Polyoxometalates in solution: speciation under spotlight

Polyoxometalates (POMs) are a large group of anionic polynuclear metal-oxo clusters with discrete and chemically modifiable structures. In most aqueous POM solutions, numerous, and often highly negatively charged, species of different nuclearities are formed. It is rather difficult to determine the dominant POM species or their combination, which is responsible for the specific POM activity, during a particular application. Thus, the identification of all individual speciation profiles is essential for the successful implementation of POMs in solution applications. This review article summarizes species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations. The ion-distribution diagrams over a wide pH range are presented in a comprehensive manner. These diagrams are intended for the targeted use of POMs, and in a clear form shows species that are in equilibrium at the given pH value. Thus, the data accumulated in this review can serve as both a starting point and a complete reference material for determining the composition of POM solutions. Some examples are highlighted where the POM speciation studies led to a detailed understanding of their role in applications. In doing so, we aim to motivate the POM community for more speciation studies and to make the subject more comprehensible, both for synthetic POM chemists and for scientists with different backgrounds interested in applying POMs in biological, medical, electrochemical, supramolecular and nanochemistry fields, or as homogeneous catalysts and other water-soluble materials.


Nadiia I. Gumerova
Nadiia Gumerova is currently a postdoctoral research fellow at the Department of Biophysical Chemistry at the University of Vienna (Austria). She received her PhD degree in Inorganic Chemistry in 2015 from the Vasyl' Stus Donetsk National University (Ukraine). During her PhD studies, Nadiia Gumerova investigated the solution chemistry of isopoly-and heteropolytungstates, including formation conditions of Anderson type polyoxometalates. In 2017, she received the Lise Meitner Fellowship from Austrian Science Fund and joined the group of Prof. Annette Rompel at the University of Vienna (Austria). She has an enduring interest in the areas of organic-inorganic hybrid polyoxometalate-based materials for their biological application as additives for macromolecular crystallography and as enzyme inhibitors.

Solution and solid-state structural chemistry of polyoxometalates (POMs)
Polyoxometalates (POMs) are a large group of discrete, mostly anionic polynuclear metal-oxo clusters amenable to a variety of chemical transformations. [1][2][3] POMs are generally characterized in the solid state prior to dissolution, and this structural information is used as the framework on which the solution chemistry is developed. The compound isolated in crystalline form may not necessarily be the one with highest abundance in solution. For example, di-and tri-molybdates can be easily crystallized from an acidified molybdate solution at pH B 7, 4,5 but they are not represented as discrete solution species, and the dominating anion at this pH is [Mo VI 7 O 24 ] 6À . 6 In solution, POMs form species that can be protonated and undergo redox processes, which contribute to the utmost importance of the speciation characterization. For the application and/or investigation of POM complexes in aqueous solution, a thorough insight of the solution chemistry by identifying all equilibrium constants and individual speciation profiles is essential in order to understand the reaction mechanism and tune the application conditions accordingly. In rare cases where the solution chemistry is presented in literature, the solution details are often not of the main interest for the authors, and are described only in the supplementary materials. The rapidly growing number of POMs application in solution, especially their catalytic 7 and biological ones, [8][9][10][11] entails the need for a deeper understanding and analysis of the fundamental relationship between POM's structural behavior in the solid state and in solution. This is currently an acute drawback, often leading to an incorrect interpretation and erroneous determination of structure-activity relationships. To greatly facilitate the selection of a suitable POM cluster with classical addenda atoms (V V , Nb V , Ta V , Mo VI and W VI ) for any deliberate and purposeful use in solution, this review will serve as a guide for better understanding of the POM behavior in the liquid phase. The POM solution equilibria presented in details can and should be used to interpret the results obtained in POM solution application for fundamental vision and full understanding of the POM nature.

Speciation in chemistry: key factors affecting POMs solution behavior
The most concise definition of chemical speciation is as follows: composition, concentration, and oxidation state of each of the chemical forms of an element present in a sample. 12,13 The term ''speciation'' is also used to describe the distribution of species in a particular sample, where it is synonymous with the ''species distribution''. 14 This notion has been accepted in such diverse fields as toxicology, clinical chemistry, geochemistry, environmental chemistry, biochemistry and inorganic chemistry. New developments in analytical instrumentation and methodology (electronical, vibrational, X-ray absorption and nuclear magnetic resonance spectroscopy, mass-spectrometry, electrochemistry) allow identifying and quantifying the species present in solution.
The key factors affecting POMs speciation and the mechanism of isopoly-and heteropolyanions formation are the added acid and metal concentrations, kind of interactions and the range of chemical conditions (ionic strength, buffer type, presence of potential heteroatoms, type of countercations, etc.) under which the dissolution takes place. It is not the pH alone that determines POMs speciation; rather, it is the ratio of acid (usually strong inorganic acids such as HCl, HNO 3  concentration. This ratio determines the 'degree of protonation', Z, which is defined as the average number of protons bound to monomeric oxo-metalate in solution. 1 It is widely used in speciation studies, and we will refer to it in the cases where no pH regions are given and Z is used by the authors. Z is defined as the ratio q/p in the general eqn (1): in case of M = V V , l = 1, n = 4, k = 3; and M = Mo VI or W VI , l = 1, n = 4, k = 2.
The main results of speciation studies are distribution curves that represent the percentages, partial mole fractions (a) or equilibrium concentrations of the different chemical species present in solution under given conditions. 15 Concentration distribution curves are generally presented as a function of a single variable, such as pH. For distribution curves of POMs, the 'degree of protonation' Z is commonly used as one single variable. The equilibrium concentrations of various species are calculated by solving the mathematical system of mass balance equations constructed for each component, and these mass balance equations are then solved iteratively for the concentrations of the free components. 16 It should be noted that the thermodynamic equilibrium constants are based on activities that depend on temperature and pressure. Most reported stability constants for POMs are considered as stoichiometric constants, which are expressed as equilibrium concentration quotients, and thus they are valid only at a given ionic strength (m, M) and in a given solvent. As an example, the concentration equilibrium constants lg K c for the formation of Ni-centered Anderson-type polyoxomolybdate (POMo) eqn (2) is calculated according to eqn (3): Potentiometric studies (Fig. 1A) allowed to calculate lg K c for each anion in solution and based on these values the ion distribution diagram depending on the acidity [a, mole fraction = f (Z)] was built (Fig. 1B). 17 To avoid questioning of inconsistent results presented by various authors, throughout this review we deliberately display distribution diagrams without considering concentration or mole fraction and only focus on the pH range in which a particular POM species is stable. As an example, the maximum mole fraction at pH 5.5 for [Mo VI 7 O 24 ] 6À is B75% reported by Cruywagen et al., 6 and is B100% by Maximovskaya et al. 18 Only species confirmed by several methods (preferably NMR spectroscopy among them) and/or groups are included to our analysis and as a result to the diagrams. The speciation analysis for POM with V V , Nb V , Ta V , Mo VI and W VI as addenda atoms and for majority of archetypes is provided; however, due to lacking a ''data base'' for POM anions in solution, some speciation works might have been unintentionally overlooked.
Since the majority of POMs have been formed and studied in water, this review focuses on speciation in aqueous solutions, where many applications take place, e.g. catalysis and biological application. Although the possible supramolecular assembly of POMs with H-binding and counterions is an important topic for POM chemistry and application, this aspect is not covered here. We first discuss the state of the art and methodology in studying polyanions in solution, which often is a complex issue. This section is followed by the detailed description of POM speciation mostly according to the solution pH and organized by their addenda atoms (V V , Nb V , Ta V , Mo VI and W VI ). The common trends in POM solution behavior with the aim of predicting stability trends based on the addenda atom type and structure are discussed throughout the text. Some POM applications performed without POM stability proofs are examined.

