Monoﬂuorophosphates—New Examples and a Survey of the PO 3 F 2 − Anion

: During a systematic study of monoﬂuorophosphates, i.e., compounds comprising the tetrahedral anion PO 3 F 2 − , twelve, for the most part new, compounds were obtained from aqueous solutions. Crystal structure reﬁnements based on single crystal X-ray diffraction data revealed the previously unknown crystal structures of CdPO 3 F(H 2 O) 2 , Cr 2 (PO 3 F) 3 (H 2 O) 18.8 , Pb 2 (PO 3 F)Cl 2 (H 2 O), (NH 4 ) 2 M (PO 3 F) 2 (H 2 O) 2 ( M = Mg, Mn, O–P–F = 104.8(1.7) ◦ , using a dataset of 88 independent PO 3 F 2 − anions or entities. For those crystal structures of monoﬂuorophosphates where hydrogen bonding is present, in the vast majority of cases, hydrogen bonds of the type D –H ··· F–P ( D = O, N) are not developed.


Introduction
The family of monofluorophosphates comprising the PO 3 F 2− anion was introduced by Lange more than 90 years ago [1]. In the PO 3 F 2− anion, the fluorine atom is directly bound to the phosphorus atom. However, in the literature, compounds with discrete PO 4 3− and F − anions are also sometimes incorrectly described as "fluorophosphates", e.g., Mn 2 PO 4 F [2] or Na 2 FePO 4 F [3]. These compounds correctly belong to the family of 'phosphate fluorides'. Various preparation methods for monofluorophosphates as well as applications of this family of compounds as additives in toothpastes, wood preservatives, corrosion inhibitors, solubility inhibitors for lead in potable water sources, or as active agents against osteoporosis or caries during biomineralization of fluoroapatite were summarized some time ago [4]. More recently, some monofluorophosphates were also shown to exhibit excellent nonlinear optical (NLO) behaviour [5].
In his seminal paper, Lange emphasized the chemical relationships between monofluorophosphates and sulfates in terms of solubilities and reaction behaviours. In fact, the PO 3 F 2− anion is isoelectronic with the SO 4 2− anion, and both anions have a tetrahedral shape, as later evidenced by the very first structure determination of a monofluorophosphate [6]. Prior to this first experimental proof about the structure and shape of the PO 3 F 2− anion, it was assumed that monofluorophosphates are isomorphic with corresponding sulfates [7]. It should be noted that also the terms isomorphic/isomorphism still are found in literature to express structural relationships, but their use is not recommended any longer [8]. More appropriate terms are isostructural/isostructurality or synonymous isotypic/isotypism. Meanwhile, numerous other monofluorophosphates were synthesized and structurally determined, but not for all reported monofluorophosphates corresponding sulfates even exist or show isotypism with existing sulfates.
The current study was undertaken to add more examples of structurally determined monofluorophosphates with inorganic cations to the already existing list of this family of compounds. Although some of the monofluorophosphates investigated during this study have previously been reported and their powder diffraction data deposited in the International Centre for Diffraction Data's (ICDD) powder diffraction file (PDF) [9], structural details of corresponding phases are still missing up to now. As it turned out, some of the data compiled in the PDF at that time are incorrect (wrong space groups, wrong unit cell volume) and were revised during the current study. Moreover, results of the present crystal structure analyses were used under special emphasis to review structural characteristics (bond lengths and angles, point group symmetries) and hydrogen-bonding features of the PO 3 F 2− anions as well as their structural relationships to corresponding sulfates. O and F atoms differ only in one electron and thus have very similar atomic form factors for X-rays. Consequently, a distinction of the two atom types on the basis of X-ray diffraction methods alone is not free from ambiguity, as was recently shown for minerals that were claimed to comprise monofluorophosphate groups [10]. Nevertheless, the result of the current structure evaluation for the PO 3 F 2− anion is a useful tool to correctly assign F and O atoms in monofluorophosphates, as exemplified by using structure data of a published crystal structure with incorrectly assigned F and O atoms.

Syntheses and Single Crystal Growth
For syntheses of NH 4 + -containing monofluorophosphates, the starting compound (NH 4 ) 2 PO 3 F(H 2 O) was prepared according to Schülke and Kayser [11] and its purity checked with X-ray powder diffraction (XRPD). One gram of this material was dissolved in 10 mL of a methanol/water mixture (1:1 v:v). Then, 80 mg solid AgNO 3 were added to this solution to precipitate the phosphate anions present due to incomplete conversion or partial hydrolysis of the PO 3 F 2− anion. The yellow Ag 3 PO 4 was filtered off, and the filtrate was repeatedly checked for PO 4 3− anions by adding a few drops of an AgNO 3 solution until no more clouding was observed in the filtrate, ensuring that all PO 4 3− anions were removed. Then, 10 mL of a solution consisting of 500 mg of the respective metal chloride in methanol/water (1:1 v:v) were added to the monofluorophosphate solution. The excessive Ag + ions were precipitated as AgCl and filtered off. To the remaining clear filtrate 100 mL of an acetone/methanol solution (2:1 v:v) were slowly added, resulting in flocculent precipitates in all cases. The respective suspensions were stirred for one hour and then were filtered. The obtained solids were washed with methanol and acetone and then dried in an exsiccator overnight. XRPD revealed an amorphous state for the obtained materials, except for (NH 4 ) 2 Mg(PO 3 F) 2 (H 2 O) 2 that was obtained as a pure polycrystalline phase (cf. PDF entry #00-059-0045).
For single crystal growth of NH 4 + -containing monofluorophosphates, 100 mg of the as-precipitated solids were dissolved in 10 mL of a methanol/water mixture (1:1 v:v), to some extent under mild warming. The clear solutions were allowed to evaporate for 2-4 days until full dryness. In all cases, the majority of material was still amorphous, and only few single crystals were found to be suitable for X-ray analysis. This way, single crystals of (NH 4 ) 2 M(PO 3 F) 2 (H 2 O) 2 (M = Mg, Co, Mn), (NH 4 ) 2 Ni(PO 3 F) 2 (H 2 O) 6 , NH 4 Cr(PO 3 F) 2 (H 2 O) 6 and NH 4 Cu 2 (H 3 O 2 )(PO 3 F) 2 could be obtained from the respective metal salt solution. Single crystals of Cr 2 (PO 3 F) 3 (H 2 O) 18 4

