Monofluorophosphates—New Examples and a Survey of the PO₃F²⁻ Anion

Abstract

During a systematic study of monofluorophosphates, i.e., compounds comprising the tetrahedral anion PO₃F²⁻, 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 CdPO₃F(H₂O)₂, Cr₂(PO₃F)₃(H₂O)_18.8, Pb₂(PO₃F)Cl₂(H₂O), (NH₄)₂M(PO₃F)₂(H₂O)₂ (M = Mg, Mn, Co), NH₄Cr(PO₃F)₂(H₂O)₆, NH₄Cu₂(H₃O₂)(PO₃F)₂, (NH₄)₂Zn(PO₃F)₂(H₂O)_0.2, and (NH₄)₂Zn₃(PO₃F)₄(H₂O), as well as redeterminations of ZnPO₃F(H₂O)_2.5 and (NH₄)₂Ni(PO₃F)₂(H₂O)₆. From the previously unknown crystal structures, CdPO₃F(H₂O)₂ (space group P$\overline{1}$), Cr₂(PO₃F)₃(H₂O)_18.8 (P$\overline{1}$), Pb₂(PO₃F)Cl₂(H₂O) (Pnma), NH₄Cr(PO₃F)₂(H₂O)₆ (R$\overline{3}$m), (NH₄)₂Zn(PO₃F)₂(H₂O)_0.2 (C2/c), and (NH₄)₂Zn₃(PO₃F)₄(H₂O) (I$\overline{4}$3d) each crystallizes in an unique crystal structure, whereas compounds (NH₄)₂M(PO₃F)₂(H₂O)₂ (M = Mg, Co) crystallize in the (NH₄)₂Cu(PO₃F)₂(H₂O)₂ type of structure (C2/m) and (NH₄)₂Mn(PO₃F)₂(H₂O)₂ in a subgroup thereof (P2₁/n, with a klassengleiche relationship of index 2), and NH₄Cu₂(H₃O₂)(PO₃F)₂ (C2/m) crystallizes isotypically with natrochalcite-type KCu₂(H₃O₂)(SO₄)₂. A survey on the PO₃F²⁻ anion, including database entries of all inorganic compounds comprising this group, revealed mean bond lengths of P–O = 1.506(13) Å, P–F = 1.578(20) Å, and angles of O–P–O = 113.7(1.7)° and O–P–F = 104.8(1.7)°, using a dataset of 88 independent PO₃F²⁻ 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.

1. Introduction

The family of monofluorophosphates comprising the PO₃F²⁻ anion was introduced by Lange more than 90 years ago [1]. In the PO₃F²⁻ anion, the fluorine atom is directly bound to the phosphorus atom. However, in the literature, compounds with discrete PO₄³⁻ and F⁻ anions are also sometimes incorrectly described as “fluorophosphates”, e.g., Mn₂PO₄F [2] or Na₂FePO₄F [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 PO₃F²⁻ anion is isoelectronic with the SO₄²⁻ 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 PO₃F²⁻ 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 PO₃F²⁻ 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 PO₃F²⁻ 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.

2. Materials and Methods

2.1. Syntheses and Single Crystal Growth

For syntheses of NH₄⁺-containing monofluorophosphates, the starting compound (NH₄)₂PO₃F(H₂O) 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 AgNO₃ were added to this solution to precipitate the phosphate anions present due to incomplete conversion or partial hydrolysis of the PO₃F²⁻ anion. The yellow Ag₃PO₄ was filtered off, and the filtrate was repeatedly checked for PO₄³⁻ anions by adding a few drops of an AgNO₃ solution until no more clouding was observed in the filtrate, ensuring that all PO₄³⁻ 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 (NH₄)₂Mg(PO₃F)₂(H₂O)₂ that was obtained as a pure polycrystalline phase (cf. PDF entry #00-059-0045).

For single crystal growth of NH₄⁺-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 (NH₄)₂M(PO₃F)₂(H₂O)₂ (M = Mg, Co, Mn), (NH₄)₂Ni(PO₃F)₂(H₂O)₆, NH₄Cr(PO₃F)₂(H₂O)₆ and NH₄Cu₂(H₃O₂)(PO₃F)₂ could be obtained from the respective metal salt solution. Single crystals of Cr₂(PO₃F)₃(H₂O)_18.8 were likewise harvested from the batch containing the ammonium-chromium monofluorophosphate solution. From the original ammonium zinc monofluorophosphate solution, three different types of single crystals were isolated, viz. ZnPO₃F(H₂O)_2.5, (NH₄)₂Zn(PO₃F)₂(H₂O)_0.2 and (NH₄)₂Zn₃(PO₃F)₄(H₂O).

