X-ray structural studies of K-6[(CoW12O40)-W-II]center dot similar to 16H(2)O and K-5[(CoW12O40)-W-III]center dot similar to 16H(2)O and structural trends along the [XW12O40](n-) series, where X = P-V, Si-IV, Co-III and Co-II

The X-ray crystal structures of K-6[(CoW12O40)-W-II].similar to 16H(2)O and K-5[(CoW12O40)-W-III].similar to 16H(2)O are reported. The compounds are i sostructural, hexagonal, space group P6(2)22, and have Z 3 with a 19.118(7) , c 12.383(9) Angstrom, V 3919.6(35) Angstrom(3), and a 19.111(2), c 12.509 (2) Angstrom, V 3956.6(9) Angstrom(3), respectively. Both anions exhibit th e standard alpha-Keggin structure, which consists of a central tetrahedrall y coordinated Co-II or Co-III, surrounded by four groups of three edge-shar ing octahedra (W3O13 subunits) which are linked in turn to each other and t o the central CoO4 tetrahedron by shared oxygen atoms at the vertices. Alth ough thermogravimetric analyses show that there are c. 16 water molecules p er formula unit in both compounds, only six and three water molecules for t he 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-I II, 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 mo re isolated with increasing size of the central heteroatom from P-V to Co-I I, while the bonding within an individual W3O13 subunit becomes weaker. Ext ended-Huckel molecular orbital calculations are used to examine stability c hanges 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 central ly located H+ ions, but no steric effects. The studies show that no major c hanges occur in the overall stability of the framework, but that there is a redistribution in the contributions of the two types of bridging oxygen at oms to the stability of the framework, which parallels the progressive isol ation and weaker bonding in the W3O13 subunits.