(H 2 O).
Single crystals of CdPO 3 F(H 2 O) 2 and Pb 2 (PO 3 F)Cl 2 (H 2 O) were obtained from metathesis reactions. For this purpose, 200 mg Ag 2 PO 3 F (prepared according to [4]) were dissolved in 10 mL of water; equimolar amounts of CdCl 2 and PbCl 2 , respectively, were added to the solution, resulting in an immediate precipitation of AgCl. The suspension was stirred for two hours, AgCl filtered off, and the filtrate allowed to evaporate until complete dryness. CdPO 3 F(H 2 O) 2 was obtained as a single phase material, whereas only few single crystals of Pb 2 (PO 3 F)Cl 2 (H 2 O) could be isolated. In the latter batch, polycrystalline 2PbCO 3 ·Pb(OH) 2 was also determined by XRPD next to a dark-brown to metallic film deposited at the surface of the glass. The formation of the film points to silver that apparently was also present in the filtrate and was reduced to its metallic form during evaporation.

Single Crystal Diffraction and Structure Analysis
Single crystals were optically preselected under a polarising microscope, embedded in perfluorinated polyether for protection from air and humidity and mounted on MiTeGen MicroLoops TM for the diffraction studies. Experimental details of the data collections and refinements are collated in Table 1.
All crystal structures were initially solved with SHELXS [12] and refined with SHELXL [13]. For the renewed refinement of ZnPO 3 F(H 2 O) 2.5 and (NH 4 ) 2 Ni(PO 3 F) 2 (H 2 O) 6 , the original atom labelling and atomic coordinates (as starting parameters) were resumed from the original structure reports [14,15]. For NH 4 Cu 2 (H 3 O 2 )(PO 3 F) 2 , atom labelling and coordinates were adopted from isotypic KCu 2 (H 3 O 2 ) 3 (SO 4 ) 2 [16]. In cases where H atom positions were clearly discernible from difference Fourier maps, the corresponding sites were refined with soft restraints on N-H or O-H bond lengths. In cases where H atom positions could not be unambiguously located, H atoms were not considered in the final model, but are part of the chemical formula, X-ray density, etc. These cases apply to Pb 2 (PO 3 F)Cl 2 (H 2 O), Cr 2 (PO 3 F) 3 (H 2 O) 18.8 and (NH 4 ) 2 Zn(PO 3 F) 2 (H 2 O) 0.2 . In the crystal structure model of the chromium compound severe disorder of the free water molecules (i.e., the non-coordinating or structural water molecules) is observed, both in terms of occupational and positional disorder; the same applies for the partly hydrated zinc compound. Site occupation factors (s.o.f.) for these O sites were refined freely without restraints or constrains. Disorder was also observed for NH 4 Cr(PO 3 F) 2 (H 2 O) 6 and NH 4 Cu 2 (H 3 O 2 )(PO 3 F) 2 where the N atom of the ammonium cation is situated on a position with site symmetry 3m and 2/m, respectively, which results in a symmetry-restricted disorder of the corresponding ammonium H atoms. Finally, in the crystal structure of (NH 4 ) 2 Zn 3 (PO 3 F) 4 (H 2 O), the N site of the ammonium cation and the O site of the water molecule share the same fully occupied site with a 2/3 occupation by N and a 1/3 occupation by O.
Further details of the crystal structure investigations may be obtained from The Cambridge Crystallographic Data Centre (CCDC) on quoting the depository numbers listed at the end of Table 1. The data can be obtained free of charge via www.ccdc.cam.ac. uk/structures. Table 2 lists selected bond lengths and angles for all crystal structures, and Table 3 gives numerical details of hydrogen bonding.

Vibrational Spectroscopy
The infrared (IR) spectrum of a powdered CdPO 3 F(H 2 O) 2 sample was recorded as a KBr pellet in the spectral range between 4000 and 400 cm −1 employing a Bruker-EQUINOX-55 FTIR-instrument (Billerica, MA, USA). Raman spectra down to 100 cm −1 , were measured using the FRA 106 Raman accessory of an IF66 Bruker spectrophotometer (Billerica, MA, USA). Radiation from a Nd:YAG solid-state laser (1064 nm) was used for excitation. The spectral resolution was ± 4 cm −1 in both measurements.

Thermogravimetry (TG)
A Netzsch TG209 F1 thermobalance (Selb, Germany) was used for measurement using a corundum crucible in flowing nitrogen atmosphere and a heating rate of 20 • C/min.

Results
In the current study, only (NH 4 ) 2 Mg(PO 3 F) 2 (H 2 O) 2 and CdPO 3 F(H 2 O) 2 were obtained as pure and crystalline phases and in amounts sufficient for the application of other current analytical methods (vibrational spectroscopy, thermogravimetry). All other monofluorophosphates either were obtained in form of multi-phase material or in form of only few single crystals next to amorphous material. This allowed in all cases the determination of the crystal structure but prevented further analytical measurements.

