AB-INITIO STUDY OF THE RELATIVE BASICITY OF THE EXTERNAL OXYGEN SITESIN M2W4O194- (M = NB AND V)

Geometry optimizations were carried out for the.cis and trans forms of Nb2W4O194- The energy difference between the two conformations was fo und to be 2.3 kcal mol-l, the structure in which the two Nb atoms are in the cis formation being the most stable. Several isomers of the Nb2 W4O19H3- anion were also studied for the cis form, suggesting that the oxygen bonded to two Nb atoms is the most basic center. The energetic determination of the oxygen basicities in hexametalates was compared with the indirect and less computationally demanding methodology based upon molecular electrostatic potential (ESP) distributions, The ESP d istribution in HNb2W4O193- suggests that a second proton should fix on to an OW2 oxygen site and that the ONb terminal O's remain the most ba sic terminal centers. In V2W4O194-, the unique OV2 was identified as t he most basic center. Although at variance with the niobotungstate ani on, the most basic center does not support the highest net negative ch arge. The terminal OV oxygen sites were found to be the least basic te rminal O's.