Shivram S. Garje1 and Vimal K. Jain*2

1 Department of Chemistry, University of Pune, Pune - 411 007, 2 Chemistry Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India

Abstract The chemistry of organo-arsenic, -antimony and -bismuth compounds has been reviewed. General features of 15 elements and the stereochemistries adopted by these elements are discussed briefly. This review covers the chemistry of tri- and pentavalent organometallic compounds of arsenic, antimony and bismuth as as -metal and multiply bonded, heterocyclic and coordination compounds. Pharmaceuticals and environmental aspects of these compounds have also been discussed. Emerging trends in the field have been brought out in the end.

Introduction The field of organoarsenic chemistry has a long history that started as early as 1760 when Cadet de

Glaussicourt isolated tetramethyldiarsine (Me2As-AsMe2), generally known as Cacodyl, in 1760 [1]. It took about a century to establish the composition of Cadet's compound. Trimethylantimony [2] and triethylbismuth [3] were the first organometallics of antimony and bismuth reported by Löwig and Schweizer in 1850. Considerable attention was paid when the potential of organoarsenicals such as 3,3'-diamino-4,4'- dihydroxy arsinobenzene (Salvarsan base) as therapeutic agent against the treatment of was reported by in 1910 [3a], As a natural extension to this development, compounds of higher homologues (Sb and Bi) were also investigated. However, with the discovery of Penicillin as an effective cure for syphilis in 1943 and the inhibition of organoarsenicals in war, interest in these compounds declined for quite some time. During this , the progress in this field was rather slow and steady but gained momentum only in the last 15 years or so when their potential as MOCVD precursors in material science was realized. Besides these developments, compounds of these elements have biocidal properties and find extensive applications in organic synthesis [4] and industry. Apart from their diverse applications, the area of basic research has also made great stride in recent years due to their interesting structural features and synthetic challenges [5-7], This review intends to bring out some salient features of the chemistry of organoarsenic, antimony and bismuth compounds and to find out emerging trends in this field. The review is an overview of the field and is by no means comprehensive. References to the more detailed review articles which cover specific aspects are given in the text.

General features of group '15' elements , , arsenic, antimony and bismuth constitute the 'group 15' of the . These members are sometimes referred as "pnictides", but this name is not widely used since it is not 2 approved by IUPAC. They have the following general outer electronic configuration: ns , ηρΛ ηpy\ ηpz\ Some of the physical properties of these elements are given in Table 1. Although, there is a progressive variation in physical properties from nitrogen to bismuth, some properties such as bond energies, , etc. vary irregularly. Nitrogen differs considerably from the remaining group 15 elements in that (i) it can form strong pK-p„ multiple bonds, and (ii) its inability to increase coordination number beyond four. Other group 15 elements may have coordination number five or six by employing one or two outer d orbitals in bonding. On descending the group, there is a gradual increase in the metallic character. A slow increase in the electropositive character and a gradual decrease in the ionization potential from phosphorus to bismuth to the formation of bismuth cation, Bi'+.

All the group 15 elements form trivalent compounds of the types R3M, R2MX, RMX2 and MX3 (R = organic group and X = inorganic lieand) with an inert pair of . Nitrogen and phosphorus compounds are generally tetrahedral (sp hybridization) with a stereochemically active pair usually referred as "". In the case of bismuth compounds, the lone pair is stereochemically inactive and remains almost always in the 6s orbital. It is usually called as "inert pair". Compounds of arsenic and antimony can adopt either of these configurations [8].

45 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview

These elements exhibit a higher valency of five. However, arsenic(V) and bismuth(V) compounds are less common as compared to phosphorus(V) and antimony(V) compounds. This is attributed to the effect of electron penetration [9], As compared to 3s electrons of phosphorus (where 3d shell is empty), the 4s electrons of arsenic penetrate within the 3d10 shell and are held more firmly. Similarly, the 6s electrons in bismuth penetrate within the 4/4 shell and are held firmly compared to the 5s electrons of antimony, where the 4/ shell is empty. Pentavalent compounds usually show trigonal bipyramidal geometry with sp3d hybridization. In contrast, pentaphenylantimony has a square pyramidal structure [10]. Some typical coordination geometries exhibited by the group 15 organometallics are shown in Scheme 1.

