X-Ray Structural Studies of K6[CoII W12O40]·~16H2O and K5[CoIII W12O40]·~16H2O and Structural Trends Along the [XW12O40]– Series, where X = PV, SiIV, CoIII and CoII

Abstract

The X-ray crystal structures of K₆[Co^II W₁₂O₄₀]·~16H₂O and K₅[Co^IIIW₁₂O₄₀]·~16H₂O are reported. The compounds are isostructural, hexagonal, space group P 6₂22, and have Z 3 with a 19.118(7), c 12.383(9) Å, V 3919.6(35) ų , and a 19.111(2), c 12.509(2) Å, V 3956.6(9) ų , respectively. Both anions exhibit the standard α-Keggin structure, which consists of a central tetrahedrally coordinated Co^II or Co^III , surrounded by four groups of three edge-sharing octahedra (W₃O₁₃ subunits) which are linked in turn to each other and to the central CoO₄ tetrahedron by shared oxygen atoms at the vertices. Although thermogravimetric analyses show that there are c. 16 water molecules per formula unit in both compounds, only six and three water molecules for the Co^II and Co^III compounds, respectively, could be crystallographically located. The others are presumably zeolitic and highly disordered in nature. Structural differences between the anions on replacement of Co^II by Co^III , as well as comparisons with the related [PW₁₂O₄₀]^3– and [SiW₁₂O₄₀]^4– ions, which contain (formally) P^V and Si ^IV , respectively, are discussed. This comparison indicates that the W₃O₁₃ subunits become progressively more isolated with increasing size of the central heteroatom from P^V to Co^II , while the bonding within an individual W₃O₁₃ subunit becomes weaker. Extended-HÜckel molecular orbital calculations are used to examine stability changes in the polyoxotungstate framework, using the actual polyoxotungstate geometries of the anions, on variation in size of the central heteroatom. These are compared to that in the [H₃W₁₂O₄₀]^5– ion, which has two centrally located H⁺ ions, but no steric effects. The studies show that no major changes occur in the overall stability of the framework, but that there is a redistribution in the contributions of the two types of bridging oxygen atoms to the stability of the framework, which parallels the progressive isolation and weaker bonding in the W³O¹³ subunits.