Methods of POM investigation in solution
The ambiguity of POMs equilibria in solution requires appropriate experimental techniques and careful interpretation of the results. Ideally, a number of different complementary and orthogonal experimental techniques are necessary to study and understand the distribution of species. However, this extensive characterization is rarely done. An exceptional example for a detailed species analysis is an investigation of acidified orthotungstate solution using electrospray-ionization massspectrometry (ESI-MS), 183 W-NMR and Raman spectroscopy. 19 The polycondensation product, heptatungstate [W VI 7 O 24 ] 6À , has been proven by NMR and Raman spectroscopies to be the main species in an equilibration mixture at pH o 7, but fails to be detected by ESI-MS due to its ESI-induced dissociation into Lindqvist [W VI 6 O 19 ] 2À anion (for more details see Section 3.4.1). The reason leading to the different results obtained via NMR, Raman and MS is the instability of [W VI 7 O 24 ] 6À upon ionization. This study also shows that ESI-MS is mainly applicable to stable polyanions, and that the method of investigation should be selected based on its strengths and weaknesses and by taking into account the specific characteristics of a particular POM.
In order to obtain a spectrum as a fingerprint and assign it to a species, the species must be either homogeneous in solution or solid. The POM solution to be examined requires observation over a varying pH, time range and, if applicable, temperature and concentration range. Currently, solution studies no longer use only potentiometry and vibration spectroscopy as their exclusive methods for characterizing metal oxide systems, but various advanced methods complement these studies, including multinuclear NMR spectroscopy, smallangle X-ray scattering (SAXS), X-ray absorption spectroscopy (XAS, consisting of Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES)) investigations as well as mass-spectrometry. We briefly summarize the characterization methods in context of their application in POM systems. A detailed description of the basic principles of the method, the recording, evaluation and interpretation of the data can be found in the specialized literature. In the following, the methods for examining POMs in solutions are presented in the order of their chronological appearance.

Potentiometry
Potentiometric titration was one of the first methods that was used to investigate POM speciation. 1 In principle, the method involves measuring the hydrogen ion concentration in polyanion solutions as a function of the acid or base added (Fig. 1A) and the total metal ion concentration. 16,20 The values for the formation constants and stoichiometric coefficients are determined by titration, followed by the iterative calculation of the constants of one species in the presence of another and the entire system being described by calculations in an iterative process. The formation constants K c are the basis for species distribution diagrams (Fig. 1B) showing various types of POM that exist at different pH values. However, great care should be taken to ensure that a true equilibrium has been reached for each measurement, that the activity coefficient quotients are reasonably constant (use of supporting electrolyte) and that the potentials of the liquid transitions can be controlled. At the moment, potentiometry is outdated due to the availability of other advanced methods that provide a more accurate picture of the processes in solution.

