Coordination Chemistry of Vanadium Aquo Complex Ions in Oxidation States +II, +III, +IV, and +V: A Hybrid-Functional DFT Study

Abstract

The vanadium aquo complexes: [V(H₂O)₆]²⁺, [V(H₂O)₆]³⁺, [VO(H₂O)₅]²⁺, and [VO₂(H₂O)₃]⁺. H₂O containing Vⁿ⁺: n = +II, +III, +IV, and +V ion respectively with only H₂O as ligands are the most prevailing ionic species in their aqueous type medicinal and biological fluid matrices, and all-vanadium redox flow battery (VRFB) systems. Since, they tend to display particular configurations with distinctive electronic stabilities, understanding how each adjacent vanadium ion stabilizes itself with specific hydration number and acquires unique equilibrium structure is very indispensable. With this as a major objective, the coordination chemistry of all these four hydrated vanadium complexes are studied here thoroughly by applying a hybrid-functional DFT method. It is found that all the theoretically derived bond lengths (Th.) of each optimized complex ion agree reasonably with the experimental values (Exp.): (a) [V(H₂O)₆]²⁺: V−OH₂ Th. 2.0 Å, Exp. 2.1 Å; (b) [V(H₂O)₆]³⁺: V−OH₂ Th. 1.98 Å, Exp. 1.99 Å; (c) [VO(H₂O)₅]²⁺: equatorial V−OH₂ Th. 2.03 Å, Exp. 2.03 Å, trans V−OH₂ Th. 2.17 Å, Exp. 2.20 Å, and V=O Th. 1.57 Å, Exp. 1.59 Å; (d) [VO₂(H₂O)₃]⁺. H₂O: V=O Th. 1.6 Å, Exp. 1.6 Å, and V−OH₂ Th. 2.0 Å, Exp. 2.0 Å. Similarly, the bonding patterns and a closed 3D coordination geometry of each complex ion revealed through the theoretically generated electron density map (mapped with the total density) are also very reliable. The importance and originality of this study lies in deriving all the structural data sets and characteristic coordination geometry of each hydrated vanadium complex ion theoretically as they are very essential while modelling VRFB simulator.