Single crystals of CdPO₃F(H₂O)₂ and Pb₂(PO₃F)Cl₂(H₂O) were obtained from metathesis reactions. For this purpose, 200 mg Ag₂PO₃F (prepared according to [4]) were dissolved in 10 mL of water; equimolar amounts of CdCl₂ and PbCl₂, 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. CdPO₃F(H₂O)₂ was obtained as a single phase material, whereas only few single crystals of Pb₂(PO₃F)Cl₂(H₂O) could be isolated. In the latter batch, polycrystalline 2PbCO₃·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.

2.2. 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. Details of data collections and structure refinements.

Compound		CdPO3F(H2O)2	Cr2(PO3F)3(H2O)18.8	Pb2(PO3F)Cl2(H2O)	ZnPO3F(H2O)2.5	(NH4)2Co(PO3F)2(H2O)2	(NH4)2Mg(PO3F)2(H2O)2
Formula weight		246.40	736.88	601.27	208.38	326.99	292.37
Temperature/°C		23	−173	23	−173	23	23
Radiation; λ/Å		Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073
Diffractometer		SMART CCD	APEX-II CCD	SMART CCD	APEX-II CCD	CAD-4	SMART CCD
Crystal dimensions / mm		0.14 × 0.10 × 0.04	0.10 × 0.10 × 0.01	0.35 × 0.05 × 0.04	0.12 × 0.02 × 0.02	0.05 × 0.05 × 0.01	0.04 × 0.04 × 0.01
Crystal colour; shape		colourless; fragment	green; plate	colourless; needle	colourless; rod	violet; plate	colourless; plate
Space group, no.		P$\overline{1}$, 2	P$\overline{1}$, 2	Pnma, 62	P$\overline{1}$, 2	C2/m, 12	C2/m, 12
Formula units Z		2	4	4	4	2	2
a/Å		5.27680(10)	11.5937(16)	20.4864(14)	7.6020(2)	13.386(3)	13.374(2)
b/Å		6.66970(10)	15.292(2)	5.3967(4)	7.6490(2)	5.3476(9)	5.3541(8)
c/Å		7.7037(2)	15.360(2)	6.9722(5)	9.4671(3)	7.390(2)	7.3852(11)
α/°		65.5060(10)	83.804(6)	90	88.633(2)	90	90
β/°		85.9190(10)	84.203(6)	90	88.888(2)	114.02(2)	113.758(3)
γ/°		75.3940(10)	82.579(6)	90	87.182(2)	90	90
V/Å3		238.584(9)	2674.1(6)	770.84(10)	549.58(3)	483.2(2)	484.01(13)
μ/mm−1		4.867	1.118	44.496	4.733	2.169	0.576
X-ray density/g·cm–3		3.430	1.830	5.181	2.518	2.247	2.006
Absorption correction		multi-scan; SADABS	multi-scan; SADABS	multi-scan; SADABS	multi-scan; SADABS	numerical; HABITUS	multi-scan; SADABS
Trans. coef. Tmin; Tmax		0.549; 0.829	0.778; 0.972	0.023; 0.269	0.545; 0.749	0.884; 0.934	0.887; 0.922
Range θmin–θmax		2.91–35.51	1.34–26.00	3.09–30.49	2.15–43.26	3.02–29.96	3.01–28.29
Range	h	−8–7	−14–14	−25–29	−14–14	−18–18	−17–17
	k	−10–10	−18–18	−7–7	−14–14	−7–7	−7–7
	l	−12–12	−18–18	−9–9	−18–18	−10–10	−9–9
Measured reflections		20950	80926	8539	77904	2799	3204
Independent reflections		2055	10504	1291	8190	772	660
Obs.reflections [I > 2σ(I)]		1927	6720	1176	7431	540	517
Ri		0.0235	0.0731	0.0390	0.0329	0.0668	0.0525
Number of parameters		87	681	59	198	55	55
Ext. coef. (SHELXL)		0.0173(9)	-	0.00298(15)	0.0042(4)	-	-
Diff. elec. dens. max; min		0.51 (0.70, Cd1);	1.24 (0.94, O8W);	2.33 (0.80, Pb1);	1.34 (1.52, O8);	0.44 (0.49, O1);	0.57 (0.71, O2);