Electronic and vibrational spectroscopy
Typically, the addenda ions in POMs have d 0 electronic configuration, and as a result, only one absorption band occurs in the UVvis between 190 and 400 nm due to the oxygen-to-metal charge transfer transition. 21,22 The spectra of the reduced ''heteropoly blue'' complexes show intervalence charge-transfer transitions, e.g., Mo V -Mo VI at B700 nm. 23 Electronic spectroscopy implies practically no structural information, but it is one of the easiest ways to check POM stability in solution. 24 (Fig. 2). 25 A detailed description of the hydrolytic stability of {PW 12 } is given in Section 3.4.2.1.
Infrared, Raman and resonance Raman spectroscopy are broadly used in POM chemistry as diagnostic fingerprints. Similarities of spectral band positions, shapes and relative intensities for two compounds strongly indicate that both have identical structures. The characteristic spectrum region is between 1000 and 400 cm À1 where absorptions due to metaloxygen stretching vibrations occur.
A good agreement between the spectra of crystalline and dissolved polyanions shows that the structure of the dissolved anion is the same as that observed in the solid state. Raman spectroscopy is more frequently used for aqueous solution studies of POMs [26][27][28][29] and IR spectroscopy for studies in both aqueous and nonaqueous solvents. 30
2.3.1. Addenda nuclei 51 V NMR. So far, the largest number of measurements for POMs has been carried out at 51 V, a core nucleus with relatively high sensitivity, which provides spectra with line widths in the range from B10 to B800 Hz in diamagnetic polyanions. The chemical shifts (reference VOCl 3 ) in isopoly-and heteropolyvanadates fall in the range between À400 to À600 ppm. Even relatively small structural variations result in separable peaks due to the broad chemical shift range. 33,34 95 Mo NMR. Despite the existence of two NMR-active isotopes, 95 Mo (NA = 15.87%; I = 5/2) and 97 Mo (NA = 9.46%, I = 5/2), Mo NMR is less frequently used due to their low natural abundance and their low gyromagnetic ratios: g( 95 Mo) = À1.751 Â 10 7 rad s À1 T À1 and g( 97 Mo) = À1.788 Â 10 7 rad s À1 T À1 . The 95 Mo nucleus is generally preferred over 97 Mo because of its lower quadrupolar moment. Compared with 183 W NMR, the 95 Mo NMR signals from a typical asymmetric POMo environment are strongly broadened due to the quadrupole moment, which complicates spectral measurements and their interpretation. The earlier measurements of the 95 95 Mo enrichment (96%) and, subsequently, 95 Mo NMR was applied to study aqueous Mo VI solutions. 18 183 W NMR. Despite its low sensitivity, the 183 W NMR is of unique importance in studying polyoxotungstates (POTs). Narrow NMR lines of 183 W with nuclear spin I = 1/2 allow to observe constants of the indirect spin-spin coupling, i.e. 2 J(W-P), 2 J(W-W), which provide structural information. The use of high field spectrometers significantly reduces the sensitivity limitations, although a concentrated sample solution (B1 mol L À1 ) and a long acquisition time are still required. A saturated solution of sodium tungstate is recommended as a reference for the chemical shift. 32 For POTs, the range of 183 W-chemical shifts lays between +260 and À300 ppm and even up to À670 ppm, if the POTperoxocomplexes are taken into account. 32 When looking at reduced POTs or at POTs with incorporated paramagnetic ions even larger chemical shifts from +2500 to À4000 ppm are observed. 93 Nb (NA = 100%; I = 9/2) and 181  Although 17 O is difficult to observe on account of its low natural abundance (0.04%) and its negative quadrupole moment Q( 17 O) = À26 mB, these disadvantages are compensated by its large chemical shift ranging from 1200 to À100. 1,31 To overcome the low natural abundance of 17 O (0.04%), target POMs can be enriched with H 2 17 O. This allows studying the rates of oxygen-isotope exchange between solvent and POM molecules sites and has been proven useful for understanding of POMs equilibria. 36 2B) and the majority of its derivatives, each form is represented only by one signal in the chemical shift range between À15 and À2.5 ppm (relative to 85% H 3 PO 4 ), which allows to directly detect several coexisting species and to determine their concentrations. 38 Other nuclei. 11 B (NA = 80.42%; I = 3/2), 39 19 F (NA = 100%; I = 1/2), 40 27 Al (NA = 100%; I = 5/2) 41 and 195 Pt (NA = 33.7%; I = 1/2) 42 NMR spectroscopy are not as often used, but equally important for POM solution investigation.
If suitable NMR-active cations are present, NMR measurements can also probe cation-POM interactions in solution, which can affect the transformation between POM species. 43 Using 7 Li, 44 23 Na 45 and 133 Cs 46 NMR spectroscopy, the Nyman group successfully studied the cationic association with various POMs in solution.

Mass-spectrometry (MS)
Electrospray-ionization mass-spectrometry (ESI-MS) is suitable for the elucidation of solution phase equilibria of stable upon ionization anions, since it enables semi-quantitative detection of both cationic and anionic species in aqueous solvents with excellent detection limits. POMs are ideal candidates for massspectrometry studies since they exhibit complex isotopic envelopes resulting from the high number of stable isotopes as for tungsten ( 100 Mo, 9.6%), and are intrinsically charged. 47,48 However, the experiments must be carefully designed in order to obtain reliable data without overinterpretation of gas phase data for the solution chemistry. 49 While ESI-MS does not yield any information beyond the mass-to-charge ratio of the analyte, it has high sensitivity and does not impose too many requirements on the system to be analyzed, and timeresolved data can be obtained on very dilute solutions. ESI-MS measurements have been used for comprehensive POM speciation studies with all kind of addenda atoms and significantly contribute to the speciation analysis given in detailed in Section 3.

Small angle X-ray scattering (SAXS)
SAXS is a well-established non-destructive method for probing the size, shape, reactivity, and interactions of dissolved species. 50 This method is very powerful, but so far, an underutilized technique to obtain speciation information on POM solutions. SAXS is fundamentally similar to X-ray crystallography, where a sample is irradiated by a collimated monochromatic X-ray beam. 51 Like nanoparticles and quantum dots, many POMs exhibit high net charge, contain high electrondensity elements (W, Mo and other metals) and therefore scatter X-rays strongly. Since POMs are molecular by definition, solutions in which the clusters are stable must be absolutely monodisperse, and their X-ray scattering data can be simulated very accurately by applying solid-state crystal structure data sets. To date, many POM classes have been thoroughly investigated using SAXS, including POMs of group V (Nb V and Ta V ), 50,51 POTs 52 and their complexes with actinides, 53 large reduced POMs 54 as well as POM supramolecular assemblies. 55 SAXS can be successfully used to investigate speciation in catalytic systems, as one representative example, the Cocontaining POTs speciation as a function of pH, buffer salts, and addition of a chemical oxidant during water oxidation catalysis has been carefully studied. 56

Other methods
POMs have a rich electrochemistry associated with both reduction of tungsten or molybdenum 57 and redox-reaction of heterometals (i.e., incorporated cobalt, ruthenium, iridium, or nickel). These characteristic redox wave peaks can be used to identify the number of terminal oxygen atoms, metastable hydrolysis fragments, new isomers and reduced anions. 58,59 Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) are valuable techniques to probe both the local coordination environment and the oxidation state of POM's atoms either in solution or in solidstate materials. Each kind of atom in the POM cluster can be accessed individually and an average spectrum for each element is observed. 60 Despite XAS (X-ray absorption spectroscopy) being a powerful technique, there are just a limited number of examples for their usage in POM structure analysis. 61,62 Dynamic light scattering (DLS) is aimed to determine whether particles are formed in solutions and, if present, to examine their size. 63 DLS has found its broadest usage in monitoring POM stability during catalytic reactions (e.g. water splitting systems 64 ).