CdPO 3 F(H 2 O) 2
All atoms in the crystal structure of CdPO 3 F(H 2 O) 2 are located on general sites. The cadmium cation exhibits a distorted octahedral coordination sphere defined by two cisaligned water molecules (OW1, OW2) and four O atoms from four PO 3 F 2− anions. Two [CdO 4 (OH 2 ) 2 ] octahedra share an edge to form a dimer {Cd 2 O 6 (OH 2 ) 4 }; adjacent dimers are linked by corner-sharing with PO 3 F 2− groups into layers extending parallel (001). An intralayer hydrogen bond between two water molecules (OW1 and OW2) consolidates this arrangement. Neighbouring layers are held together by medium-strong to weak and partly bifurcated hydrogen bonds between both water molecules and O1 and O2 atoms of the monofluorophosphate anions ( Figure 1).

CdPO3F(H2O)2
All atoms in the crystal structure of CdPO3F(H2O)2 are located on general sites. The cadmium cation exhibits a distorted octahedral coordination sphere defined by two cisaligned water molecules (OW1, OW2) and four O atoms from four PO3F 2− anions. Two [CdO4(OH2)2] octahedra share an edge to form a dimer {Cd2O6(OH2)4}; adjacent dimers are linked by corner-sharing with PO3F 2− groups into layers extending parallel (001). An intralayer hydrogen bond between two water molecules (OW1 and OW2) consolidates this arrangement. Neighbouring layers are held together by medium-strong to weak and partly bifurcated hydrogen bonds between both water molecules and O1 and O2 atoms of the monofluorophosphate anions ( Figure 1). On the basis of the known structural data, it is possible to perform an analysis of the vibrational-spectroscopic behaviour of the PO3F 2− anion present in the CdPO3F(H2O)2 crystal structure, using the simple site-symmetry approximation [17][18][19][20]. Since the monofluorophosphate anion is located on a general C1 position, the symmetry of the "free" PO3F 2− anion (C3v) was correlated with its site symmetry (C1), as shown in Table 4. From these On the basis of the known structural data, it is possible to perform an analysis of the vibrational-spectroscopic behaviour of the PO 3 F 2− anion present in the CdPO 3 F(H 2 O) 2 crystal structure, using the simple site-symmetry approximation [17][18][19][20]. Since the monofluorophosphate anion is located on a general C 1 position, the symmetry of the "free" PO 3 F 2− anion (C 3v ) was correlated with its site symmetry (C 1 ), as shown in Table 4. From these results, it becomes evident that, under site symmetry conditions, the three double degenerated E modes are split, and all vibrations present IR and Raman activity. The FTIR spectrum of CdPO 3 F(H 2 O) 2 is quite simple (Figure 2a) and can be clearly correlated with the results of this analysis. In the Raman spectrum, a more reduced number of bands was observed which, notwithstanding, was useful to additionally support the performed assignments, which are shown in Table 4 and briefly commented on as follows: - Regarding the vibrations of the water molecules, the O-H stretchings are seen as a relatively broad and clearly splitted band due to the presence of two crystallographically different water molecules. The positions of these bands are characteristic for the presence of hydrogen bridges of medium strength [20], in agreement with the results of the structure analysis. Interestingly, the corresponding deformational mode, δ(H 2 O), shows also splitting signals. - The antisymmetric ν(PO 3 ) vibration was not observed in the Raman spectrum, whereas in the IR spectrum it is very strong and broad. In accord with the predictions of the site-symmetry analysis two components can be seen. The corresponding symmetric stretching vibration is the strongest Raman band in both compounds and is also relatively strong in the IR spectrum. - The ν(P-F) vibration can be clearly identified in the spectra, lying at somewhat higher energy than that observed in the solution Raman spectrum (795 cm −1 ) [20]. -For the deformational modes only δ(PO 3 ) could be identified, clearly split in the IR spectra as predicted (cf. Table 4), whereas no signals for the δ(FPO 3 ) mode could be found. In the Raman spectrum of a PO 3 F 2− solution, both vibrations are reported at the same energy (520 cm −1 ) [20], although in the case of crystalline Hg 2 PO 3 F, both vibrations were identified at slightly different wavenumbers, with ν 5 > ν 3 [21]. - The corresponding ν 6 -PO 3 -rocking mode was only identified in the Raman spectrum, as a very weak band.  The TG curve of CdPO3F(H2O)2 and the associated difference curve ist depicted in Figure 2b. The dihydrate starts to decompose with an onset temperature of 144 °C accompanied with a first dehydration step that can be grouped into two separated events. Considering a mass loss of 7.3% per water molecule in the formula unit, the first dehydration event is associated with the loss of about one water molecule (maximum in the difference curve at 159 °C and a mass loss of 6.8%), followed by a second event (maximum in the difference curve at 177 °C) with the release of about one-third of a water molecule (2.9%) relative to the formula unit. The second dehydration step (maximum in the difference curve at 270 °C) is indicated by the release of about two-thirds of a water molecule per formula unit with a further mass loss of 5.8%. The formation of the anhydrous compound is completed at 280 °C (expected overall mass loss 14.6%, observed 15.5%). Above this temperature, the remaining phase(s) gradually decompose(s), and Cd2P2O7 [22] was identified by PXRD as the only crystalline reaction product obtained at 1000 °C. For a clear interpretation of this last decomposition step, coupled mass-spectroscopic studies of the gaseous products released during the TG experiment and temperature-dependent PXRD would have been required. In general, monofluorophosphates show a rather complex thermal decomposition behaviour, as exemplified by the cases of (NH4)2Mg(PO3F)2(H2O)2 [23] with Mg2P4O12, of Ag2PO3F [4] with Ag4P2O7 and Ag3PO4, and of SrPO3F(H2O) [24] with Sr2P2O7 and Sr5(PO4)3F, respectively, as the final products.

