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

The X-ray crystal structures of K6[CoII W12O40]·~16H2O and K5[CoIIIW12O40]·~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) Å3 , and a 19.111(2), c 12.509(2) Å, V 3956.6(9) Å3 , respectively. Both anions exhibit the standard α-Keggin structure, which consists of a central tetrahedrally coordinated CoII or CoIII , 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 CoII and CoIII compounds, respectively, could be crystallographically located. The others are presumably zeolitic and highly disordered in nature. Structural differences between the anions on replacement of CoII by CoIII , as well as comparisons with the related [PW12O40]3– and [SiW12O40]4– ions, which contain (formally) PV 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 PV to CoII , 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.