Polyoxovanadates (POVs)
This section is divided into isopoly-and heteropolyvanadates. First, the general behavior of POVs in solution is described, and second, the characteristics of tri-, tetra-, penta-and decavanadates, as well as polyoxophosphovanadates are detailed in the order of increasing nuclearity.
3.  constructed about thirty years ago as a result of rigorous research using 51 V NMR investigations in conjunction with potentiometric titration studies. 72,73 The proposed speciation model ( Fig. 4) postulates, that in aqueous vanadium solutions of pH 4 6, polymers with one to five V atoms (Fig. 3) Table 1. ''previously unknown'' polyoxovanadate anions and cations, which might have been formed in gas phase due to ionization.
Under acidic conditions (pH 5.4) in Schneider's insect medium (SIM: Na 2 HPO 4 , 0.7 g L À1 ; MgSO 4 , 3.7 g L À1 ; KCl, 1.6 g L À1 ; KH 2 PO 4 , 0.45 g L À1 ; NaCl, 2.1 g L À1 and 20 proteinogenic amino acids), the decomposition of {V 10 } is substantially slower and after 2 hours, only 0.06 molar equivalent of {V 4 } (Fig. 3E) was detectable and the rest remained as {V 10 }. 93 Recently EXAFS and XANES spectroscopic investigations were applied to examine the V oxidation state in {V 10 } solution. 61 Under the physiological temperature of 37 1C {V 10 } began to decompose after about 1 h at pH 7.5 (2 mM Tris-HCl buffer), while at room temperature {V 10  The authors, without considering the {V 10 } decomposition into mono-, di-and tetravanadates during the incubation, made a conclusion about the superiority of decavanadate as an antibacterial agent, which in fact definitely requires a more careful study.
An example of the proper use of decavanadate is when applied as an inhibitor for bovine pancreatic ribonuclease A (RNase A). 103 Before evaluating the thermodynamic parameters for the {V 10 } binding to RNase A applying isothermal titration calorimetry (ITC), a stock solution of decavanadate was prepared with a pH between 3 and 5 and checked after 24 hours to verify that equilibrium was established. All experiments have been performed at 25 1C in 30 mM sodium succinate buffer (pH 6.0), which, in accordance with the IPOVs ion-distribution diagram (Fig. 4) and {V 10 } decomposition scheme (Fig. 5), should not lead to decavanadate decomposition. As a result, it is justified to conclude that the intact cluster interacts with RNase A.
Crans and co-authors recently conducted an even more thorough analysis of decavanadate speciation to study its effect on the growth of Mycobacterium smegmatis (M. smeg) and Mycobacterium tuberculosis (M. tb). 101 51 V NMR spectra were recorded at pH between 5.8 and 6.8, where there should not have been any decavanadate hydrolysis according to Fig. 4. The spectra showed 100% intact decavanadate {V 10 } in stock solution and growth media with {V 10 } but without bacterial cells. The addition of growth media containing cells to {V 10 } stock solution immediately caused some decomposition of {V 10 }. 101 These results lead to the conclusion, that {V 10 } can interact with some cell components, leading to {V 10 } decomposition and emphasize the importance to investigate the influence of macromolecules on POM speciation. Ramos et al. 97 Fig. 6A)have been identified by pH-potentiometry and high-field 31 P and 51 V NMR spectroscopy to be present between pH 1 and 10 ( Fig. 7 and Table 2 6B) by 51 V NMR and EPR spectroscopic studies in aqueous solution and in Luria-Bertani (LB) medium (pH 7.4; tryptone, 10 g L À1 ; yeast extract, 5 g L À1 ; NaCl, 10 g L À1 ) to understand mixed valence HPOVs chemoprotective activity against the alkylating agent diethylsulphate (DES) in Escherichia coli DH5a cultures. 108 6B) is more stable in LB than in pure aqueous solution, and is able to react with increasing amounts of DES. According to ion-distribution diagram (Fig. 7), fully oxidized {PV 14 } (Fig. 6A) should be stable between pH 3.5 and 5.5. The tested reduced analog [H 6 5À also decomposes and rearranges rapidly in LB (pH = 7.4), resulting in complete absence of chemoprotective activity against DES. Decomposition products, [V V O 4 ] 3À and {V 10 }, react poorly, or even do not react, with the alkylating agent. The observation of chemoprotective activity for [(CH 3 ) 4 6A) against DES is clearly dependent on the chemical nature and stability of the soluble species in the culture medium after addition of the alkylating agent.

Polyoxoniobates (PONbs) and polyoxotantalates (POTas)
In contrast to POVs, POMos and POTs, polyoxoniobates (PONbs) and polyoxotantalates (POTas) can be stabilized only under basic conditions due to their high negative charge, and their chemistry was initially dominated by the Lindqvist anion [M 6 O 19 ] 8À (M = Nb V , Ta V ) in solution and by its alkali salts in the solid state. 1 Owing to the narrow working pH region (47), the low solubility, and the low reactivity of niobate and tantalate species, the progress of PONb and POTas chemistry is far behind that of POMo or POT chemistry. 109 This chapter is devoted only to isopolyniobates (IPONbs) and -tantalates (IPO-Tas) due to the lack of information about heteropoly PONbs and POTas stability and speciation in solution.
3  Fig. 8B) 111,112 were for a long time the only known PONbs. Spectroscopic and potentiometric studies have shown that the Lindqvist ion is indeed the dominant species of Nb in solutions at pH higher than 7 and room temperature ( Fig. 9). 113 109 their solution behavior needs to be investigated in more details. The speciation diagram for IPONbs is rather incomplete and includes four species {Nb 6 }, {Nb 7 }, {Nb 10 } and {Nb 24 } ( Fig. 9 and Table 3). It should be noted that aqueous polyniobate chemistry at pH lower than 5 remains to be explored.
The maximum intensity of grey color in each box with a single species corresponds to its maximum concentration in the chosen pH region. The grey boxes along the y-axis are positioned according to increasing nuclearity, but do not show the domination over other species at a certain pH range. The structures of the species are presented in Fig. 3, 5 and 6. The 51 V chemical shifts, formation constants and pK a values are summarized in Table 2.  5) and decaniobate {Nb 10 } (Fig. 8B). 117 At pH higher than 12 the hexaniobate ion {Nb 6 } is deprotonated and at lower pH values (Fig. 9), it carries between one and three protons, probably located on the m 2 -bridging O b oxygen atoms (Fig. 8A). According to 17 O NMR measurements and density functional theory (DFT) calculations the protonated states of {Ta 6 } are all observed at lower pH values -monoprotonated species dominate at pH 10.5 and di-protonated at pH 9. 118 Hexaniobate {Nb 6 } (Fig. 8A) in its various protonation states has been characterized in solution and in the solid-state (Table 3). While all hexatantalate protonation states have been a Stoichiometric coefficients and formation constants for:   29,[113][114][115]118,119,126 The range of pH o 5 is not studied well and the precipitation of Nb V 2 O 5 ÁnH 2 O is usually observed. The maximum intensity of grey color in each box with a single species corresponds to its maximum concentration in the chosen pH region. The grey boxes along the y-axis are positioned according to increasing nuclearity, but do not show the domination over other species at a certain pH range. The structures of IPONbs are depicted in Fig. 8 and their formation constants are given in Table 3.  (Fig. 9). 29 The decaniobate ion showed no sign of protonation between pH 6-10, in contrast to the hexaniobate ion which is protonated between pH 8 and 13. 115 The conversion of {Nb 10 } to {Nb 6 } was studied by 17 Fig. 8D) upon adding only alkali chloride salts, even in buffered (1 M HEPES, pH = 7) neutral solutions. 127 The rate of {Nb 10 } to {Nb 24 } conversion increases in raw Cs + 4 Rb + 4 K + 4 Na + 4 Li + and cation concentration and indicates that the alkali cations open the compact {Nb 10 } structure and are primarily responsible for driving the reaction. 127