Cr2(PO3F)3(H2O)18.8
The crystal structure of Cr2(PO3F)3(H2O)18.8 is rather complex, with five Cr sites (two of which (Cr1, Cr2) are situated on inversion centres on Wyckoff positions 1a and 1h, respectively), with six PO3F 2− anions (one of which (P6) shows positional disorder of the F6 atom over two set of sites), with 24 O atoms associated with aqua ligands of the Cr(III) atoms, and with 14 crystal water molecules (five of which (O1W-O5W) are positionally and occupationally disordered over multiple sites).
The five chromium(III) atoms are solely ligated by water molecules in an octahedral manner and are isolated in the crystal structure. The corresponding Cr-Owater distances are normal and agree with other [Cr(OH2)6] octahedra, e.g., like those found in alums [25]. The TG curve of CdPO 3 F(H 2 O) 2 and the associated difference curve ist depicted in Figure 2b. The dihydrate starts to decompose with an onset temperature of 144 • C accompanied with a first dehydration step that can be grouped into two separated events. Considering a mass loss of 7.3% per water molecule in the formula unit, the first dehydration event is associated with the loss of about one water molecule (maximum in the difference curve at 159 • C and a mass loss of 6.8%), followed by a second event (maximum in the difference curve at 177 • C) with the release of about one-third of a water molecule (2.9%) relative to the formula unit. The second dehydration step (maximum in the difference curve at 270 • C) is indicated by the release of about two-thirds of a water molecule per formula unit with a further mass loss of 5.8%. The formation of the anhydrous compound is completed at 280 • C (expected overall mass loss 14.6%, observed 15.5%). Above this temperature, the remaining phase(s) gradually decompose(s), and Cd 2 P 2 O 7 [22] was identified by PXRD as the only crystalline reaction product obtained at 1000 • C. For a clear interpretation of this last decomposition step, coupled mass-spectroscopic studies of the gaseous products released during the TG experiment and temperature-dependent PXRD would have been required. In general, monofluorophosphates show a rather complex thermal decomposition behaviour, as exemplified by the cases of (NH 4 ) 2 Mg(PO 3 F) 2 18.8 The crystal structure of Cr 2 (PO 3 F) 3 (H 2 O) 18.8 is rather complex, with five Cr sites (two of which (Cr1, Cr2) are situated on inversion centres on Wyckoff positions 1a and 1h, respectively), with six PO 3 F 2− anions (one of which (P6) shows positional disorder of the F6 atom over two set of sites), with 24 O atoms associated with aqua ligands of the Cr(III) atoms, and with 14 crystal water molecules (five of which (O1W-O5W) are positionally and occupationally disordered over multiple sites).
The five chromium(III) atoms are solely ligated by water molecules in an octahedral manner and are isolated in the crystal structure. The corresponding Cr-O water distances are normal and agree with other [Cr(OH 2 ) 6 ] octahedra, e.g., like those found in alums [25].
[Cr(OH 2 ) 6 ] octahedra associated with Cr3, Cr4, and Cr5 are arranged in (011) (Figure 3). Although H atoms could not be located for the water molecules, it is evident that hydrogen bonding is the crucial force for stabilising the stacking arrangement in this crystal structure. Within an anionic {[Cr(OH 2 ) 6 ] 3 (PO 3 F) 6 13.6 } 3+ layers one might expect also O-H···F hydrogen bonds in this crystal structure. However, a clear localisation of (disordered) H atoms will be possible only by the application of neutron diffraction, provided that crystals large enough for this diffraction technique can be grown.
[Cr(OH2)6] octahedra associated with Cr3, Cr4, and Cr5 are arranged in (011) layers that are sandwiched by the six monofluorophosphate anions, giving an overall composition of {[Cr(OH2)6]3(PO3F)6} 3− 13.6} 3+ layers one might expect also O-H···F hydrogen bonds in this crystal structure. However, a clear localisation of (disordered) H atoms will be possible only by the application of neutron diffraction, provided that crystals large enough for this diffraction technique can be grown.
The formula of this compound can also be written as [Cr(H2O)6]2(PO3F)3·6.8H2O. Considering full occupancy of the positionally and occupationally disordered five crystal water sites, this would result in a ‚19-hydrate', i.e., [(Cr(H2O)6]2(PO3F)3·7H2O. This amount of water is close to that reported for highly hydrated violet chromium(III) sulfates that are described to contain about 18 water molecules [26]. However, crystal structure determinations of corresponding chromium(III) sulfate hydrates have not been performed up to now.    6 ] 2 (PO 3 F) 3 ·7H 2 O. This amount of water is close to that reported for highly hydrated violet chromium(III) sulfates that are described to contain about 18 water molecules [26]. However, crystal structure determinations of corresponding chromium(III) sulfate hydrates have not been performed up to now.