Trivalent organo-arsenic, -antimony and -bismuth compounds

Relatively few primary (RMH:) and secondary (R:MH) have been reported possibly due to their high , high volatility and tendency to decompose. The number of hydrides known for these follow a trend, As > Sb » Bi. These hydrides can be prepared in several ways; reduction being the most common method [11-13], Thus, reduction of arsonic, arsinic and their salts with a variety of reducing agents such as dust with hydrochloric [11], amalgamated zinc [12] readily afford primary

and secondary . Halo-arsines and - can be reduced readily by LiAlH4 or Li/NaBH4 [13].

Table 1. Some selected physical properties of group 15 elements

Physical property Ν Ρ As Sb Bi 7 15 33 51 83 Atomic weight 14 30.97 74.92 121.75 208.98 4 2 Electronic configuration [He]2sV [Ne]3sV [Ar]3i/° [Ktw; [Xe]4/ 5i/°6s 4s2 4p3 5s 5ρ 6p3 1st Ionization 14.5 11.0 9.8 8.6 7.3 potential 3.0 2.1 (2.18)* 2.0 (2.19)* 1.9 (2.06)* 1.9 (2.14)* (Pauling) Radii: Ionic 1.71 (N"3) 2.12 2.22 2.45 (Sb"3) 1.08 (Bi+3) (Ρ"3) (As"3) 0.92 (Sb+3) Covalent 0.70 1.10 1.20 1.36 1.46 (for trivalent state) (°C) -196 280 633 (s) 1380 1560 (°C) -210.1 44.1 (wh.) 817 (gr., 36 630.5 (gr) 271.3 590 (r) atm.) (g/cmJ) 0.81 1.82 (wh.) 5.73 (gr.) 6.67 9.80 Oxidation states -3 to +5 ** 3, 5 3, 5 3, 5 3, 5 Abudance in 's crust 19 1120 1.8 0.20 0.008 by weight (in ppm) Where wh. = white, r = red, gr. = grey, s = sublimes, atm. = atmospheric pressure; *= recent values computed by Batsanov; ** = e.g. -Ill in NH3, -II in NH2NH2, -I in NH2OH, 0 in N2, +1 in N20 (nitrous ), +11 in NO (), +111 in HN02 (nitrous acid), +IV in N02 (), +V in HN03 ().

A very large number of tertiary -arsines, -stibines and -bismuthines have been synthesized and several of them have been fully characterized [14, 15], The most common method for synthesizing these derivatives is the reaction of the Grignard reagent [16] with MX3 (eqn. 1). However, when the Grignard reagent is difficult to obtain, organolithium reagents are employed [17], Apart from these reagents, a number of other transmetallation reactions have been employed to prepare R3M. The reagents which have been used in transmetallation include organo- [18], -zinc [19], -cadmium, [20], -mercury [21] and - [22] compounds. The Wurtz-Fittig reaction is seldomly used to prepare alkyl derivatives, however, it has been utilized to prepare aryl or cyclic derivatives [23] (eqn. 2).

3RMgX + MXj f R3M + 3MgX2 (1)

46 Shivram S. Garje and Vimal K. Jain Main Group Metal Chemistry

M(lll) Miio^. (Pk

Pyramidal Tetrahedral Trigonal bipyramidal 2 3 sp SP3 sp dz2

e.g. RQM R3M-^M Ph2Bi(S2COPr')


Square pyramidal Octahedral Pentagonal bipyramidal 3 3 3 sp dx2.y2 spV sp d

e.g. PhSb(S2PPh2)2 Sb[S2P(OEt)2]3 Bi(S2COPr')3

M(IV) yM lllO^..

Tetrahedral Trigonal bipyramidal Square pyramidal 3 3 3 sp sp dz2 sp dx2.y2

+ e.g. [R4M] X" R5M, R3MX2 Ph5Sb


Octahedral spV

[SbPh6]", [SbMe4F]n

Scheme 1

47 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview

,CH2Br MeAsCU sMe (2) 4 Na CH2Br Most of the trialkyl derivatives are volatile liquids with disagreable odours and are air sensitive. The lower members are spontaneously flammable in air. On the other hand, the triaryl derivatives are and are stable in air. Tertiary-arsines, -stibines and -bismuthines, R3M all have pyramidal structure with the average bond angle (CMC) becoming smaller from arsenic to bismuth. For example, ZCMC in (p- CICsH^M is 104°, 97°, 93° for Μ = As, Sb, Bi, respectively [24]. These molecules have high inversion barriers (~ 200 KJmor1). Chiral arsines RR'R"As have been isolated using both chemical and biological methods [25], In recent years, several of the alkyl derivatives have found wide applications as precursors in the formation of compound such as GaAs, InSb, etc.[26-29].