Polyoxomolybdates (POMos)
Since the 1960s the speciation in acidified solutions of [Mo VI O 4 ] 2À with different concentration and ionic strength has been a subject of comprehensive investigation by potentiometry, NMR spectroscopy, and ESI-MS and was extensively elaborated by Tytko and Glemser, 128 Cruywagen, 6 Pettersson, 129 Maksimovskaya 18 Fig. 10A), which shows its maximum concentration at pH B 5 and can    (Fig. 10C). The structure of the latter species is believed to be the same as that of the [H 2 Mo VI 8 O 28 ] 6À anion, which was crystallized from aqueous solution with isopropylammonium as countercation. 130 Later Maksimovskaya and Maksimov have shown using the same methods, 95 Mo and 17 O NMR, that only heptamolybdate and octamolybdate co-exist in the pH region from 5.5 to 2.5. 18 Moreover, they showed evidence of the di-protonated anion {Mo 7 6 Equilibrium constants obtained by different research groups based on potentiometric and NMR spectroscopic studies under the same conditions agree quite well ( Table 4). The ion-distribution diagram presented here (Fig. 11) is based on potentiometric, Raman and NMR spectroscopic investigations (  (Fig. 10A), have been reported to be simultaneous present at neutral pH, that makes the analysis of their equilibrium important for understanding their function in biological or catalytic application complicated. Ng  (Fig. 10B), as was suggested by Tytko. 135 When studying the sorption of isopolyoxomolybdates into layered double hydroxides by in situ real time infrared spectroscopy, the coexistence of [Mo VI O 4 ] 2À and {Mo 7 } was observed only in the pH range from 5.5 to 5.7 in 0.1 M solution of Mo VI , whereas at pH above 5.7, the predominant anion is the monomeric one. 144 Investigation of the hydrolysis of the phosphodiester bond in the DNA model substrate bis(p-nitrophenyl)phosphate (BNPP, 25 mM) in the presence of Na 6 [Mo VI 7 O 24 ] (25 mM) at 50 1C in the absence of buffer showed that the maximal cleavage reaction was observed at pH = 5.3. 145 95 Mo NMR confirmed that the predominant species at this pH is {Mo 7 } (Fig. 10A), which promotes the hydrolysis. Furthermore, 95 (Fig. 14A), which was the final compound observed at the end of hydrolytic reaction. 146 3.3.1.3. Hexamolybdate [Mo VI 6 O 19 ] 2À and its stability in aqueous solutions. In addition to the five species shown in the iondistribution diagram for IPOMos (Fig. 11), the Lindqvist type hexamolybdate anion [Mo VI 6 O 19 ] 2À ({Mo 6 }) exists in solid state and has the same structure as its niobate analog (Fig. 8A). The   6,18,128,131,132 The maximum intensity of grey color in each box with a single species corresponds to its maximum concentration in the chosen pH region. The grey boxes along the y-axis are positioned according to increasing nuclearity, but do not show the domination over other species at a certain pH range.  6 ] 3+ , that are well characterized by potentiometry, and the equilibrium constants have been determined for different ionic media by various authors (Table 4). 137,149,150 The total molybdate concentration must be lower than 10 À4 M for the mono-and binuclear species to be predominantly present in aqueous solution.  (Fig. 12). 149 3.3.1.5. Computational investigation of IPOMos formation. The speciation diagram for IPOMos does not show species with two to six molybdenum ions (Fig. 11). It is possible that these species exist at low concentration as intermediates in the process of forming larger IPOMos. DFT calculations together with ESI-MS experiments provide 151,152 insights into the possible formation mechanism of the Lindqvist [Mo VI 6 O 19 ] 2À ion (Fig. 8A), which is preferably formed in organic media and is undergoing a transformation in aqueous solution. 1 (Fig. 10E)) exist in solution and in solid state (Table 4). Low-nuclearity IPOMos, which are unstable in solution, tend to form polymeric structures with infinite two-dimensional chains that have been synthesized in solid state reactions 154,155 or hydrothermally (Table 5). 156,157 In situ Raman spectroscopy confirmed that between 170 and 190 1C and pH 7 and 5, chain-like or discrete molecular structures of dimolybdates [Mo VI 2 O 7 ] 2À (Fig. 13B) and trimolybdates [Mo VI 3 O 10 ] 2À (Fig. 13D) are preferentially formed, 158 whereas heptamolybdate {Mo 7 } dominates under ambient pressure at 25 1C (Fig. 11). Longterm equilibrium in solutions of {Mo 7 } leads to its transformation and crystallization of different products depending on temperature and time ( Fig. 13 and Table 5), which should undoubtedly be taken into account when using heptamolybdate. The addition of structuredirecting reagents such as big cage-like organic cations ( This section is divided into parts according to the type of heteroatom and ends with sections on the stability of the two most common archetypes -Keggin (Fig. 14C) and Wells-Dawson (Fig. 14G).    Fig. 14A, and was isolated in solid state in all protonation states (Table 6). 169 Fig. 14G) Mo VI atoms start to form. Besides, the {P 2 Mo 5 } (Fig. 14A) and {PMo 11 } (Fig. 14D) series, protonated trilacunary species {PMo 9 } ( Fig. 14E and F) 178 The results of ESI-MS studies on the pH-dependence of phosphomolybdate solutions from pH between 1.7 and 10.2 confirm the presence of both mono-and trilacunary species. 173 The confusion in the literature about the existence of mono-{PMo 11 } and trilacunary {A-PMo 9 } complexes is understandable, since their spectroscopic characteristics ( 31 P NMR shifts and Raman spectra ( , has been determined from both potentiometric and 31 P NMR measurements. 174,175 In the system with phosphite, phenyl-and methylphosphonate anions structures similar to the H x [P V 2 Mo VI 5 O 23 ] (6Àx)À (x = 0-2) anion 169 (Fig. 14A) dominate. 174 It was shown that partial oxidation of phosphite to phosphate occurs, especially in the thin-walled NMR sample tubes when exposed to fluorescent light. In {X 182 These investigations established the formation of two series of complexes: colorless anions with two As atoms and five or six Mo atoms,  Table 6.  Fig. 14E and Table 7). Pettersson suggests for {As 2 Mo 5 } the same structure as for {P 2 Mo 5 } (Fig. 14A), 182 while the {As 2 Mo 6 } complex was isolated in all protonated states and characterized by SXRD (Table 7 and Fig. 14B). Applying large-angle X-ray scattering (LAXS) investigations on {AsMo 9 } complexes showed them to have the same basic trilacunary structure (Fig. 14E) (Fig. 14C) firstly appears 1 min and 23 s after the acidified molybdate solution is mixed with the acidified phosphate solution at pH 1, which is the optimal acidity for the {PMo 12 } formation. 188 The UV-vis, IR and   1.8. 193,194 The yield of both aand b-forms (at pH 4.0 and 1.2, respectively) is independent of molybdate concentrations between 0.015 and 0.100 M. Later, using ESI-MS the formation of molybdosilicates was detected between pH 1 and 2. 195 The decomposition of {SiMo 12 } is 50% completed immediately after dissolution at pH 4.1. 191 As the pH is increased, the decomposition also proceeds rapidly and at pH 5.  196 Thus, the stability of the lacunary anion relative to the Keggin anion, is higher for molybdogermanates than for molybdosilicates, which leads to a higher number of sandwich derivatives for molybdogermanates. 197 At pH above 2, the stability of {XMo 12 } anions can be increased by the addition of organic solvents. 198 Heteropolyanions with protonated amino acids as cations can be stable up to pH 6.2, which can provide valuable insights for correct application of these POMos in medical usage. 199 In general, complexes based on lacunary HPOMos are more unstable in aqueous solution than, for example, their tungsten analogs, 197 and there is still insufficient data on lacunary HPOMos stability in solution. Heteropolymolybdates follow a generally accepted trend -the substitution of addenda atoms with transition metals in Keggin clusters reduces the net charge and stabilizes therewith the anions (Table 8) Fig. 14C) and stay intact in aqueous solution between pH 4 and 10. 200 3.3.2.5. Hydrolytic stability of HPOMos with Anderson structure. HPOMos with Anderson structure can be synthesized from aqueous solution between pH 4 and 5 and are stable in near neutral aqueous media. 1,201 There are only a few papers discussing the hydrolytic stability of the Anderson POMos with Al III , 202 Cr III , 203 Ni II 17 and Ga III 204 as a central heteroatom (Fig. 16A and  B). UV-vis studies revealed that [Cr III (OH) 6 Mo VI 6 O 18 ] 3À is stable in a fairly narrow pH range of 4.50 to 5.83 (Fig. 17). 203 In solutions with an aluminum concentration of 1 mM and ratio [Mo]/[Al] between 0.1 and 1, [Al III (OH) 6 Mo VI 6 O 18 ] 3À is the predominate species over IPOMos (octa-and heptamolybdate) between pH 2 and 5 (Fig. 17). 202 According to potentiometric and 27 Al NMR data, the formation constant for this species is lg K c = 50.95 AE 0.04 in 0.6 M NaCl medium at 25 1C based on reaction (4): and then at Z 4 1.29 converted to {Mo 7 } (Fig. 1B) accompanied by the release of Ni 2+ (8) and a decrease in acidity (9) The described decomposition pathway could be principally applied to Anderson HPOMos with different di-and trivalent ions. Noteworthy, the organic functionalization can significantly increase the stability of Anderson HPOMos. Thus, the alkoxy-functionalized Anderson gallatomolybdate (Fig. 16B) is reported by ESI-MS to be stable up to pH 9 (Fig. 17). 204 (Fig. 16C) has been verified by Raman and UV-vis spectroscopies and showed that the Cocontaining HPOMos is the predominant species between pH 1 and 4.5 (Fig. 17). 212

