text: The metals of groups 8 and 9 feature the most extensive library of reactivity with yellow arsenic documented in the scientific literature, with particular focus on reactions of iron and cobalt complexes with As. Much like the chromium and molybdenum sandwich complexes, (CpFe(CO)] complexes of iron react with yellow arsenic to produce analogous bimetallic products featuring "triple-decker" geometry. These reports also detail the isolation of a key intermediate, pentaarsaferrocene ([CpFe(μ-As)]). This intermediate, isolobal to ferrocene, replaces one of the cyclopentadienyl ligands with a cyclic As ligand that features As-As bond lengths of 2.312 Å (in line with delocalized As-As double bonds). This "sandwich-forming" reactivity can be meaningfully tuned by introducing bulkier ligands. Modifying the cyclopentadienyl groups with much bulkier derivatives produces a vastly different set of products. First, a butterfly complex with a central As unit is formed. Irradiation with light leads to further CO elimination and the formation of a bridged butterfly complex, which then rearranges into a unique complex featuring a central As moiety. This ligand, formally tetraanionic, forms an eight-membered ring bridging four iron atoms in total.
  Much of the same reactivity, including formation of butterfly and sandwich compounds, has been described for cobalt complexes in the presence of yellow arsenic. Beyond these compounds, the history of reactivity of cobalt and yellow arsenic dates back to 1978, when Sacconi and coworkers reported the reaction of cobalt tetrafluoroborate and yellow arsenic in the presence of 1,1,1-tris(diphenylphosphinomethyl)ethane. The resulting complex features a cyclic As moiety bridging two cobalt centers, of which the former is assigned formally as a 3π-electron system. The reaction of [Cp*Co(CO)] dimer with yellow arsenic was shown by Scherer et al. to produce a wide variety of isolable products, featuring a mixture of linking arsenic moieties including cyclobutane-like and butterfly type complexes. Analogous reactions with rhodium complexes are also known.