Arsenic in the solid state can be found as gray, black, or yellow allotropes. These various forms feature diverse structural motifs, with yellow arsenic enabling the widest range of reactivity. 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.
Gray arsenic or metallic arsenic is the most stable allotrope of the element at room temperature, and as such is its most common form. [1] This soft, brittle allotrope of arsenic has a steel grey, metallic color, and is a good conductor. [2] The rhombohedral form of this allotrope is analogous to the phosphorus allotrope black phosphorus. In its α-form, As6 rings in chair confirmations are condensed into packed layers lying perpendicular to the crystallographic c axis. Within each layer, the vicinal As-As bond distances are 2.517 Å, while the layer-to-layer As-As bond distances are 3.120 Å. The overall structure displays a distorted octahedral geometry, resulting in the largely metallic properties of this allotrope. Upon sublimation at 616 °C, the gas phase arsenic molecules lose this packing arrangement and form small clusters of As4, As2, and As, though As4 is by far the most abundant in this phase. [1] If these vapors are condensed swiftly onto a cold surface (<200 K), solid yellow arsenic (As4) results due to the lack of energy required to form the rhombohedral gray arsenic lattice. Conversely, condensation of arsenic vapors onto a heated surface generates amorphous black arsenic. The crystalline form of black arsenic can also be isolated, and the amorphous form can be annealed to return to the metallic gray arsenic form. Yellow arsenic can also be returned to the gray allotrope in a facile manner through application of light or by returning the molecule to room temperature. [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 . Two examples of the reactivity of gray arsenic towards transition metals are known. [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.
Black, or amorphous arsenic (chemical formula Asn) is synthesized first through the sublimation of gray arsenic followed by condensation onto a heated surface. This structure is thought to be the arsenic analogue of red phosphorus. The structure of black arsenic in its crystalline phase, while not synthesized in its pure form, is by extension analogous to black phosphorus, and takes on an orthorhombic structure built from As6 rings. Black arsenic has as-yet been synthesized only in the presence of atomic impurities including mercury, [5] phosphorus, and oxygen, though a pure form of black arsenic was found in the Copiapó region of Chile. 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. [6]
Rapid condensation of arsenic vapors on to a cold surface results in the formation of yellow arsenic (As4), 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. [1] 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, As4 has demonstrated competent reactivity across the d-block of the periodic table.
The first activation of a main group compound by yellow arsenic was reported in 1992 by West and coworkers, involving the reaction of As4 with a disilene compound, tetramesityldisilene, to generate a mixture of compounds including a butterfly structural motif of bridging arsenic atoms. [7] Notably, the product mixture obtained in this reaction differs from the analogous reaction with P4 that produces the butterfly compound alone, highlighting that the reactivity of yellow arsenic and white phosphorus cannot be considered identical. The first organo-substituted As4 compound was produced by Scheer and coworkers in 2016 via reaction with the CpPEt radical. [8] Analogous to the butterfly compound obtained by the West group, the product obtained in this reaction featured a bridging As4 motif that reversibly returned As4 and the parent radical in the presence of light or heat. 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.
Other reactions of main group compounds with yellow arsenic have been shown to involve units of arsenic with more than four atoms. In reaction with the silylene compound [PhC(NtBu)2SiN(SiMe3)2], an aggregation of As4 was observed to form a cage compound of ten arsenic atoms, including a seven-membered arsenic ring at its center. [9]
Among the early (group 4 and 5) transition metal elements, few examples of arsenic activation has been reported to date. 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. [11] Trace amounts of a zirconium dimer bridged by a (μ,η2:2:1-As5)-moiety were also reported in this study, which described the complexes as possible reagents for As4 transfer. In group 5, arsenic activation has been more widely explored, with complexes of both niobium and tantalum known. [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. Notable examples include the formation of triple-decker complexes [(CpRMo)2(μ,η6-As6)] and [{CpRCr}2(μ,η5-As5)] via reaction of the corresponding molybdenum and chromium dimers with yellow arsenic. [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. Both of these reactions necessitate harsh reaction conditions like boiling xylene to overcome the high barriers to activation of As4. Conversely, utilization of more sterically demanding ligands on the metal center enabled reactions in milder conditions with molybdenum and chromium. Cummins' Mo(N(tBu)Ar)3 catalyst, also known to split the N-N triple bond in dinitrogen, reacts with yellow arsenic to form a terminal arsenic moiety triple-bonded to the metal center - one of only several compounds known to contain a terminal arsenic atom. [15] Complexes with metal-metal multiple bonds also enable mild As4 activation parameter. A chromium-chromium quintuply-bonded species reported by Kempe reacts with yellow arsenic to form a crown complex in which the four arsenic atoms form an approximately tetrahedral structure, with each chromium atom bonding to three arsenic atoms. [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. Much like the chromium and molybdenum sandwich complexes, (CpRFe(CO)2]2 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 ([CpRFe(μ5-As5)]). [18] This intermediate, isolobal to ferrocene, replaces one of the cyclopentadienyl ligands with a cyclic As5 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 As4 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 As8 moiety. This ligand, formally tetraanionic, forms an eight-membered ring bridging four iron atoms in total. [17]
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 As3 moiety bridging two cobalt centers, of which the former is assigned formally as a 3π-electron system. [19] The reaction of [Cp*Co(CO)]2 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. [20] Analogous reactions with rhodium complexes are also known. [21]
Among the group 10 and 11 elements, nickel and copper feature most prominently in literature reactions with yellow arsenic. Nickel tetrafluoroborate salts react analogously to cobalt complexes in the presence of triphos to form a sandwich structure with a central cyclic As3 moiety. Much like iron, the reaction of nickel cyclopentadienyl carbonyl complexes with As4 yields a variety of bi- and multi-metallic products depending on the size of the attending ligands, though the nature and geometric structure of these compounds differ from those observed with iron. [19] These include trimers with bridging As4 and As5 moieties in cubane structural arrangements when smaller Cp ligands are employed, and distorted hexagonal prism complexes with two nickel fragments and four arsenic atoms when bulkier Cp groups are introduced.