Polyoxotungstates (POTs)
Polyoxotungstate species are overall more stable than their molybdate analogues due to the enthalpy factor of the condensation process when the tetrahedrally coordinated W VI or Mo VI ions expand their coordination spheres from four to six during POM formation reaction. [1][2][3] Part of this effect can be rationalized in terms of the slightly larger force constant of the W-O bonding as compared to that of the Mo-O bonding. 213 Owing to the existence of a number of simultaneous equilibria, some of which are very slow to establish, and the metastability of some anions, speciation in isopolytungstates (IPOTs) solutions has not been studied as extensively as in Mo VI or V V systems. However, the solution stability of the phosphotungstate anion [P V W VI 12 O 40 ] 3À ({PW 12 }, Fig. 2B) was investigated as thoroughly as no other POM species. 38 3.  217,218 and lately by ESI-MS. 19,219 When an aqueous solution of tungstate is acidified, condensation reactions lead to the formation of IPOT anions, of which the following types are certainly present in solution and in the solid phase (  , Fig. 18D). Studies of how the pH affects aqueous solutions of [W VI O 4 ] 2À are difficult to perform, partly due to very slow equilibria, and they are often problematic to  (Fig. 18B), which have been isolated in numerous salts (Fig. 19 and Table 9). At pH lower than 5, equilibria are established very slowly, and most IPOTs anions are metastable, with the exception of the metatungstate anion a-[H 2 W VI 12 O 40 ] 6À (Fig. 18C). Acidification of paratungstate solutions leads to ''pseudometatungstates'', which ultimately form the stable metatungstate anion (Fig. 18C). From solutions suggested to contain ''pseudometatungstates'', the salt K 6 H 4 [W VI 11 O 38 ]Á11H 2 O, with an anion of C s symmetry (Fig. 18E), has been crystallized. 220 However, the 183 W NMR spectrum of the similar acidified paratungstate solution shows a species with 11 resonances of equal intensity, which has tentatively been assigned to [H 7 W VI 11 O 40 ] 7À , a lacunary form of paratungstate-B (Fig. 18F), and also two lines attributed to decatungstate {W 10 } (Fig. 18D), once known as tungstate-Y, a species that is stabilized in nonaqueous solvents.