Mono- and di-organometal halides of arsenic, antimony and bismuth R3.nMX,, (η = 1 or 2; Μ = As, Sb or Bi) are well known [1], They can be prepared in several ways and each method of preparation is rather specific to the metal. Reduction of arsenic and by dioxide in the presence of iodide ion in HCl or HBr has been conveniently used to prepare the haloarsines [30], A Friedel-Crafts type of reaction between and diarylamines or diarylethers is a good method for the preparation of certain heterocyclic chloroarsines [31]. Thermal decomposition ofR3SbX2 and R2SbX3 is commonly employed for the preparation of organohalostibines. It has been observed that compounds with larger organic groups tend to have lower decomposition temperatures. Thermal stability appears to decrease in the order CI > Br > I. Recently, this method has been employed to prepare sterically demanding halostibines (eqn. 3) [32], Another reaction to prepare organometal(IIl) halides is a redistribution reaction between R3M and MX3 in appropriate stoichiometry (eqns. 4, 5) [33-36], Alkylation or arylation of MX3 with Grignard reagent or organolithium is seldom used to prepare organometal(lll) halides, R3.„MXn due to the difficulties in controlling the reaction. However, a number of less reactive organometallics such as R4Sn [36, 37], RtPb [38], R2Zn [39], R2Cd [40] have been used for this purpose. Δ

R3SbX2 • R2SbX + RX (3)

(R = neopentyl or Me3SiCH2; X = Br, Δ ~ 250°C; X = I, Δ~ 170°C)

2MR3 + MC13 • 3R2MC1 (4)

MRJ + 2MC13 3RMC12 (5) (M = As, Sb or Bi)

Haloarsines are pyramidal molecules, whereas the antimony and bismuth derivatives are associated in the state. Diphenylantimony fluoride is a with fluorine bridges and has a pseudo trigonal bipyramidal configuration around antimony. MeSbI2 (I) is a linear chain with a distorted octahedral configuration around antimony [41], Me Me λ


Organometal(III) halides are reactive compounds which readily undergo oxidation and hydrolysis and exhibit varying degree of thermal and photolytic instability. The hydrolysis of dihaloarsines yields either the arsonous acids, RAs(OH)2 or their anhydrides (RAsO)„ [42] whereas, the hydrolysis of monohalides yields (R2M)20 [43]. The organometal(lll) halides are useful starting materials for the synthesis of a wide variety of organometal(III) derivatives. They react with various nuclophiles such as RM, RO, RS", 2 S \ RC02", CN, R2N", SCN", OCN", NJ", etc. Thus, alkoxides [44-46], carboxylates [47, 48], amides [49, 50], azides [51], pseudohalides [51], thiolates [52], organochalcogenides [53], dithiocarbamate and xanthate [54-56] derivatives have been prepared and characterized.

48 Shivram S. Garje and Vimal K. Jain Main Group Metal Chemistry

Triorgano-arsenic, -antimony and -bismuth compounds act as Lewis bases or donors (soft or class b donors). Thus, a myriad of transition metal complexes have been synthesized and many of them have been fully characterized [57], The alkyl derivatives are stronger donors than the aryl compounds. The ability of the ligands decreases in the order As > Sb > Bi. The poor donor ability of bismuthines is largly due to predominating V character of the lone pair in these compounds. Therefore, relatively few coordination complexes of bismuth have been reported thus far. Examples include [Ni(CO)3(BiR3)] (R = Et [58] or Bu' [59]), [Mo(CO)4(BiEt3)2] [58].

