Allotropes of arsenic

In particular, reaction of yellow arsenic with main group and transition metal elements results in compounds with wide-ranging structural motifs, with butterfly, sandwich and realgar-type moieties featuring most prominently.

[1] Relatively few in-situ reactions have been reported involving gray arsenic due to its low solubility, although it reacts in air to form gaseous As2O3 .

[3][4] In these reactions, cyclopentadienyl complexes of molybdenum, tungsten and chromium proceed via loss of carbon monoxide to react with gray arsenic and form mono-, di-, and triarsenic compounds.

Mechanical exfoliation of the mineral found in Chilean caves, arsenolamprite, revealed a molecular structure with high in-phase anisotropy and potential as a semiconducting material.

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.

This characteristic makes the CpPEt2As4 complex a uniquely suitable "storage" molecule for yellow arsenic, as it is stable when stored at room temperature in the dark, but can release As4 in thermal or photochemical solutions.

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 As4 moiety in η1:1-fashion.

[10][12] Investigation of the electron density topology in a phosphorus/arsenic/niobium-containing system demonstrated the unique η2-bonding configuration in these complexes, in which an arsenic-phosphorus double bond binds side-on to a niobium center.

Reactions of yellow arsenic with the group 6 transition metals largely proceed through thermolytic carbon monoxide elimination in chromium and molybdenum carbonyl complexes.

[14][13] These remarkable structures feature three planar-rings arranged in parallel fashion to result in an idealized D5h point group for the chromium complex.

Conversely, utilization of more sterically demanding ligands on the metal center enabled reactions in milder conditions with molybdenum and chromium.

[16] 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 As4.

Nickel tetrafluoroborate salts react analogously to cobalt complexes in the presence of triphos to form a sandwich structure with a central cyclic As3 moiety.

Molecular structures of arsenic allotropes. Top left: Gray (metallic) arsenic, rhombohedral structure. Bottom left: Black arsenic, orthorhombic structure. Right: Yellow arsenic, tetrahedral configuration. [ 1 ]
Image of gray, or metallic arsenic
Gray, or metallic arsenic, pictured under an argon atmosphere
Structure of gray arsenic, depicting rhombohedral structure of arsenic atoms
Molecular structure of gray arsenic
Reactions of gray arsenic, adapted from ref. 1 and detailed in refs. 3 and 4. Organometallic complexes of chromium, molybdenum and tungsten react with gray arsenic to form mono-, di- and triarsenic compounds.
Reactions of gray arsenic. Organometallic complexes of chromium, molybdenum and tungsten react with gray arsenic to form mono-, di- and triarsenic compounds. [ 1 ] [ 3 ] [ 4 ]
Molecular structure of black arsenic
Molecular structure of yellow arsenic
Selected reactions forming butterfly compounds of arsenic and main group elements. [ 8 ] [ 9 ] Cp PEt = C 5 (4-EtC 6 H 4 ) 5 ), Cp * =(η 1 -Me 5 C 5 )
Laplacian of electron density, representing the topology of electron density in a niobium/arsenic/phosphorus complex reported by Spinney et al. [ 10 ]
Chromium and molybdenum triple-decker "sandwich" arsenic complexes [ 13 ] [ 14 ]
Reactivity of yellow arsenic with iron complexes featuring bulky cyclopentadienyl moieties, resulting in the formation of butterfly complexes followed sequentially by an As 8 realgar-like central structure linking two iron fragments. [ 1 ] [ 17 ]