Aging of tungstate solution. Conversions of paratungstates.
It is a well-known experimental fact that, from aged fully equilibrated tungstate solutions close to neutral and slightly acidic pH (Fig. 19), the isolation of paratungstate B {W 12 O 42 } (Fig. 18B) prevails. 218 Potentiometric studies in the [W VI O 4 ] 2À -H + system with different aging times (0-20 000 min from the preparation) show that heptatungstate (Fig. 18A) and possibly unstable hexatungstate transform over time into protonated forms of paratungstate B (Fig. 18B), the existence of which was proven by crystallization, 223 Taking together the results obtained by ESI-MS, 183 W NMR and Raman spectroscopy, it must be concluded that the heptatungstate (Fig. 18A) anion is the main species in freshly prepared solutions under nearly neutral conditions (pH = 5.8 and 6.8). 19 (Fig. 18B) in neutral to slightly acidic solutions, indicating that after conversion paratungstate B tends to crystallize immediately. 218 The reverse transition is also possible as upon prolonged heating of a Na 10 (Fig. 18A). To conclude, heptatungstate {W 7 } (Fig. 18A) is more stable in solution, while paratungstate B {W 12 O 42 } (Fig. 18B) is more likely to crystallize from solution.  6 (Fig. 18G), have been isolated from concentrated weakly alkaline aqueous tungstate solutions, but there is no evidence that these anions are stable in solution. These penta-and hexatungstates contain {WO 6 } octahedra with free octahedra faces showing three terminal oxygen atoms in an unstable fac configuration ( Fig. 18H and G) (Fig. 18A), and therefore might exist in such low concentrations that they cannot be detected experimentally. 218 Concentrated  (Fig. 18B). So far, the stability of high nuclearity species was only demonstrated in the gas phase with ESI-MS. The synthetic procedures for synthesizing these IPOTs require the presence of the sulfite anion, and control experiments without sulfite do not allow the isolation of the compounds in high yields. Once again, not only pH and acidity affect the speciation of POMs, but also the substance present in the medium including choice of countercation, aging time and concentration.  Fig. 8A) and decatungstate anions {W 10 } (Fig. 18D) are well characterized in non-aqueous solutions. The {W 10 } complex becomes stable in the presence of many organic solvents such as acetonitrile, 1,4-dioxane, methanol, dimethylsulfoxide and N,N-dimethylformamide. 230 All IPOTs that are stable in . Heteropolytungstates (HPOTs) are a rapidly growing class of POM chemistry with the largest number of synthesized compounds due to the variety of stably lacunary precursors that can be easily formed from Keggin (Fig. 20) and Dawson (Fig. 24) archetypes and combined with d-or f-metals as well as with organic ligands ( Table 8). The speciation and self-assembly of POMs are intimately related, therefore this section is of particular interest not only for targeted application, but also for the development of new synthetic strategies.  (Fig. 20E-H), followed by formation of dior trilacunary anions (Fig. 20I-K).
{PW 12 }. The hydrolytic conversions of {PW 12 } (Fig. 20A) have been studied for more than a century and most recently were thoroughly reviewed by Maksimovskaya and Maksimov. 38 The analysis of 95 previously published investigations made it  (Fig. 21). 38 A few explanatory remarks need to be added to this scheme, while a detailed discussion is given in ref. 38 (Fig. 20M). 235 At pH higher than 7 three isomers of the trilacunary anion A-a-, A-b-, B-a-[P V W VI 19 O 34 ] 9À (see isostructural molybdates for a-isomers in Fig. 14E and F (Fig. 20A) is stable in dilute solutions below pH 4.5, but the band g-isomer ( Fig. 20B and C) are metastable, slowly converting to the a-form (Fig. 23). 1 (Fig. 23). 237 (Fig. 19). 238,239 The b-{SiW 12 } complex upon alkalization forms three short-lived isomers of the b-[Si IV W VI 11 O 39 ] 8À anion ( Fig. 20F-H), 239 b 1 (C s symmetry), b 2 (C 1 ), and b 3 (C s ), and the isomerization pathway is irreversible in solution and finally yields a-[Si IV W VI 11 O 39 ] 8À (Fig. 20E). Above pH 8, the trilacunary anions a-[Si IV W VI 9 O 34 ] 10À (see isostructural molybdates Fig. 14E and F) Fig. 14E and F) between pH 9 and 10 are metastable, leading to monomeric silicate and tungstate (Fig. 23). 238 g-[Si IV W VI 12 O 40 ] 4À (Fig. 20C) is stable only in non-aqueous or mixed water-ethanol or dioxane media. Upon dissolution of g-{SiW 12 } in acidic aqueous media (pH o 4), only its aand b-forms were detected. 240 At pH between 4 and 7, the highly reactive 241 g-[HSi IV W VI 10 O 36 ] 7À (Fig. 20I) is present in freshly prepared solutions (Fig. 23). Then the band finally the a-isomer of {SiW 11 240 Another labile g lacunary complex has been determined by ESI-MS as an intermediate during the transformation from b-{SiW 11 } to g-{SiW 10 }. 243 The unexpected labile precursor g-{SiW 9 } is capable of a direct bg isomerization via a rotational transformation (Fig. 23) (Fig. 20A) and 8% b-{Al 2 W 11 } (Fig. 20B). 41 The introductory data on pH stability of some sandwich-like HPOTs archetypes is described in (Table 8) (Fig. 20A), should not be considered as intact species which will undergo at least partial hydrolysis. Due to their good synthetic accessibility they are often used for Fig. 19 Speciation of isopolyotungstates in an aqueous solution with the concentration of 0.01-0.1 M W VI based on works. 132,215,217,218 The maximum intensity of grey color in each box with a single species corresponds to its maximum concentration in the chosen pH region. The grey boxes along the y-axis are positioned according to increasing nuclearity, but do not show the domination over other species at a certain pH range.  (Fig. 20A)) counterbalanced by their pronounced hydrolysis. A recent theoretical study by Carbo and co-authors also demonstrates that Keggin anions with lower net charge have higher affinity towards proteins. 249 The selective peptide bond hydrolysis for human serum albumin, oxidized insulin chain B, myoglobin and cytochrome c by a range of Ce(IV)-substituted and Zr(IV)-substituted Lindqvist, Keggin and Wells-Dawson POMs was investigated by Parac-Vogt's group along with speciation in solution detailed in her recent review. 250 For instance, a combination of low-temperature 31 P DOSY NMR spectroscopy and theoretical calculations allowed authors to conclude that in an aqueous solution with pD 6.4 the Keggin-type dimer [{a-P V W VI 11 Zr IV (m-OH)(H 2 O)} 2 ] 8À used as enzyme mimetic for the hydrolysis of the phosphoester bonds in the DNA model substrate BNPP is present in equilibrium with its monomeric form. 251 In the proposed reaction mechanism, BNPP initially coordinates to monomeric {Zr IV P V W VI 11 } in a monodentate fashion, which results in hydrolysis of the first phosphoester bond in BNPP and formation of nitrophenyl phosphate.
Many reactions applying POM catalysts are conducted in pure water, 7 and so the hydrolytic stability must be taken into account. A good example is given by Neumann and co-authors investigating a sandwich Krebs type HPOT Na 12 Fig. 24A),   38 The maximum intensity of grey color in each box with a single species corresponds to its maximum concentration in the chosen pH region. The grey boxes along the y-axis are positioned according to increasing nuclearity, but do not show the domination over other species at a certain pH range. The abbreviation {PW 11  , which is stable from pH 1 to 8. Since no detailed investigation of speciation was reported for {P 2 W 18 }, the hydrolytic conversion is presented as a scheme without exact pH ranges (Scheme 1). Contant and Thouvenot studied the isomerism and stability of Wells-Dawson tungstates with P V and As V as heteroions. 258 As for the Keggin anions, band g-isomers have structures in which one and two {W 3 } groups, respectively, have been rotated by 601. Structures in which the central {W 6 } belts are staggered rather than eclipsed are denoted with an asterisk, and out of those, only the g*-isomer has been confirmed. 1 Spectroscopic (IR, Raman, UV-vis, 183 W and 31 P NMR), polarographic and kinetic measurements showed that the a isomers are the only species stable in aqueous solution and all other forms isomerize to it. 258 All isomers of {X 2 W 18 } (X = P V and As V ) under alkaline hydrolysis firstly form monolacunary compounds. The rate of hydrolysis follows the trend for countercations K + 4 Na + 4 Li + and for isomers a o b o g o g*.   (Fig. 24A), {P 2 W 12 } (Fig. 24D) and {P 2 W 15 } (Fig. 24C) to affect aquaporin-3 (AQP3) activity and impair melanoma cell migration has been recently tested. 259 The careful speciation investigation of respective POT clusters at physiological pH 7.4 (phosphate-buffered saline medium; NaCl, 137 mmol L À1 ; KCl, 2.7 mmol L À1 ; Na 2 HPO 4 , 10 mmol L À1 ; KH 2 PO 4 , 1.8 mmol L À1 ) and in water by 31 [260][261][262][263] In the presence of aurone synthase CgAUS 264 {TeW 6 } was even functionalized by carboxylic fragment of protein glutamic acid on the m 2 oxygen atoms, which proves the reactivity of Anderson POTs and demonstrates a special kind of flexibility with respect to both geometric and functional properties. 262 All these examples show the importance of speciation analysis in physiological systems in the presence of biomacromolecules. The double-side trisfunctionalized [Cr III ((OCH 2 ) 3 CC 2 H 5 ) 2 W VI 6 O 18 ] 3À is a predominant species in neutral pH between 5.7 and 7 almost without decomposition fragments, which was shown by ESI-MS. 265

