Cadmium sulfite hexahydrate revisited

The present structural revision of the title compound, tetracadmium tetrasulfite hexahydrate, [Cd4(SO3)4(H2O)5]·H2O, is a low-temperature upgrade (T = 100 K and R = 0.017) of the original room-temperature structure reported by Kiers & Vos [Cryst. Struct. Commun. (1978). 7, 399–403; T = 293 K and R = 0.080). The compound is a three-dimensional polymer with four independent cadmium centres, four sulfite anions and six water molecules, five of them coordinated to two cadmium centres and the remaining one an unbound solvent molecule which completes the asymmetric unit. There are two types of cadmium environment: CdO8 (through four chelating sulfite ligands) and CdO6 (by way of six monocoordinated ligands). The former groups form planar arrays [parallel to (001) and separated by half a unit cell translation along c], made up of chains running along [110] and [10], respectively. These chains are, in turn, interconnected both in an intraplanar as well as in an interplanar fashion by the latter CdO6 polyhedra into a tight three-dimensional framework. There is, in addition, an extensive network of hydrogen bonds, in which all 12 water H atoms act as donors and eight O atoms from all four sulfite groups and two water molecules act as acceptors.

In combination with transition metals the anion can generate interesting structures, many of them reported in pion-  Kiers & Vos, 1978, in a R.T.,low resolution structure determination (T: 293 K, R:0.080) and for which we present herein an upgrade, by way of a low temperature data refinement (T: 100 K, R: 0.017).
The structure (shown in Fig. 1) is a three-dimensional polymer with four independent cadmium centres, four sulfite anions and six water molecules, five of them coordinated to two cadmium centres and the remaining one, an unbound solvate which completes the asymmetric unit.
The cadmium environments in the structure split naturally into two types, viz.: two CdO 8 , centred at Cd1 and Cd2 and achieved through four chelating sulfite bites each, and two CdO 6 , centred at Cd3 and Cd4 and where no chelating bites whatsoever take part, the donor O atoms being either bridging (sulfite) or monocoordinated (aqua) ( Table 1).
The two octacoordinated cadmium centres are comparable, but due to multiple chelation the corresponding CdO 8 polyhedra are difficult to describe by any regular model. However, both centres present a similar "tetrahedral" environment of ligands, the one around Cd1 being more flattened and describable as something midway a tetrahedral and a square planar arrangement. The one around Cd2, instead, is much more biased towards a tetrahedral shape.
In this regard the groups are adequate for a Vector Bond Valence treatment (hereafter VBV, Harvey et al., 2006), a novel approach tending to a simpler description of multidentate binding, in which the action of each ligand is replaced by a single interaction vector, the Vector Bond Valence (or VBV), derived from the individual bond valences (Brown & Altermatt, 1985) of the coordinating atoms.
Even though for the four-ligand coordination geometry the VBV model would not predict a definite geometry for the four VBV vectors, the requirement of a bond valence of ~2 for both cations and a nil resultant of their vectorial sum would still be in force.
supplementary materials sup-2 These requirements are satisfactorily fulfilled in both cases, with a scalar Bond Valence of 2.017 and 1.949, and a resultant VBV of 0.047 and 0.084 valence units for Cd1 and Cd2, respectively. Also the geometries of the (distorted) tetrahedra are correctly described by the VBV vectors, with the flattened Cd1 polyhedron presenting two large angles between trans VBV (126.8 (1) and 130.8 (1)°), and a much tighter span for the rest (Range: 94.9 (1)-111.7 (1)°) while the tetrahedron centred at Cd2 presents a close angle distribution throughout (Range: 100.9 (1)-115.9 (1)°).
The remaining cadmium centres Cd3 and Cd4, lacking any chelating ligand in their polyhedra, present rather regular octahedral arrangements (Table 1).
The anions coordinate through all their three donor O atoms, though not through sulfur, in µ 3 , µ 4 and µ 5 modes (Fig. 2).
In addition to the diversity in cation environments, there is a more profound difference setting apart these two types of polyhedra, and it consists in their quite diverse structural function.
On one side, both CdO 8 groups join to each other forming two sets of straight chains (See Fig. 3 for details) at z = 0, running along [110], and z = 1/2, running along [110], (A and B in Fig. 5; see below) both orientations subtending and angle of 98.0 (1)° to each other. Inspection of Fig. 3 reveals that the chains embed the crystallographic symmetry centres at sites X (at 1/2,0,1/2) and Y (at 0,1/2,1/2). There is, however, an extra, nearly perfect (though non crystallographic) pseudo centre midway the former two at site Z = 0.255,0.263, 1/2, relating Cd1 with Cd2, and SO 3 (1) with SO 3 (3). The degree of local pseudo symmetry involved can be assessed by the least squares fit of the Cd1, Cd2, SO 3 (1) and SO 3 (3) group (built up around the pseudo centre) and its inverted image, which gives a mean deviation of 0.11 (1)Å and a maximum of 0.14 (1)Å for the O31-O33 pair (Fig. 4). All six water molecules in the structure are involved in H-bonding through their twelve H atoms as donors ( Table 2). The acceptor role is covered by eight O atoms coming from the four sulfite groups and two water molecules. The sulfite anions participate in a rather uneven way, e.g.: sulfite(1) through only one H-bond involving O31, sulfite(2), through two bonds, both involving O22, sulfite(3) and sulfite(4) through three bonds each, via O13 and O33 (twice) for the former, and by way of O14 and O24 (twice) for the latter. Among the water molecules, only one aqua participates as an acceptor (O4W, bound to Cd4), the remaining one being the crystal water O6W, which receives two bonds, and thus completes the scheme. The complexity of this H-bonding scheme turns almost impossible any meaningful representation of the network to which it gives rise, for which a detailed packing figure including them has been spared, for the sake of clarity.

Experimental
The compound was obtained by slow inter diffusion of Na 2 SO 3 and Cd(CH 3 CO 2 ) 2 aqueous solutions in (1:1) molar ratio.
The connecting path between the two vessels was filled with an aqueous solution of NaCH 3 CO 2 , in order to minimize concentration gradients. After several weeks of unperturbed diffusion a crop of colourless, prismatic crystals of the title compound was obtained.