Poly[diaquatetra-μ-selenocyanato-cadmium(II)dipotassium(I)]

In the title compound, [CdK2(NCSe)4(H2O)2]n, the cadmium(II) cation is situated on a twofold rotation axis and is coordinated in a slightly distorted tetrahedral geometry by two symmetry-related μ-1,1,1,3 and two-symmetry related μ-1,1,3,3 bridging selenocyanate anions, all of which are Se bonded. These bridging selenocyanate anions are further coordinated to two symmetry-related potassium ions. Each of the potassium ions is coordinated by one terminally bonded water molecule and six selenocyanate anions, two of which are crystallographically independent. The asymmetric unit consists of one cadmium and one potassium cation, two bridging selenocyanate anions and one water molecule. The polymeric subunits are further connected via the selenocyanate anions into a three-dimensional coordination network. In this coordination network, intramolecular hydrogen bonds between neighbouring water molecules can be found.

In the title compound, [CdK 2 (NCSe) 4 (H 2 O) 2 ] n , the cadmium(II) cation is situated on a twofold rotation axis and is coordinated in a slightly distorted tetrahedral geometry by two symmetry-related -1,1,1,3 and two-symmetry related -1,1,3,3 bridging selenocyanate anions, all of which are Se bonded. These bridging selenocyanate anions are further coordinated to two symmetry-related potassium ions. Each of the potassium ions is coordinated by one terminally bonded water molecule and six selenocyanate anions, two of which are crystallographically independent. The asymmetric unit consists of one cadmium and one potassium cation, two bridging selenocyanate anions and one water molecule. The polymeric subunits are further connected via the selenocyanate anions into a three-dimensional coordination network. In this coordination network, intramolecular hydrogen bonds between neighbouring water molecules can be found.  Guo & Mak (1998 Table 1 Hydrogen-bond geometry (Å , ). Data collection: X-AREA (Stoe & Cie, 2008); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97.

Related literature
We gratefully acknowledge financial support by the State of Schleswig-Holstein and the Deutsche Forschungsgemeinschaft (Project 720/3-1).

Poly[diaquatetra--selenocyanato-cadmium(II)dipotassium(I)]
T. Reinert, J. Boeckmann, I. Jess and C. Näther (2010a,b)) we have reacted cadmium(II) dinitrate with potassium selenocyanate and pyrimidine in water in order to prepare cadmium selenocyanato coordination polymers with pyrimidine as co-ligand. In this reaction single crystals were obtained, which were identified as the title compound by single-crystal X-ray diffraction.
The cadmium cation is coordinated by Se atoms of four selenocyanato anions in a slightly distorted tetrahedral geometry.
The angles around the K atoms range between 63.10 (2)° and 155.51 (19)° (Tab. 1). The large K-Se and K-K distances are not unusual and similar values can be found in related structures (Shi et al., 2007;Couhorn & Dronskowski, 2004). The Cd and K atoms are connected by the bridging selenocyanato anions into a three-dimensional coordination network (Fig. 2).
It must be noted that the present bridging modes of the selenocyanato anions are observed for the first time in a coordination polymer and similar coordination modes can be found for azido anions (El Fallah et al., 2008;Guo & Mak, 1998).

Experimental
Cd(NO 3 ) 2 x 4H 2 O was obtained from Merck, KNCSe and pyrimidine were obtained from Alfa Aesar. 1 mmol (174 mg) Cd(NO 3 ) 2 x 4H 2 O, 2 mmol (288 mg) KNCSe, 4 mmol (320 mg) pyrimidine and 3 ml acetonitrile were reacted in a closed snap-cap vial without stirring. After the mixture had been standing for several days in the dark at room temperature colourless block shaped single crystals of the title compound were obtained.