9mfg(2012)

Arsenic has three main allotropes: the grey one whose atoms adopt a layered arrangement, the yellow one that consists of As₄ tetrahedra, and the black amorphous form. Yellow arsenic is the most toxic and the most unstable; it decomposes quite readily, especially when exposed to light. It resembles the white allotrope of the lighter pnictide phosphorus — also a toxic and unstable tetrahedral species — but unlike P it can't be stored in solution owing to its greater instability. It is still possible to use As₄ in chemistry by freshly preparing it from the grey allotrope — but its light-sensitivity makes working with it particularly challenging.

For these reasons, there has been limited research on the reactivity, properties, and potential uses of yellow arsenic. Now, a team of scientists in Germany led by Manfred Scheer at the University of Regensburg have prepared a relatively stable silver complex featuring two As₄ ligands. The complex can be stored at −30 °C without decomposing and can be used as an As₄ transfer agent (pictured). Reaction of a silver(I) salt ([Ag(I)(CH₂Cl₂)](pftb), where pftb is perfluoro-tert-butoxy-aluminate, Al{OC(CF₃)₃}₄⁻) with freshly prepared yellow arsenic produced the salt [Ag(I)(As₄)₂](pftb). The compound, which is stable in light, was characterized by electrospray ionization mass spectrometry, Raman spectroscopy and X-ray single-crystal diffraction; theoretical calculations also provided further insight into its structure. In the same way as As₄ resembles P₄, [Ag(As₄)₂]⁺ exhibits a linear structure and an electron density that are similar to those of its phosphorus counterpart.

Credit: © 2012 WILEY

The two As₄ ligands are attached to the silver centre through side-on coordination. Despite some slight distortions caused by their interaction with the silver atom, both As₄ tetrahedra remain essentially intact, which means they are only weakly coordinated. This weak interaction enables [Ag(As₄)₂]⁺ to act as a source of As₄ — something that is impractical with As₄ in its free form. On reaction of [Ag(I)(CH₂Cl₂)](pftb) with the gold complex (PPh₃)AuCl in stoichiometric conditions, one As₄ ligand was transferred to the gold centre to form [(PPh₃)AuAs₄]⁺, also obtained as a salt with pftb.