Investigation of commercial sodium cacodylate trihydrate:: penta-μ-aqua-disodium(I) bis(dimethylarsenate) and di-μ-aqua-bis[triaquasodium(I)] bis(dimethylarsenate)

Crystals from commercial samples of sodium cacodylate trihydrate, NaO2As(CH3)(2)center dot 3H(2)O, were analyzed by singlecrystal X-ray diffraction and two phases were identified, viz. penta-mu-aqua-disodium(I) bis(dimethylarsenate), {[Na-2(H2O)(5)](C2H6AsO2)(2)}n, (I), and di-mu-aqua-bis[triaquasodium(I)] bis(dimethylarsenate), [Na-2(H2O)(8)](C2H6AsO2)(2), (II). Both (I) and (II) form layered structures in which hydrated Na+ ions form layers in the ab plane, the cacodylate ions being located in between the layers. In (I), the two nonequivalent Na+ ions (located at twofold axes) and the three non-equivalent aqua ligands (one of which also lies on a twofold axis) form infinite polymeric layers, but in (II), layers of discrete centrosymmetric [Na-2(H2O)(8)](2+) ions are present. One of the commercial samples analyzed contained almost exclusively crystals of the tetrahydrate (II), while another sample consisted of a mixture of the two phases.