Although R3M usually acts as donors, however, when R is a strong electron withdrawing group, such molecules behave as strong Lewis acids. Thus, complexes of M(CF3)3 (M = As or Sb) with pyridine have been isolated [60]. A progressive replacement of organic group by halides in R3M leads to stronger Lewis acidity which decreases in the order Bi > Sb > As. For example, bismuth forms a complex of the type

[PhBiBr2.2Py] with pyridine [61]. MeSbh (I) [41] and PhSbX2 (X = Br or I) [62] are associated by Sb-X intermolecular contacts.

Pentavalent organo-arsenic, -antimony and -bismuth compounds The of pentavalent arsenic [63], antimony [64] and bismuth [65] compounds has been reviewed. The metal atom in these compounds acquires usually pentacoordination with a trigonal bipyramidal configuration, the exception being Ph5Sb which has a square pyramidal configuration [10]. The pentaorganometal compounds (R5M) are usually prepared by the reaction of R3MX2 or R4MX with the desired organolithium or Grignard reagents. These compounds are monomeric. A number of spirocyclic arsenic compounds such as (II) and (III) have also been reported [66]. The Lewis acidity of the pentavalent organometal(V) compounds decreases in the following order: Bi > Sb > As

RM > R2M > R3M > R4M


Most of the monoorganometal(V) halides are unstable compounds which tend to disproportionate at room temperature. The vibrational spectrum of PhAsCL indicates a trigonal bipyramidal configuration with the phenyl group occupying an equatorial position [67]. Contrary to the monoorganometal(V) halides, compounds with ligands such as alkoxides and spirocyclic compounds (prepared by the reaction of the appropriate arsonic acid with two equivalents of 1,2-dihydroxy compounds, e. g. (IV) [63]) are stable

[68] and can be distilled in vacuo (e. g., [MeAs{0C(Me)2C(Me)20}2], b. p.: 131-132°C/3 mm [69]). Unlike RSbCL, their adducts are stable under ambient conditions and have a six-coordinate antimony atom [70].


Ο R?C- \ / ~~CR As R2C. / \ C!FL Ό Ο" IV

A number of diorganometal(V) halides have been reported, they are more stable than RMX4. The vibrational spectral studies on R2ASC13 (R = Me or Ph) have suggested that these compounds have a trigonal

49 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview

bipyramidal structure with the R groups occupying the equatorial positions [67], Although similar structures

have been suggested for R2SbX3 compounds on the basis of spectroscopic data [71], the X-ray structural analysis on Ph2SbCl3 revealed a dimeric structure with bridges (V) [72], Strikingly, the bromides, Ph2SbBr3, Ph2SbBr2CI and Ph2SbBrCl2 have a distorted trigonal bipyramidal geometry (VI) (where Μ = Sb, X = Br or CI) [73]. The R2SbX3 compounds form a number of adducts with oxygen donor ligands to give complexes of the type R2SbX3.L. The X in trihalides can be substituted with various anionic oxygen donor ligands such as alkoxides, carboxylates, ß-diketonates, oxinates, etc. The alkoxides of arsenic are volatile

liquids (e. g. Me2As(OMe)3, b. p. 32°C/0.2 mm; Ph2As(OMe)3, b. p. 163°C/1.0 mm [69]).

Ph Ph X CI, .CI R R A'/ X—M^ R R CI vp« X X Ph Ph VI VII

The triorganometal(V) compounds have been studied in considerable detail. They are usually

prepared by the oxidation of R3M with under anhydrous conditions. Spectroscopic and X-ray analyses revealed a trigonal bipyramidal geometry for R3MX2 (VII) [74, 75]. The halides in these complexes can be substituted with a variety of ligands such as pseudohalides, alkoxides, carboxylates, ß-diketonates, etc. The tetraorgano-arsenic and -antimony halides, RjMX are ionic compounds, RiM+X" and are usually referred as arsonium and stibonium salts in which the metal atom adopts a tetrahedral geometry. However, compounds with oxygen ligands are covalent derivatives and have a trigonal bipyramidal geometry. For

example, Me4As(OR), (OR = OMe, OEt, ONMe2, ONCHMe, etc.) [76] are distillable liquids, while the structure of Ph4Sb(OMe), a solid, has been established by single X-ray analysis [77],

Metal- metal bonded compounds A wide variety of compounds containing either homo- or hetero- metal-metal bond have been prepared and studied. The strength of the M-M bond decreases markedly from arsenic to bismuth. These

compounds include, tetraorganodimetallic species, R2M-MR2 (Me2As-AsMe2 being the first organometallic compounds isolated as early as 1760 [1]), linear tri- and tetrametallated species and ring compounds. The area is dominated by arsenic compounds with fewer examples of antimony and bismuth compounds [78]. Various methods used for the preparation of these compounds are depicted in equations 6 to 9 [79-84], The metal-metal bonds in these compounds are cleaved by halogens, HX, oxygen, sulfur, , with the reactivity increasing in the order: Μ = As < Sb < Bi.

