Ab Initio Study of the Relative Basicity of the External Oxygen Sites in M2W4O194-(M = Nb and V)

Geometry optimizations were carried out for the cis and trans forms of Nb 2 W 4 O 19 4 - . The energy difference between the two conformations was found to be 2.3 kcal mol - 1, the structure in which the two Nb atoms are in the cis formation being the most stable. Several isomers of the Nb 2 W 4 O 19 H 3 - 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 distribution in HNb 2 W 4 O 19 3 - suggests that a second proton should fix onto an OW 2 oxygen site and that the ONb terminal O's remain the most basic terminal centers. In V 2 W 4 O 19 4 -, the unique OV 2 was identified as the most basic center. Although at variance with the niobotungstate anion, the most basic center does not support the highest net negative charge. The terminal OV oxygen sites were found to be the least basic terminal O's.