X-Ray Structural Studies of K_6[Co~IIW_12O_40]·approx 16H_2O and K_5[Co~IIIW_12O_40]·approx 16H_2O and Structural Trends Along the [XW_12O_40]~(n-) Series, where X = P~V, Si~IV, Co~III and Co~II

摘要

X-Ray crystal structures of K_6[Co~IIW_12O_40]·approx 16H_2O and K_5[Co~IIIW_12O_40]·approx 16H_2O are reported. The compounds are isostructural, hexagonal, space group P6_222, and have Z3 with a 19.118(7), c 12.383 (9)A, V3919.6(35)A~3, and a 19.111(20, c 12.509(2)A, V39.56.6(9) A~3, respectively. Both anions exhibit the standard #alpha#-Keggin structure, which consistes of a central tetrahedrally Co~II or Co~III, surrounded by four groups of three edge-sharing octahedra (W_3O_13 subunits) which are linked in turn to each and to the central CoO_4 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 presembly 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 [POW_12O_40]~(3-) and [SoW_12O_40]~(4-) ions, which contain (formally) P~V and Si~V, respectively, are discussed. This comparison indicates that the W_3O_13 subunits become progressively more isolated with increasing size of the central heteroatom from P~V to Co~II, while the bonding within an individual W_3O_13 subunit becomes weaker. Extended-Huckel molecular orbital calculations are used to examine stability changes in the polyoxotungstate framwork, using the actual polyoxotungstate geometries of the anions, on variation in size of the central heteroatom. These are compared to that in the [H_3W_12O_40]~(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 redustribution 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 W_3O_13 subunits.