Pb 2 (PO 3 F)Cl 2 (H 2 O)
The crystal structure of Pb 2 (PO 3 F)Cl 2 (H 2 O) comprises two unique lead(II) atoms, two chloride anions, one monofluorophosphate anion and one water molecule of an aqua ligand. Except one oxygen atom of the latter (O2; H atom(s) could not be determined), all other atoms are situated on a mirror plane (Wyckoff position 4c). The two lead(II) atoms exhibit different coordination environments whereby in each case the F atom of the monofluorophosphate anion is not part of the first coordination sphere (shortest Pb-F distances are 3.624(3) Å for Pb1 and 3.912(3) Å for Pb2). Pb1 has a coordination number of 7 (considering distances less than 3.5 Å) and is bonded to two monofluorophosphate O atoms, the O atom of the aqua ligand and four chloride anions. Pb2 has a coordination number of 9, with one very short and two short distances to monofluorophosphate O atoms, two bonds to chloride anions and two pairs of long bonds to monofluorophosphates O atoms. The corresponding [Pb1O 2 (H 2 O)Cl 4 ] and [Pb2O 7 Cl 2 ] polyhedra are irregular and share vertices and edges to build up a three-dimensional framework structure (Figure 4). The water molecules protrude into the interstices present in this framework. The monofluorophosphate tetrahedron shares all its O atoms with the framework, whereby the P and F atoms are also oriented towards the interstices of the framework. The next nearest distance between two water molecules in this section of the structure amounts to 3.102(7) Å; three shorter distances to the monofluorophosphate F atom (2.848(8) Å and twice 3.007(4) Å) are also present, making O-H···O and also O-H···F hydrogen bonding interactions possible. Since H atoms could not be determined, detailed hydrogen bonding interactions cannot be provided.
The crystal structure of Pb2(PO3F)Cl2(H2O) comprises two unique lead( chloride anions, one monofluorophosphate anion and one water molecule and. Except one oxygen atom of the latter (O2; H atom(s) could not be de other atoms are situated on a mirror plane (Wyckoff position 4c). The two exhibit different coordination environments whereby in each case the F ato ofluorophosphate anion is not part of the first coordination sphere (shor tances are 3.624(3) Å for Pb1 and 3.912(3) Å for Pb2). Pb1 has a coordinatio (considering distances less than 3.5 Å) and is bonded to two monofluoroph oms, the O atom of the aqua ligand and four chloride anions. Pb2 has a coor ber of 9, with one very short and two short distances to monofluorophosp two bonds to chloride anions and two pairs of long bonds to monofluoro atoms. The corresponding [Pb1O2(H2O)Cl4] and [Pb2O7Cl2] polyhedra are share vertices and edges to build up a three-dimensional framework struct The water molecules protrude into the interstices present in this framewo fluorophosphate tetrahedron shares all its O atoms with the framework, w and F atoms are also oriented towards the interstices of the framework. Th distance between two water molecules in this section of the structure amou Å; three shorter distances to the monofluorophosphate F atom (2.848 (8) 3.007(4) Å) are also present, making O-H···O and also O-H···F hydrogen bo tions possible. Since H atoms could not be determined, detailed hydrogen actions cannot be provided.

ZnPO 3 F(H 2 O) 2.5
The crystal structure of ZnPO 3 F(H 2 O) 2.5 has been determined previously from a single crystal X-ray data set at room temperature, using a CAD-4 four-circle diffractometer equipped with a point detector. Since only parts of the water hydrogen atoms could be located at that time [14], the crystal structure model remained incomplete, in particular in terms of hydrogen-bonding interactions. The current re-refinement unambiguously revealed all hydrogen atoms, making a complete assignment of hydrogen-bonding interactions possible.
Two of the three unique zinc cations (Zn2, Wyckoff position 1h and Zn3, Wyckoff position 1a) are located on inversion centres; all other atoms are in general sites. Zn1 has a tetrahedral coordination environment and is bonded to the O atoms of four monofluorophosphate tetrahedra. Two such units dimerise into an inversion-symmetric {Zn1 2 (PO 3 F) 6 6 } units to define a three-dimensional framework structure. O-H···O hydrogen bonding of medium strengths between the coordinating water molecules and the monofluorophosphate O atoms reinforces this arrangement. There is an additional crystal water molecule (O2) present in the structure acting both as a donor and an acceptor group. O2 donates weak hydrogen bonds (partly bifurcated) to two ligand water O and one monofluorophosphate O atoms, and accepts medium-strong hydrogen bonds of two coordinating water molecules ( Figure 5).     [27]. Instead of the Ccentred unit cell for this structure type, PDF entry #00-045-0355 for (NH 4 ) 2 Co(PO 3 F) 2 (H 2 O) 2 reports a primitive unit cell (without further assignment of possible space groups) with lattice parameters of a = 12.3817(1), b = 5.3449(5), c = 7.3894(6) Å, β = 98.930(8) • , V = 483.10 Å 3 . Comparison with the current single crystal X-ray study (Table 1) revealed virtually the same lengths of the b and c axes and the same unit cell volume. Preparation, chemical analysis as well as infra-red spectroscopic measurements and thermal behaviour of (NH 4 ) 2 Mg(PO 3 F) 2 (H 2 O) 2 were already reported some time ago, without determination of the crystal structure. The originally given lattice parameters (a = 15.476(4), b = 5.372(1), c = 13.416(3) Å, β = 118.76(1) • ; determined from polycrystalline material using a Guinier camera) and two possible space groups (Cc or C2/c) [23] do not match with the current single crystal data with a halved unit cell volume (978 Å 3 for [23] versus 484 Å 3 in the current singly crystal study) and space group C2/m. Nevertheless, the deposited X-ray powder diffraction data (PDF entry #00-039-029) can be indexed with the actual halved cell. Rietveld refinement of (NH 4 ) 2 Mg(PO 3 F) 2 (H 2 O) 2 unambiguously showed the correctness of the halved cell in space group C2/m (PDF entry #00-059-0045; no structure data given).

(NH 4 ) 2 Mn(PO 3 F) 2 (H 2 O) 2
Although manganese is part of the first-row transition metals, (NH 4 ) 2 Mn(PO 3 F) 2 (H 2 O) 2 does not adopt the (NH 4 ) 2 Cu(PO 3 F) 2 (H 2 O) 2 structure type in space group C2/m described above for the transition metals cobalt and copper. The manganese compound shows a group-subgroup relation with the (NH 4 ) 2 Cu(PO 3 F) 2 (H 2 O) 2 structure type, crystallizing in space group P2 1 /n that is a klassengleiche subgroup of index 2 [28]. Hence, some of the sites and/or groups in the higher-symmetric space group C2/m have a reduced symmetry or split into two positions in the P2 1 /n structure.

