text: Rapid condensation of arsenic vapors on to a cold surface results in the formation of yellow arsenic (As), consisting of four arsenic atoms arranged in a tetrahedral geometry analogous to white phosphorus. Though it is the only soluble form of arsenic known, yellow arsenic is metastable: at room temperature, or in the presence of light, the structure quickly decomposes to adopt the lower-energy configuration of gray arsenic. For this reason, extensive care is required to maintain yellow arsenic in a state suitable for reaction, including rigorous exclusion of light and maintenance of temperatures below −80 °C. Yellow arsenic is the allotrope most suited for reactivity studies, due to its solubility (low, but comparatively large relative to the metallic allotrope) and molecular nature. In comparison to its lighter congener, phosphorus, the reactivity of arsenic is relatively underexplored. Research investigating reactions with arsenic are primarily concerned with the activation of main group and transition metal compounds; in the case of transition metal complexes, As has demonstrated competent reactivity across the d-block of the periodic table.