The reaction of the copper complex [L2Cu(NCMe)] (L2 = [{N(C6H3iPr2-2,6)C(Me)}2CH]) with yellow arsenic yields the As4-bridged dimer [{L2Cu}2- (μ,η2:2-As4)]. [22] The four-atom arsenic moiety in this complex was deemed to be "intact" yellow arsenic through the use of density functional theory calculations determining the change in bond critical points between the free and bound arsenic molecules. Specifically, only a small shift was observed in the bond critical points between arsenic atoms involved in binding to copper; the remaining bond critical points were very similar to free yellow arsenic.
In chemistry, a phosphaalkyne is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general formula R-C≡P. Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne (H-C≡P) has been observed in the interstellar medium.
Phosphinidenes are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The "free" form of these compounds is conventionally described as having a singly-coordinated phosphorus atom containing only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties. In the last few decades, several strategies have been employed to stabilize phosphinidenes, and researchers have developed a number of reagents and systems that can generate and transfer phosphinidenes as reactive intermediates in the synthesis of various organophosphorus compounds.
Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation C5H+5(Cp+) and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles.
A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.
The phosphaethynolate anion, also referred to as PCO, is the phosphorus-containing analogue of the cyanate anion with the chemical formula [PCO]− or [OCP]−. The anion has a linear geometry and is commonly isolated as a salt. When used as a ligand, the phosphaethynolate anion is ambidentate in nature meaning it forms complexes by coordinating via either the phosphorus or oxygen atoms. This versatile character of the anion has allowed it to be incorporated into many transition metal and actinide complexes but now the focus of the research around phosphaethynolate has turned to utilising the anion as a synthetic building block to organophosphanes.
In chemistry, aluminium(I) refers to monovalent aluminium (+1 oxidation state) in both ionic and covalent bonds. Along with aluminium(II), it is an extremely unstable form of aluminium.
Nontrigonal pnictogen compounds refer to tricoordinate trivalent pnictogen compounds that are not of typical trigonal pyramidal molecular geometry. By virtue of their geometric constraint, these compounds exhibit distinct electronic structures and reactivities, which bestow on them potential to provide unique nonmetal platforms for bond cleavage reactions.
Hexaphosphabenzene is a valence isoelectronic analogue of benzene and is expected to have a similar planar structure due to resonance stabilization. Although several other allotropes of phosphorus are stable, no evidence for the existence of P6 has been reported. Preliminary ab initio calculations on the trimerisation of P2 leading to the formation of the cyclic P6 were performed, and it was predicted that hexaphosphabenzene would decompose to free P2 with an energy barrier of 13−15.4 kcal mol−1, and would therefore not be observed in the uncomplexed state under normal experimental conditions. The presence of an added solvent, such as ethanol, might lead to the formation of intermolecular hydrogen bonds which may block the destabilizing interaction between phosphorus lone pairs and consequently stabilize P6. The moderate barrier suggests that hexaphosphabenzene could be synthesized from a [2+2+2] cycloaddition of three P2 molecules. Currently, this is a synthetic endeavour which remains to be conquered.
Aluminium(I) nucleophiles are a group of inorganic and organometallic nucleophilic compounds containing at least one aluminium metal center in the +1 oxidation state with a lone pair of electrons strongly localized on the aluminium(I) center.