(NH4)2Ni(PO3F)2(H2O)6
In contrast to the ammonium transition metal monofluorophosphate dihydrates (NH4)2M(PO3F)2(H2O)2 (M = Mg, Mn, Co, Cu) described in the preceding sections, the nickel compound crystallizes with six water molecules. (NH4)2Ni(PO3F)2(H2O)6 is a member of the vast family of Tutton salts with general formula M I 2M II (XO4)2(H2O)6. Typical crystal-chemical features of Tutton salts have been reviewed in various reports, e.g., ([29], and references therein). In short, the unit cell of a Tutton salt comprises two formula units and is made up of one M II site (here Ni) located on a centre of inversion (Wyckoff position 2a) and surrounded by six water molecules in the form of a slightly distorted octahedron, one XO4 tetrahedron (here PO3F), and one ammonium cation (for cases with other M I cations distorted M I O8 polyhedron are present). Hydrogen bonds of medium strengths between the building units of the type O-H···O and, as a peculiarity in the case of (NH4)2Ni(PO3F)2(H2O)6, also of the type O-H···F generate a three-dimensional network structure. The crystal structure of (NH4)2Ni(PO3F)2(H2O)6 has previously been determined based on a X-ray diffraction data set recorded at room temperature using a CAD-4 fourcircle diffractometer and a point detector. Since the crystal intensities dropped by up to   6 . Typical crystal-chemical features of Tutton salts have been reviewed in various reports, e.g., ([29], and references therein). In short, the unit cell of a Tutton salt comprises two formula units and is made up of one M II site (here Ni) located on a centre of inversion (Wyckoff position 2a) and surrounded by six water molecules in the form of a slightly distorted octahedron, one XO 4 tetrahedron (here PO 3 F), and one ammonium cation (for cases with other M I cations distorted M I O 8 polyhedron are present). Hydrogen bonds of medium strengths between the building units of the type O-H···O and, as a peculiarity in the case of (NH 4 ) 2 Ni(PO 3 F) 2 (H 2 O) 6 , also of the type O-H···F generate a three-dimensional network structure. The crystal structure of (NH 4 ) 2 Ni(PO 3 F) 2 (H 2 O) 6 has previously been determined based on a X-ray diffraction data set recorded at room temperature using a CAD-4 four-circle diffractometer and a point detector. Since the crystal intensities dropped by up to 73% of their initial values during the long-lasting data collection [15], it was decided to re-refine the crystal structure with CCD data at 100 K for an improved model. In principle, the current low-temperature data confirm the previous room-temperature data, however with much higher precision as indicated by standard uncertainties for bond lengths and angles about three to five times smaller. The crystal structure of (NH 4 ) 2 Ni(PO 3 F) 2 (H 2 O) 6 is depicted in Figure 8.  Figure 6; O-H···F hydrogen bonding is indicated by green lines.

NH4Cr(PO3F)2(H2O)6
PDF entry #00-044-0535 reports the same R-centred cell for NH4Cr(PO3F)2(H2O)6 but with space group R3 instead of R-3m determined from the present single crystal X-ray data. In the crystal structure, isolated [Cr(OH2)6] octahedra (point group symmetry -3m) are organised in layers parallel (001) and are sandwiched by double layers of PO3F 2− anions (point group symmetry 3m) along the [001] stacking direction. The disordered ammonium cations (site symmetry -3m) are situated between the PO3F 2− anions in the middle of the monofluorophosphate double layer. Strong hydrogen bonds between the [Cr(OH2)6] octahedra and the O atoms of the monofluorophosphate groups link the chromium and monofluorophosphate layers together. Ammonium cations additionally hydrogen-bond to the O atoms within a monofluorophosphate double layer (Figure 9).  Figure 6; O-H···F hydrogen bonding is indicated by green lines.

NH 4 Cr(PO 3 F) 2 (H 2 O) 6
PDF entry #00-044-0535 reports the same R-centred cell for NH 4 Cr(PO 3 F) 2 (H 2 O) 6 but with space group R3 instead of R3m determined from the present single crystal X-ray data. In the crystal structure, isolated [Cr(OH 2 ) 6 ] octahedra (point group symmetry 3m) are organised in layers parallel (001) and are sandwiched by double layers of PO 3 F 2− anions (point group symmetry 3m) along the [001] stacking direction. The disordered ammonium cations (site symmetry 3m) are situated between the PO 3 F 2− anions in the middle of the monofluorophosphate double layer. Strong hydrogen bonds between the [Cr(OH 2 ) 6 ] octahedra and the O atoms of the monofluorophosphate groups link the chromium and monofluorophosphate layers together. Ammonium cations additionally hydrogen-bond to the O atoms within a monofluorophosphate double layer (Figure 9).

NH4Cu2(H3O2)(PO3F)2
The ammonium copper compound crystallizes isotypically with KCu2(H3O2)(PO3F)2 [30] in the natrochalcite structure type [16]. The copper cation (site symmetry 2/m; Wyckoff position 4e) is surrounded by six O atoms and shows its characteristic tetragonally distorted octahedral coordination owing to the Jahn-Teller effect. Neighbouring [CuO6] polyhedra share common edges to form chains parallel to [010]. Adjacent chains are bridged by the monofluorophosphate tetrahedra (site symmetry m), sharing exclusively the O atoms into (001) layers. The disordered ammonium cation (with the N atom situated on Wyckoff position 2d with site symmetry 2/m) is located between adjacent layers and links them through hydrogen bonding to the monofluorophosphate O atoms. Additional hydrogen bonds, albeit of weak nature, develop between the non-disordered part of the {H3O2} − group and the F atom of the monofluorophosphate anion. The crystal structure is shown in Figure 10. It is well known that natrochalcite-type compounds contain such {H3O2} − groups where a positionally disordered H atom with half-occupation (here H2O) sits between two OH − groups. Since the features of the resulting hydrogen bonding system, including a clear location of hydrogen atoms by neutron diffraction, was reported for isotypic KCu2(H3O2)(SO4)2, we refer to the original description [16] for further details.