R2AsCl + R2ASH -*R2AS-ASR2 (6)

Na/Li RASC12 [RAs]„ (n = 5, 6) (7)


Na/THF (8)

1 ηSI P A: THF I v I Ar2SbLi + R3SbBr2 Ar2Sb" -Sb R3-Sb" Ar2 (9)

Apart from homo metal-metal bonded compounds, many compounds containing arsenic, antimony or bismuth to main group or transition metal bonds have been reported [5, 6]. diorgano- arsinides, -stibinides and -bismuthinides are useful intermediates which can be prepared by the reactions of

50 Shivram S. Garje and Vimal K. Jain Main Group Metal Chemistry

Ar„R3.nM, MR„Xj.„ with the alkali metal or R2MH with butyllithium in THF. A large number of σ-bonded group III compounds particularly of aluminium, and (eqns. 10, II) have been isolated due to their significance as single source precursor in the preparation of semiconductors [85-92], Similarly, , tin, and [86] compounds have been synthesized and characterized. σ-Bonded transition metal group 15 elements have also been reported (eqn. 12) [88],

Ph2InCl + 2As(SiMe3)3_ [Ph2InAs(SiMe3)2]2 + 2Me3SiCl (10)

Bu'2GaCl + LiAsBu': Bu'2GaAsBu'2 + LiCl (11)

MeSbBr2 + 2Na[M(CO)3Cp] MeSb[MCp(CO)3]2 + 2NaBr (12) Μ = Cr, Mo, W

Multiply bonded compounds Besides the compounds containing single metal-metal bond, there are species which contain multiply metal-metal bonds. The arsa-, stiba-, bisma-arenes (VIII) are analogous to pyridine with reduced basicity. Thus they can act like arenes and can be coordinated to metal in a hexa-hepto fashion (IX) [93]. With bulky organic groups, compounds containing P=S, As=As, P=Sb double bonds have been isolated [87]. For example, compound (X) shows a trans planar structure of CAs=AsC fragment.

Μ = As, Sb, Bi VIII IX

The Wittig reaction is an important reaction in organic chemistry; which involves phosphorus + ylides and represents a double bonded structure (R3M=CR'2) and a dipolar structure (R3M -C"R'2). The heavier homologues of phosphorus ylides have also been synthesized and studied. The number of ylidic species known decrease in the following sequence: Ρ > As » Sb > Bi. The arsenic, antimony and bismuth ylids can be prepared in several ways which include the following methodologies (i- iii),

(i) treatment of R3(R'CH2)MX with a base (eqn. 13) [94], NaNH2/THF Ph3AsMeBr > Ph3AsCH2 (13)

(ii) by the reaction of the diazo compounds with R3M, preferably in the presence of Cu(HFacac)2 (eqn. 14) [95]. Ph Ph ,Ph V ^Ph

Cu(HFacac)2 Ph3M + Ph3M (14)

Ph / Ph

Μ = Sb, Bi

(iii) by the reaction of activated methylene groups with R3AsX2 (X = halide) or R3AsO (15) [96].


Ph3As=0 + CH3N03 Phrn33 AsCHN02 (15)

51 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview

Heterocyclic compounds A number of heterocyclic compounds of arsenic, antimony and bismuth containing five-, six-, seven- , etc. membered rings are known (scheme 2) [97]. In earlier reported synthesis of heterocycles, a diGrignard or a dilithio reagent was treated with a dihalide.

