Comment

The [AsO₃]^3- arsenite group shows a distinctive pyramidal geometry, due to the stereochemically active lone pair of electrons on the As^III species, with an electron configuration of [core]4s²4p¹. This geometry is quite distinct from the tetrahedral coordination invariably displayed by the [As^VO₄]^3- arsenate group. A number of minerals and synthetic compounds containing isolated pyramidal [AsO₃]^3- ions are known, examples being reinerite, Zn₃(AsO₃)₂ (Ghose et al., 1977), and the unusual arsenite-chloride finnemanite, Pb₅(AsO₃)₃Cl (Effenberger & Pertlik, 1979).

Arsenite groups may polymerize (or condense) via vertices into extended units, the simplest example of this being the [As₂O₅]^4- diarsenite group, which is found in paulmooreite, Pb₂As₂O₅ (Araki et al., 1980). In ludlockite, PbFe₄(As₅O₁₁)₂ (Cooper & Hawthorne, 1996), as many as five AsO₃ units are fused together into [As₅O₁₁]^7- units. The polymerization of arsenite groups results in the catena-arsenite chain anion, [AsO₂]^- (or [AsO₂]_n^n-), which was first definitively characterized by Zemann (1951) in the mineral trippkeite, CuAs₂O₄. A few years later, the same anion was found in the synthetic compound NaAsO₂ by Menary (1958). The trippkeite structure was redetermined to improved precision by Pertlik (1975), who also showed that the two synthetic lead catena-arsenite chlorides Pb(AsO₂)Cl and Pb₂(AsO₂)₃Cl contain the same chain anion (Pertlik, 1988), as does the mineral leiteite, ZnAs₂O₄ (Ghose et al., 1987).

We describe here the structure of ethylenediammonium catena-arsenite, (H₃NCH₂CH₂NH₃)_0.5[AsO₂], (I), which is the first example of a catena-arsenite chain accompanied by organic cations. We also describe the redetermined structure of NaAsO₂, (II).

Compound (I) (Fig. 1) shows (H₃NCH₂CH₂NH₃)²⁺ cations and anionic [AsO₂]^- chains. The geometrical parameters for the complete ethylenediammonium cation, which is generated by twofold symmetry from the unique atoms, are normal. The catena-arsenite chain is built up from three distinct atoms, with atom O1 forming the terminal As-O bond and atom O2 acting as the bridging atom. As expected, the geometry around As is pyramidal, with the As atom displaced from the least-squares plane of the basal O atoms by 0.886 (2) Å. Interestingly, the most prominent peak (1.11 e Å^-3) in the final difference Fourier map for (I) is 0.74 Å from As, approximately where the lone pair of electrons is presumed to be located, and could thus correspond to a real chemical feature. As found in other well determined catena-arsenites (Pertlik, 1975; Ghose et al., 1987), the terminal As-O_T bond in (I) [1.705 (3) Å] is distinctly shorter than the average of the bridging bonds [mean As-O_B = 1.812 (2) Å]. The O_B-As-O_B bond angle is significantly smaller than the O_B-As-O_T bond angles (Table 1).

As well as van der Waals and electrostatic forces, the organic cations and the chain anion in (I) interact by way of N-HO hydrogen bonds (Table 2). Two of the three H-N moieties make short near-linear hydrogen bonds to arsenite O-atom acceptors, whilst the third N-H group is bifurcated to two arsenite O acceptor atoms (sum of D-HA bond angles about atom H1 = 359°). Overall, O_T accepts three hydrogen bonds and O_B accepts one. These interactions help to define a structure (Fig. 2) in which the catena-arsenite chains propagate along [010] (generated by the 2₁ screw axis), crosslinked along [100] by OH-N-HO bonds. Interchain linking along [001] is via the backbone of the organic moiety. The intrachain AsAsⁱ separation in (I) is 3.1991 (4) Å [symmetry code: (i) 1 - x, y - ,  - z].

The structure of (II) (Fig. 3) is more or less the same as that determined by Menary (1958) using film methods, but with improved standard uncertainties. The Na⁺ cation is coordinated to seven O atoms (mean Na-O = 2.623 Å), all of which are parts of neighbouring anionic [AsO₂]^- chains. The resulting NaO₇ polyhedron approximates to a distorted capped trigonal prism. The Na bond valence sum (BVS) of 1.00 (Brown, 1996) is exactly in agreement with the expected value. The As geometry is again pyramidal, with the As atom displaced from the least-squares plane of the basal O atoms by 0.912 (3) Å. The As-O distances [As-O_T = 1.684 (4) Å and mean As-O_B = 1.822 (3) Å] are similar to those found for (I). As in (I), the O_B-As-O_B bond angle is significantly smaller than the O_B-As-O_T bond angles (Table 3). By comparison, Menary's (1958) results (As-O_T = 1.600 Å, and As-O_B = 1.810 and 1.947 Å) indicated a much greater degree of distortion about As.

In the unit-cell packing in (II) (Fig. 4), the catena-arsenite chains propagate along [010], as generated by b-glide symmetry, resulting in an intrachain AsAsⁱⁱ separation of 3.2121 (7) Å [symmetry code: (ii)  - x,  + y, z]. The face- and edge-sharing NaO₇ groups are sandwiched between the [AsO₂]^- chains and crosslink them in the a direction, resulting in neutral (001) slabs of stoichiometry NaAsO₂. The As^III lone-pair electrons appear to be directed into the inter-slab region. The shortest interblock AsO and AsAs contacts are 3.762 (3) and 3.6844 (7) Å, respectively. This is quite reminiscent of the situation in ludlockite (Cooper & Hawthorne, 1996), in which the [As₅O₁₁]^7- units face each other.

The geometrical parameters for the [AsO₂]^- units in (I) and (II) are broadly consistent with the equivalent data for CuAs₂O₄ and ZnAs₂O₄. In particular, the As-O_B bond lengths are clustered in the narrow range of 1.806 (2)-1.829 (3) Å. The As-O_T bond lengths show somewhat greater variability, which might be due to the different bonding situations of the O atoms in question: the O_T atom in (I) [1.705 (3) Å] only accepts hydrogen bonds, whereas the O_T atom in CuAs₂O₄ (1.765 Å) is also bonded to two Cu atoms. However, there are also some significant differences. For example, the O_B-As-O_B bond angle of 100.3° in CuAs₂O₄ is significantly larger than the O_B-As-O_T bond angle (95.9°), which is the reverse of the situation for (I), (II) and ZnAs₂O₄ (Ghose et al., 1987).

Experimental

For (I), a mixture of As₂O₃ (1 g), ethylenediamine (0.5 g) and water (10 ml) was heated to 353 K in a plastic bottle for 48 h. Upon cooling, the resultant solids were filtered off, yielding some plate-shaped crystals of (I) accompanied by substantial amounts of undissolved or recrystallized As₂O₃. We have not yet succeeded in making (I) in purer form. For (II), a commercial sample (Sigma Chemical Co.) of NaAsO₂ was recrystallized from methanol, in which it is sparingly soluble. The resulting crystal quality is poor.

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: BC1044 ). Services for accessing these data are described at the back of the journal.

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