Conclusions
In the presented survey, the ion-distribution diagrams in a wide pH range (analog to ''phase diagrams'') for POMs of V V , Nb V , Ta V , Mo VI and W VI are given. They sum up our current understanding about POM stability and transformations in aqueous media. A brief timeline of POM speciation in aqueous media highlighting the first and the most important studies is given in Fig. S1 (ESI †). POM species in solution may be different to compounds that can be crystallized from the same solutions. It is advisable to use several orthogonal methods to understand and analyze the speciation. Sometimes, conclusions of the speciation differ according to the methods applied and the research method should be selected by taking into account the characteristics of a particular POM.
Throughout, this review factors that affect speciation have been shown. Among the obvious, such as pH (Section 3), ionic strength (Section 3), countercations (Section 3. We hope as an outcome of this review that special attention will be paid to investigation of POM behavior in solution that will facilitate the deeper understanding of their role in various applications. Reliable and accurate published data are of utmost importance for any type of POM project. For deliberate application of POMs, there is a great need for stability data obtained in non-aqueous solutions, water-organic solvent mixtures, or unusual experimental conditions (pressure, temperature, or at high concentrations), as well as in the presence of amphipathic molecules or other micelle-forming organic substances, and for complex formation processes on the surfaces of solid materials. Virtually nothing is known, about the possible role of enzymes or other physiologically occurring biological macromolecules, as well as their substrates with respect to enhancing the breakdown and/or rearrangements of POM species with some exception investigating decavanadate biomolecule interactions. We are confident that the new era of POM chemistry will be opened once the POM speciation Scheme 1 Hydrolytic transformation of {P 2 W 18 }. 257 The structures of species are shown in Fig. 24. gets the attention it deserves and analyzing the behavior of the solution will become a common tool for understanding each POM application.

Conflicts of interest
There are no conflicts to declare.