Arsindole Dibenzarsole Arsenane

1,2,3,4-Tetrahydroarsinoline Scheme 2

Coordination COIT> ounds The coordination and organometallic complexes of arsenic, antimony and bismuth with xanthates, dithiocarbamates and phosphorus based acids is a topic of much current interest and this subject matter has been reviewed in several articles [98-10I], The structural diversity of these compounds has been the driving force for the current interest in these molecules. Trivalent compounds containing dithio ligands usually show anisobidentate bonding, /. e. a primary bonding through one sulfur atom, and a weaker secondary bonding through the other one. Thus, the tris isopropylxanthate derivatives, [M{S2COPr'}3] (M = As, Sb, Bi) [102] possess anisobidentate bonding with a distorted octahedral geometry for arsenic and antimony derivatives. The bismuth complex is seven coordinated, six sulfur atoms from asymmetrically coordinated ligands and a bridging sulfur atom from a neighbouring molecule, which results in a polymeric array. On the other hand, the molecular structure of [Bi{S2COEt}3] consists of a highly distorted pentagonal bipyramidal geometry with one sulfur atom and the electron pair occupying the axial positions [103]. The complexes

[As(S2CNEt2)3] [104], [Sb(S2CNEt2)3] and [Bi(S2CNEt2)3] [105] contain anisobidentate ligands and the overall geometry is distorted octahedral with an active lone pair. The X-ray crystal structures of n n n n [As(S2CNEt2)3] [104], Sb(S S)3, (S S = S2P(OR)2, R = Me [106], Et [107], Pr' [106]) and Bi(S S)3 (S S = S2P(OR)2, R = Et [108], Pr'[109]) have established that these molecules are monomeric tris chelates with anisobidentate ligands displaying distorted octahedral configuration. On the other hand, the molecular structure of [Sb(S2PPh2)3] [106] and [Bi(S2COEt)3] [103] consist of a highly distorted pentagonal pyramidal geometry with one sulfur atom and the electron pair occupying the axial positions.

Organometallic derivatives, [RBi(S2CNR'2)2] (R = Me or Ph) [54, 56] and [MeBi(S2COR')2] (R' = Me, Et, Pr') [55] have been prepared by German workers. The diphenylbismuth dithiocarbamates [54] disproportionate readily (eqn.16) to monophenylbismuth complexes, while the xanthate derivatives are stable at room temperature and the structure of [Ph2Bi(S2COPr')] has been established by X-ray diffraction [110]. The molecule has a distorted trigonal bipyramidal geometry. The phenyl groups occupy the axial positions whereas, the two sulfur atoms and the non-bonding electron pair occupy the equatorial positions.

2 Ph2BiCl + 2 NaS2CNR2 ¥ PhBi(S2CNR2)2 + Ph3Bi + 2 NaCl (16) R = Me or Et

Organometallic derivatives of arsenic and antimony, [PhM{S2P(OPr')2}2] [111] (M = As, Sb) are monomeric compounds with asymmetric monometallic biconnective ligands. In [RAs{S2P(OR')2}2](R =

52 Shivram S. Garje and Vimal K. Jain Main Group Metal Chemistry

Me, Et and R' = Et, Prn, Pr') anisobidentate bonding is observed [112]. Whereas in their monothio analogues bonding is through sulfur atom only [112, 113]. Similar situation exists in the case of xanthate and dithiocarbamate derivatives [114]. [PhSb(S2PPh2)2] [115] and [MesBi(S2PPh2)2] [116] have anisobidentate bonding with square pyramidal geometry. [Ph2Sb(S2PPh2)] [117] is a weak dimer containing eight membered Sb4S4P2 rings, in which transannular Sb...S interactions raise the antimony coordination to five leading to a distorted square pyramidal geometry. The monothiophosphate derivatives of antimony show a different behaviour. In these molecules the primary bonding is through the oxygen atom rather than the sulfur. Thus, [Sb(OSPR2)3] [118] (R = Ph, C-Hx) has primary bonding through the oxygen atom. However, in [Sb(SOCPh)3] [119], the primary bonding is through sulfur and is attributed to the borderline hard-soft acid character of Sb"' [119]. The X-ray structures of [Ph2Sb{0(X)PPh2}] (X = O, S) [120] have revealed that diphenylantimony(III) groups are linked into chains by symmetrically bonded bridging phosphinate/thiophosphinate groups. In the case of monothio derivatives, the primary bonding is through oxygen. The geometry around antimony is pseudotrigonal bipyramidal with the two phenyl groups in equatorial positions. (+, H3N CI"