NH 4 Cu 2 (H 3 O 2 )(PO 3 F) 2
The ammonium copper compound crystallizes isotypically with KCu 2 (H 3 O 2 )(PO 3 F) 2 [30] in the natrochalcite structure type [16]. The copper cation (site symmetry 2/m; Wyckoff position 4e) is surrounded by six O atoms and shows its characteristic tetragonally distorted octahedral coordination owing to the Jahn-Teller effect. Neighbouring [CuO 6 ] polyhedra share common edges to form chains parallel to [010]. Adjacent chains are bridged by the monofluorophosphate tetrahedra (site symmetry m), sharing exclusively the O atoms into (001) layers. The disordered ammonium cation (with the N atom situated on Wyckoff position 2d with site symmetry 2/m) is located between adjacent layers and links them through hydrogen bonding to the monofluorophosphate O atoms. Additional hydrogen bonds, albeit of weak nature, develop between the non-disordered part of the {H 3 O 2 } − group and the F atom of the monofluorophosphate anion. The crystal structure is shown in Figure 10. It is well known that natrochalcite-type compounds contain such {H 3 O 2 } − groups where a positionally disordered H atom with half-occupation (here H2O) sits between two OH − groups. Since the features of the resulting hydrogen bonding system, including a clear location of hydrogen atoms by neutron diffraction, was reported for isotypic KCu 2 (H 3 O 2 )(SO 4 ) 2 , we refer to the original description [16] for further details.  The O atoms of four monofluorophosphate anions tetrahedrally surround both zinc cations. The latter do not share common atoms but are bridged by the monofluorophosphate units into (10-1) layers. Within a layer disorder of the four O atoms around Zn2 over two sets of sites is observed, with atoms O4 and O8 having an occupational ratio of 0.65(3):0.35(3) for the split pair A:B. This disorder also affects the monofluorophosphate anions associated with P2 and P3 that share these O atoms with Zn2. Additionally, the water molecule (O1W) shows positional and occupational disorder. It is disordered over an inversion centre and shows an occupancy of 0.309 (12); full occupation of this site would result in a value of 0.5, leading to a formula of (NH4)2Zn(PO3F)2(H2O)0.33. Neighbouring layers are linked through intermediate ammonium cations by medium to weak hydrogen bonds to the monofluorophosphate O atoms. The crystal structure of (NH4)2Zn(PO3F)2(H2O)0.2 is displayed in Figure 11.   Figure 11. Figure 11. The crystal structure of (NH4)2Zn(PO3F)2(H2O)0.2 in a projection along [010]. Colour code of PO3F tetrahedra and H atoms as in Figure 1; O atoms of disordered crystal water molecules are given as yellow spheres. For clarity, disorder involving parts of the [ZnO4] and PO3F tetrahedra is not shown.
Whereas ZnA is exclusively bonded to four O atoms (O1) of symmetry-related monofluorophosphate anions with an equal bond length of 1.934(3) Å, ZnB is coordinated by only three monofluorophosphate O atoms at two shorter and one longer Zn-O distances. The fourth coordination site, completing a distorted tetrahedron, is occupied by the water molecule at the longest distance of 2.204 (14) Å. Again, the F atom of the monofluorophosphate tetrahedron does not take part in constructing the framework structure because it is not part of the coordination spheres around the two zinc sites ( Figure 13). However, it is involved in weak hydrogen bonding interactions as the acceptor atom with the disordered (N1H/O1W) donor group. Two more hydrogen bonding interactions of similar strength are present between the donor group and the monofluorophosphate O atoms.   4 at room temperature (PDF entry #00-044-0539; a = 11.4769(5) Å). In the crystal structure of (NH 4 ) 2 Zn 3 (PO 3 F) 4 (H 2 O) disorder is observed, affecting the zinc and the ammonium sites. The major part of the cation (ZnA; occupancy 0.75) is situated on Wyckoff position 12b with 4 site symmetry. Due to disorder around this axis, the remaining Zn cations split into four equivalent sites (ZnB) with an occupancy of 0.0625 each. The N atom of the ammonium cation (N1H) and the O atom of a water molecule (O1W) simultaneously occupy Wyckoff position 12a (located on a 4 axis) in a ratio of 0.67:0.33. The site symmetry of the PO 3 F tetrahedron is .3 with the P atom situated on Wyckoff position 16c. The disordered part of the crystal structure is shown in Figure 12.
Whereas ZnA is exclusively bonded to four O atoms (O1) of symmetry-related monofluorophosphate anions with an equal bond length of 1.934(3) Å, ZnB is coordinated by only three monofluorophosphate O atoms at two shorter and one longer Zn-O distances. The fourth coordination site, completing a distorted tetrahedron, is occupied by the water molecule at the longest distance of 2.204 (14) Å. Again, the F atom of the monofluorophosphate tetrahedron does not take part in constructing the framework structure because it is not part of the coordination spheres around the two zinc sites ( Figure 13). However, it is involved in weak hydrogen bonding interactions as the acceptor atom with the disordered (N1H/O1W) donor group. Two more hydrogen bonding interactions of similar strength are present between the donor group and the monofluorophosphate O atoms.

