The Chemistry of Antimony

Antimony is in group 15 of the periodic table, together with nitrogen, phosphorus, arsenic and bismuth. It has two stable naturally occurring isotopes: 121Sb (57.25%) and 123Sb (42.75%).3 Antimony is mostly found in ore form as either stibnite (Sb2S3) or ullmanite (NiSbS). Similar to arsenic, antimony is

Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine Edited by Gielen and Tiekink © 2005 John Wiley & Sons, Ltd generally classified as a semi-metal. The most common and stable oxidation states of the element are trivalent (SbIII) and pentavalent (SbV), with ionic radii of 0.76 and 0.62 A, respectively.3,4

The simple oxyanion forms of both antimonite (SbIII) and antimonate (SbV) have not been adequately characterized. Although it is generally believed that H3SbO3 and Sb2O5 are the correct formulae, examples in earlier literature suggested that hydrated antimonate has the formula Sb2O5 but these were later proved to contain [Sb(OH)6]^.1 Both Sb2O3 and Sb2O5 form complexes with organic acids such as tartaric acid and gluconic acid. The resulting compounds are useful as therapeutic agents. The basic properties of antimony are summarized in Table 23.1.

Trivalent antimony forms many complexes by means of Lewis acid-base reactions. The valence orbital from the metal center accommodates the incoming electron pair. In the case of haloantimony(III) complexes, monomeric haloantimony(III) anions [SbX4]~, [SbX5]2~ and [SbX6]3~ are formed by the addition of X~ (halide) ligands to SbX3.

Complexes of Sbra with group 15 elements have been investigated and their structures have been determined. For example, the crystal structure of an ionic complex [H(py)][SbI4(dmpe)] (py = pyridine, dmpe = 1,2-bis (dimethylphosphino) ethane) showed that SbIII is coordinated to four iodine and two phosphorus atoms from 1,2-bis(dimethylphosphino) ethane.5

SbIII is also coordinated to various oxygen- and sulfur-containing ligands and its coordination number can vary. For example, in the antimony complex with citric acid, various building units were observed. In Li[Sb(citrate)2 (H2O)] 2H2O and Na[Sb(citrate)2(H2O)2]-H2O, the Sbra center is coordinated to four oxygens from two citrates with each in a bidentate mode. Antimony adopts a distorted pseudo-trigonal bipyramidal geometry with an equatorial lone pair (Figure 23.1).6 8 The basic building unit of the clinically used anti-parasitic agent potassium antimony(III) tartrate (PAT) is the dimeric anion. Each Sbra is also coordinated to four oxygens from two tartrates, with each in a bidentate mode. The coordination geometry of Sbra is distorted pseudo-trigonal

Table 23.1 Basic properties of antimony

Atomic number

51

Earth occurrence

0.2 ppm

Relative atomic mass

121.75

Ground state electronic configuration

[Kr]4d105s25p3

Ionic radius (A)

SbIII(0.76), SbV(0.62)

Isotopic distribution (%)

121Sb(57.25), 123Sb(42.75)

Standard reduction potential E0 (H+)

Sbm + 3e"! Sb0 0.204 V

SbV + 2e"! SbIII 0.605 V

Major ores

Stibnite (Sb2S3), ullmanite (NiSbS)

Radioactive isotope

125Sb, f3~ emission, t1/2 = 2.76 years

Sb"

Sb11

Sb11

Figure 23.1 X-ray crystal structures of antimony(III) citrate (a) and potassium anti-mony(III) tartrate (PAT, K2Sb2(C4H2O6)2) (b) (see Plate 3)

bipyramidal (Figure 23.1).9 Sb111 has a high affinity for thiolate ligands and the X-ray crystal structures of several Sbra-thiolate complexes have been determined. In these complexes, Sb111 exhibits coordination numbers of either 3 or 4. The metal has trigonal pyramidal or bipyramidal geometry with Sb—S bond distances of ~2.4-2.6A.10 Weak interactions to other thiolate sulfurs (either intra- or inter-molecular) are observed in some of the complexes.

Like the Sb111 complexes, SbV also forms many complexes. Pentahaloantimony compounds such as SbCl5 are powerful halide acceptors, usually with the formation of the [SbX6]~ mono-ligand coordinated adduct. The sixth coordination site can also be occupied by a neutral donor ligand. One example is the SbF5(NC—CN) dinitrile complex with Sb—N bond lengths of 2.23 A.11 Antimony pentachloride can act as a powerful halide Lewis acid in the presence of AsCl3 and chloride.12

Complexes containing SbV and group 15 donor ligands have been investigated. The structures of these complexes are normally simple octahedral or distorted octahedral due to the absence of the lone pair electrons in the coordination environment, in contrast to the SbIII complexes. The X-ray crystal structure of an SbV porphyrin complex, [Sb(TTP)(OCH(CH3)2)2]Cl (TTP = meso-tetra-p-tolylporphyrin), showed that SbV is inserted into the porphyrin ring and octahedrally coordinated by four nitrogens from the porphyrin and two oxygens from 2-propanol molecules at the z-direction.13 The coordination chemistry of antimony has been extensively studied.14

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