The electronic structure and stability of the isomers of octamolybdate

The structure of the gamma-, delta-, is an element of-, and xi-isomers and the bonding in the alpha-, beta-, gamma-, delta-, is an element of-, and xi-isomers of [Mo8O26](4-) isopolyanions have been calculated using density functional theory. The optimized structures are in reasonably good agreement with those determined experimentally with the exception of the gamma-form. For this isomer, the optimization leads to a lengthening of an internal bond, and as a result, the topology of this isomer becomes identical to that of the xi-form. The electronic structure and relative stability have been probed using a bond order and valency analysis and through a decomposition of the bonding energy. The terminal Mo-O bonds possess fractional multiple bond character with similar values for those attached to four, five, and six coordinate metal atoms. The Mo-O bond order decreases as the coordination number of the oxygen increases, and a number of pseudoterminal oxygen sites have been located. The bond order analysis appears to confirm the coordination numbers of the metal atoms in the delta- and xi-isomers but suggests that additional contacts should be considered for one of the terminal oxygen atoms in the c-isomer, leading to a topology intermediate between that of the beta-isomers and of the previously predicted (beta-gamma) intermediate. Despite the range of coordination numbers, charges, and bond orders, the overall bonding capacity of the oxygen atoms, measured through the full valency index, appears similar. The alpha- and delta-isomers are predicted to be the most intrinsically stable while the beta-form is the least stable. The relative stability of the isomers is due to a balance between steric interactions and favorable atomic interactions, both of which correlate with the number of Mo-O bonds. Although the alpha- and delta-isomers possess relatively few bonds, their open structures lead to low steric crowding. The compact structure of the beta-form leads to highly unfavorable steric interactions.