Mean bond lengths and angles in the PO 3 F tetrahedron
For the statistical analysis of bond lengths and angles within a PO 3 F tetrahedron in inorganic monofluorophosphates that are compiled in the most recent version of the Inorganic Structure Database (ICSD, [31]), reliability factors R1 ≤ 0.08 for the structure model and only ordered PO 3 F groups were considered as criteria, disregarding different measurement temperatures or redeterminations. HPO 3 F − tetrahedra present in hydrogenmonofluorophosphates were not taken into account. In summary, 88 independent PO 3 F tetrahedra from 63 different monofluorophosphate phases (including the examples of the current study) were used ( Table 5). As a result, the P-F bond of 1.578 (20) Å is significantly longer than the three P-O bonds with 1.506(13) Å, and relative to the ideal tetrahedral angle of 109.47 • , the three O-P-O angles of 113.7(1.7) • are enlarged by about 4 • and the O-P-F angle of 104.8(1.7) • reduced by about the same value. The averaged values for bond lengths and angles in the monofluorophosphate PO 3 F tetrahedron differ markedly from those of the difluorophosphate PO 2 F 2 tetrahedron. Here, the two P-O and the two P-F bonds are shortened with mean values of 1.459 (27) and 1.530 (21) Å, respectively, and the O-P-O angle once more is widened to 121.2 (2.9) • , whereas the O-P-F angle of 108.7 (6) • now is closer to the ideal value (the F-P-F angle is the smallest in the PO 2 F 2 tetrahedron with 98.5 (2.6) • ) [32]).
The computed mean values of the monofluorophosphate tetrahedron can be used as a simple tool for evaluation of crystal structures with this entity. In one case (Table 5), a significant deviation in terms of bond lengths and angles was observed for the crystal structure of (NH 4 ) 3 Fe(PO 3 F) 2 F 2 [33] where one of the two distinct monofluorophosphate anions has one of the P-O bonds as the longest in the tetrahedron, a very short P- Based on the current averaged data for a PO 3 F tetrahedron, it is clear that atoms O1 and F4 were wrongly assigned and must be interchanged.

Symmetry of the PO 3 F group in crystal structures
Possible point group symmetries of a PO 3 F group in a crystal structure are 1, 3, m and 3m, the latter being the highest possible point group symmetry for this tetrahedron in the crystalline state. The vast majority of monofluorophosphate groups exhibits point group symmetry 1 (70 examples), followed by point group symmetry m (15 examples), 3m (two examples) and 3 (one example). The reported point group symmetry of 42m for the PO 3 F group in K 3 (PO 3 F)F [34] is incompatible with its molecular symmetry and consequently, this group is disordered.

Isotypism with sulfates
From the numerous phases compiled in Table 5 3 , NaFe(PO 3 F) 2 , SnPO 3 F, Ag 2 PO 3 F, and Hg 2 PO 3 F, but the majority of monofluorophosphate phases has no sulfate counterpart.

•
Hydrogen bonding with the monofluorophosphate F atom as an acceptor As discussed briefly for appropriate structures above and detailed in Table 3, hydrogen bonding involving the F atom of the monofluorophosphate anion occurs only occasionally and then only as a comparatively weak interaction. A review of the crystal structures where hydrogen bonding is possible and where all H atoms were determined revealed that this situation holds also for most other monofluorophosphates. Considering a D···F distance (D = donor atom: N, O) less than 3.2 Å and D-H···F angles greater than 130 • , as relevant for a significant hydrogen bonding interaction [35], then only for Li(NH 4 )PO 3 F, Na 2 PO 3 F(H 2 O) 10 6 is this kind of interaction realized, albeit of weak nature (Table 5). In all other monofluorophosphates capaple of hydrogen-bonding interactions either the D···F-P distances are much greater than the threshold of 3.2 Å, or the D-H···F angles are much smaller than 140 • . In these structures, D-H···O hydrogen bonds dominate or are the only hydrogen-bonding interactions.   [54] P2 1 /c, 4 1 ----KFe 2 (PO 2 F 2 )(PO 3 F) 2 F 2 [33] P1, 2 1, 1 ----RbFe 3 (PO 3 F)((PO 2 ) 2 (F 1.5 (OH) 0.5 ) 2 )F 2 [33] C2/c, 4 1 ----  [27] C2/m, 2 m ----Ba 2 Cu 2 (PO 3 F)F 6 [57] P2 1 /c, 4 1 --- * It is most unlikely that (NH 4 )Mn 3 (PO 3 F) 2 (PO 2 F 2 )F 2 and (NH 4 )Mn 3 (PO 3 F) 2 (H 2 PO 4 )F 2 crystallize in the same type of structure with virtually the same unit cell and the same space group symmetry and differ only in one of the anions, i.e., PO 2 F 2 -and PO 2 (OH) 2 − . In all likelihood, one of the crystal structure models (and the respective composition) is incorrect. Based on the available data, an evaluation was, however, not possible.

Conclusions
Single crystals of twelve and partly unknown monofluorophosphate phases were grown from aqueous solutions. Crystal structure refinements of these compounds extend our knowledge about the PO 3 F 2− anion. Based on the present crystal structure data and a complete literature search addressing monofluorophosphate structures of inorganic compounds, the following structural characteristics for the tetrahedral PO 3 F group were obtained: The P-F bond has a mean value of 1.578(20) Å and is considerably longer than the mean of the three P-O bonds of 1.506(13) Å, and the mean O-P-O angles of 113.7(1.7) • are considerably larger than the mean O-P-F angle of 104.8(1.7) • . The point group symmetry of the "free" PO 3 F group (C 3v in Schoenflies or 3m in Hermann-Maugin notation) is found with this symmetry in the solid state only in two examples. In most cases (70 examples) the point group symmetry is reduced to C 1 (1) followed by point group symmetry C s (m) with 15 examples and C 3 (3) with one example. The monofluorophosphate F atom is characterized by its isolated state in the crystal structure. In the vast majority of cases, it is not part of the coordination sphere of the cation and/or is not engaged in hydrogen bonding as an acceptor atom. Only in exceptional cases are weak interactions realized, i.e., for large cations with high coordination numbers in form of long metal-F bonds or as hydrogen bonds with long donor···F distances between 2.8 and 3.2 Å.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available in The Cambridge Crystallographic Data Centre (CCDC) and can be obtained free of charge via www.ccdc.cam.ac.uk/ structures.