Gallium monoiodide is an inorganic gallium compound with the formula GaI or Ga4I4. It is a pale green solid and mixed valent gallium compound, which can contain gallium in the 0, +1, +2, and +3 oxidation states. It is used as a pathway for many gallium-based products. Unlike the gallium(I) halides first crystallographically characterized, gallium monoiodide has a more facile synthesis allowing a synthetic route to many low-valent gallium compounds.
A Fischer carbene is a type of transition metal carbene complex, which is an organometallic compound containing a divalent organic ligand. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-backdonation from the metal centre is generally weak, the carbene carbon is electrophilic.
Phosphanides are chemicals containing the [PH2]− anion. This is also known as the phosphino anion or phosphido ligand. The IUPAC name can also be dihydridophosphate(1−).
Polyfluoroalkoxyaluminates (PFAA) are weakly coordinating anions many of which are of the form [Al(ORF)4]−. Most PFAA's possesses an Al(III) center coordinated by four −ORF (RF = -CPh(CF3)2 (hfpp), -CH(CF3)2 (hfip), -C(CH3)(CF3)2 (hftb), -C(CF3)3 (pftb)) ligands, giving the anion an overall -1 charge. The most weakly coordinating PFAA is an aluminate dimer, [F{Al(Opftb)3}2]−, which possess a bridging fluoride between two Al(III) centers. The first PFAA, [Al(Ohfpp)4]−, was synthesized in 1996 by Steven Strauss, and several other analogs have since been synthesized, including [Al(Ohfip)4]−, [Al(Ohftb)4]−, and [Al(Opftb)4]− by Ingo Krossing in 2001. These chemically inert and very weakly coordinating ions have been used to stabilize unusual cations, isolate reactive species, and synthesize strong Brønsted acids.
Cobalt compounds are chemical compounds formed by cobalt with other elements.
Carbones are a class of molecules containing a carbon atom in the 1D excited state with a formal oxidation state of zero where all four valence electrons exist as unbonded lone pairs. These carbon-based compounds are of the formula CL2 where L is a strongly σ-donating ligand, typically a phosphine (carbodiphosphoranes) or a N-heterocyclic carbene/NHC (carbodicarbenes), that stabilises the central carbon atom through donor-acceptor bonds. Carbones possess high-energy orbitals with both σ- and π-symmetry, making them strong Lewis bases and strong π-backdonor substituents. Carbones possess high proton affinities and are strong nucleophiles which allows them to function as ligands in a variety of main group and transition metal complexes. Carbone-coordinated elements also exhibit a variety of different reactivities and catalyse various organic and main group reactions.
1,3-Diphospha-2,4-diboretanes, or B2P2, is a class of 4-member cyclic compounds of alternating boron and phosphorus atoms. They are often found as dimers during the synthesis of boraphosphenes (RB=PR'). Compounds can exhibit localized singlet diradical character (diradicaloid) between the boron atoms in the solution and solid state.
Stable and persistent phosphorus radicals are phosphorus-centred radicals that are isolable and can exist for at least short periods of time. Radicals consisting of main group elements are often very reactive and undergo uncontrollable reactions, notably dimerization and polymerization. The common strategies for stabilising these phosphorus radicals usually include the delocalisation of the unpaired electron over a pi system or nearby electronegative atoms, and kinetic stabilisation with bulky ligands. Stable and persistent phosphorus radicals can be classified into three categories: neutral, cationic, and anionic radicals. Each of these classes involve various sub-classes, with neutral phosphorus radicals being the most extensively studied. Phosphorus exists as one isotope 31P (I = 1/2) with large hyperfine couplings relative to other spin active nuclei, making phosphorus radicals particularly attractive for spin-labelling experiments.
Pnictogen-substituted tetrahedranes are pnictogen-containing analogues of tetrahedranes with the formula RxCxPn4-x. Computational work has indicated that the incorporation of pnictogens to the tetrahedral core alleviates the ring strain of tetrahedrane. Although theoretical work on pnictogen-substituted tetrahedranes has existed for decades, only the phosphorus-containing species have been synthesized. These species exhibit novel reactivities, most often through ring-opening and polymerization pathways. Phosphatetrahedranes are of interest as new retrons for organophosphorus chemistry. Their strain also make them of interest in the development of energy-dense compounds.
Aluminylenes are a sub-class of aluminium(I) compounds that feature singly-coordinated aluminium atoms with a lone pair of electrons. As aluminylenes exhibit two unoccupied orbitals, they are not strictly aluminium analogues of carbenes until stabilized by a Lewis base to form aluminium(I) nucleophiles. The lone pair and two empty orbitals on the aluminium allow for ambiphilic bonding where the aluminylene can act as both an electrophile and a nucleophile. Aluminylenes have also been reported under the names alumylenes and alanediyl.
The stabilization of bismuth's +3 oxidation state due to the inert pair effect yields a plethora of organometallic bismuth-transition metal compounds and clusters with interesting electronics and 3D structures.