Protonation Equilibria of Mononuclear Vanadate:  Thermodynamic Evidence for the Expansion of the Coordination Number in VO₂⁺

Abstract

A spectrophotometric investigation of the protonation of HVO(4)(2-) has been conducted at vanadium(V) concentrations low enough (5 x 10(-5) mol dm(-3)) to prevent the formation of polynuclear ions. Equilibrium constants as well as enthalpy and entropy changes for the formation of H(2)VO(4)(-) and VO(2)(+) have been determined in ionic medium: 1.0 mol dm(-3) NaCl and NaClO(4), respectively. The percentage concentration of the neutral species, H(3)VO(4), is so low (apparently <1%) that the protonation of H(2)VO(4)(-) to form VO(2)(+) occurs virtually in a single step. The disproportionately great stability of VO(2)(+) relative to H(3)VO(4) is explained in terms of an increase in the coordination number of vanadium when the cationic species is formed. This explanation is based on a comparison of the thermodynamic quantities of the vanadium(V) species with those of various other oxyacids reported in the literature.

Towards understanding the poor thermal stability of V5+ electrolyte solution in Vanadium Redox Flow Batteries

“…This neutral molecule H 3 VO 4 (or more precisely VO(OH) 3 ) is well known to exist at room temperature and at low pH (∼3.5) [14,17]. Recently, Sadoc et al [18] reported that the hydrated VO 2 + molecule is prone to deprotonation at high temperatures (500 K), resulting in the formation of VO(OH) 2 (H 2 O) 2 + and subsequently the neutral H 3 VO 4 .…”

“…This VO 2 + species reportedly exists in its hydrated form with a six-coordinated octahedral structure in which two oxygen atoms are coordinated in cis-configuration along with four complexed water molecules (i.e. [VO 2 (H 2 O) 4 ] + ) [14]. However, our DFT based geometry optimization revealed that this octahedral vanadate ion structure is not energetically favorable and a slightly more stable structure is formed by repelling a water molecule located at trans position to the vanadyl oxygen.…”

Structural and spectroscopic study of Mg13.4(OH)6(HVO4)2(H0.2VO4)6

“…In vanadium(V) complexes, the (HVO 4 ) 2-group is well known and present in numerous structures [22][23][24]. However, until now a (HVO 4 ) 2-group was not found among purely inorganic compounds.…”

Medium Effects on51V NMR Chemical Shifts: A Density Functional Study

“…Thermodynamic data for the appropriate protonation equilibria have been taken as evidence for a corresponding expansion of the coordination number about vanadium [45] (that is, from four in 1 to six). Indeed, a minimum can be located for [VO 2 (OH 2 ) 4 ] with a distorted octahedral coordination sphere around the metal (2 a, Figure 5).…”