Atoxyl Oxyphenarsine hydrochloride Arsenamide Antitrypanosomiastic Antihelmintic



H2N Ν Antimony(V) AA Η sodium gluconate Antipyranosomal Antiteishmanial Ο

Antimony potassium tartrate (Tartaremetic) Kala-azar

Ο Scheme 3

53 Vol. 22, No. 1, 1999 The Chemistry of Organo-Arsenic,Antimony and Bismuth Compounds an Overview

Pentavalent derivatives of arsenic, antimony and bismuth with these ligands have also been reported with greater emphasis on the antimony compounds. [Me3Sb(S2CNMe2)2] [121] has anisodentate ligand whereas, in [Me3Sb(S2PPh2)2] [115], monodentate bonding through the sulfiir atom is observed, although, a weak. S(=P) and Sb intramolecular interaction leading to a distorted trigonal bipyramidal structure has been observed. The monothio analogue [Me3Sb(OSPPh2)2] [122], is a discrete monomer with a trigonal bipyramidal geometry around antimony and the ligands are bonded only through the oxygen atoms. The same trend is observed in dialkylmonothiophosphate derivatives also [123],

Dicyclohexyl phosphinic acid derivatives of antimony, {SbPh2CI[02P(C6Hn)2]}20 have been reported recently in which antimony is hexacoordinated with bridging phosphinate group and the geometry around n 1 antimony is octahedral [124], The compounds R?As(02PPh2)2 (R = Et, Pr , Pr ) are discrete molecules with monodentate bonding whereas, their antimony analogues are polymeric with bridging ligand [125],

MeAs03H2 Me2As02H Me3AsO Me4As(+

Ο (+) II μ

Arsenobetaine Arsenocholine Scheme 4

Pharmaceutical and environmental aspects Compounds of arsenic and antimony have been used as therapeutic agents for thousands of years. However, the modern work on such compounds started in 1905 when sodium -4-aminophenyl arsonate (atoxyl) was successfully used against sleeping sickness. This prompted a systematic study of organoarsenicals (scheme 3). By 1932, a total of 12,500 compounds of arsenic had been synthesized and clinically tested. With the discovery of Penicillin in 1943, there was a sharp decline in the interest of organoarsenic compounds in medicine. Still, some of the organoarsenicals are in use as , and fungicides [126]. One of the major causes of toxicity of arsenic is thought to be its ability to inhibit a variety of enzymatic reactions. The usual mode of action is often stated to be with the groups ().

Because of the similarity of arsonic acids RASOIH2 to containing the phosphonooxy group, -O-

Ρ03Η2, they enter in enzymatic reactions. If the action of the is to alkylate, acylate or phosphorylate the phospho group then, a new futile cycle can be established because the product (from arsonic acids) is spontaneously hydrolyzed to give the original RAs03H2. An arsenomethyl compound may bind to an enzyme that binds a or carboxylate [127], is the most important route by which arsenic is metabolized in living organisms. The toxicity of arsenic ranges from very low (e.g. As2S3) to extremely high (e.g. AsH3) depending on the chemical state. Although arsenic is toxic to higher living forms, some strains are capable of producing organoarsenic compounds. Some of the marine organisms such as mollusces, marine algae etc. store arsenic in several mg/kg of dry weight. Arsenic compounds with one to four methyl groups attached to the arsenic atom are common constituents in marine samples (scheme 4) [128].

Conclusions This review briefly describes various areas of research which are currently pursued in different laboratories. The survey reveals that the emerging trends in the field for future investigations appear to be: (a) precursors for MOCVD process; (b) hypercoordinated and self-assembled molecules; (c) chemistry with phosphorus based acid ligands, (d) biological aspects of these compounds

Acknowledgements The authors thanks Dr. C. Gopinathan, Head, Chemistry Division; Dr. J. P. Mittal, Director, Chemistry Group, BARC; Dr. S. B. Padhye, Head, Inorganic Chemistry Division and Dr. S. R. Gadre, Head, Department of Chemistry, University of Pune, for their encouragements.

54 Shivram S. Garje and Vimal K. Jain Main Group Metal Chemistry

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Received: June 30, 1998 - Accepted: August 24, 1998 - Accepted in revised camera-ready format: October 28, 1998