Influence of the central atom on the electronic properties of Lindqvist polyoxometalates

The roles of the central atom (X) in polyanions on the structural, electronic, and redox properties of these two species, Lindqvist polyoxomolybdate [Mo6XO18]n− (X = O, S, Se, and N, P, As), have been systematically investigated using density functional theory methods in this work. It shows that the bond lengths between Mo and central atom (Mo–X), as well as bridging and terminal atoms (Mo–Ob and Mo–Ot), increase with increasing size of the central atoms. Moreover, the oxidative capacity of Lindqvist polyoxomolybdate depends on the central atom X, [Mo6O19]2− < [Mo6SO18]2− < [Mo6SeO18]2− and [Mo6NO18]3− < [Mo6PO18]3− < [Mo6AsO18]3−. Within each row in the periodic table, the reduction energy (RE) values were computed to be more negative for X = O, S, and Se comparing with those of [Mo6XO18]3− (X = N, P, and As). The results suggest that the charge of central atom determines the RE value. Based on the analysis of NBO, the nucleophilic activity of bridging oxygen Ob and terminal oxygen Ot in Lindqvist polyoxomolybdate increase from X = O to S and Se or from X = N to P and As. In contrast to the central atom O in [Mo6O19]2−, the central atoms S, Se, P, and As contribute to the highest occupied molecular orbital (HOMO) of [Mo6XO18]n−. The trends of related properties for Lindqvist polyoxotungstates [W6XO18]n− (X = O, S, Se, and N, P, As) are similar to those of polyoxomolybdates.