[e–·Å–3] (dist./Å, atom)		−0.56 (0.65, Cd1)	−0.97 (0.43, O14W)	−1.51 (0.26, Pb2)	−0.48 (0.48, O4)	−0.34 (0.51, H2)	−0.34 (1.23, H2)
R[F2 > 2σ(F2)]		0.0136	0.0605	0.0237	0.0220	0.0280	0.0462
wR2(F2 all)		0.0297	0.2010	0.0554	0.0523	0.0662	0.1153
Goof		1.081	1.036	1.062	1.225	0.989	1.031
CSD number		2,048,140	2,048,141	2,048,144	2,048,145	2,048,134	2,048,135
Compound		(NH4)2Mn(PO3F)2(H2O)2	(NH4)2Ni(PO3F)2(H2O)6	NH4Cr(PO3F)2(H2O)6	NH4Cu2(H3O2)(PO3F)2	(NH4)2Zn(PO3F)2(H2O)0.2	(NH4)2Zn3(PO3F)4(H2O)
Formula weight		323.00	398.83	374.08	376.09	301.10	642.09
Temperature/°C		−173	−173	−173	−173	23	−173
Radiation; λ/Å		Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073	Mo K$\overline{a}$; 0.71073
Diffractometer		APEX-II CCD	APEX-II CCD	APEX-II CCD	APEX-II CCD	APEX-II CCD	APEX-II CCD
Crystal dimensions/mm		0.08 × 0.06 × 0.01	0.44 × 0.32 × 0.15	0.15 × 0.15 × 0.08	0.20 × 0.10 × 0.08	0.10 × 0.10 × 0.01	0.08 × 0.08 × 0.08
Crystal colour; shape		light-pink; plate	blue; fragment	green; plate	light-blue; parallelepiped	colourless; plate	colourless; octahedron
Space group, no.		P21/n, 14	P21/c, 14	R$\overline{3}$m, 166	C2/m, 12	C2/c, 15	I$\overline{4}$3d, 220
Formula units Z		2	2	3	2	12	4
a/Å		12.558(3)	6.2700(3)	6.5491(2)	9.1012(8)	18.9363(17)	11.3693(4)
b/Å		5.4559(19)	12.2845(6)	6.5491(2)	6.4121(5)	7.6955(7)	11.3693(4)
c/Å		7.4215(18)	9.1894(4)	25.4381(14)	7.8506(7)	20.5276(18)	11.3693(4)
α/°		90	90	90	90	90	90
β/°		99.918(5)	106.033(2)	90	116.277(3)	108.641(2)	90
γ/°		90	90	120	90	90	90
V/Å3		500.9(2)	680.27(6)	944.88(8)	410.80(6)	2834.4(4)	1469.61(16)
μ/mm−1		1.697	1.745	1.245	5.631	2.976	5.415
X-ray density/g·cm–3		2.142	1.947	1.972	3.040	2.117	2.902
Absorption correction		multi-scan; SADABS	multi-scan; SADABS	multi-scan; SADABS	multi-scan; SADABS	multi-scan; SADABS	multi-scan; SADABS
Trans. coef. Tmin; Tmax		0.636; 0.747	0.662; 0.749	0.661; 0.747	0.631; 0.748	0.889; 0.995	0.656; 0.746
Range θmin–θmax		2.98–32.99	2.34–45.97	2.40–34.71	2.89–39.98	2.88–31.00	4.39–30.00
Range	h	−18–19	−12–12	−10–10	−16–16	−27–19	−15–12
	k	−8–8	−24–24	−9–10	−11–11	−11–11	−15–15
	l	−11–11	−18–18	−40–40	−14–13	−23–29	−16–16
Measured reflections		10,452	105,648	6844	7619	11,828	6077
Independent reflections		1886	5910	557	1356	4382	362
Obs.reflections [I > 2σ(I)]		1059	5659	461	1183	3079	328
Ri		0.0881	0.0227	0.0526	0.0336	0.0392	0.0516
Number of parameters		94	129	30	57	234	33
Ext. coef. (SHELXL)		-	0.0022(9)	-	-	-	-
Flack parameter		-	-	-	-	-	0.026(15)
Diff. elec. dens. max; min		0.56 (0.63, O1)	0.53 (0.56, Ni1);	0.51 (0.77, Cr1);	1.07 (1.52, H1W);	0.78 (0.75, O1);	0.34 (0.78, O1);
[e−·Å−3] (dist./Å, atom)		−0.63 (0.72, P1)	−0.79 (0.55, Ni1)	−0.51 (0.0, Cr1)	−0.83 (0.60, Cu1)	−0.45 (1.36, Zn2)	−0.33 (0.41, Zn1A)
R[F2 > 2σ(F2)]		0.0404	0.0142	0.0232	0.0239	0.0467	0.0265
wR2(F2 all)		0.1009	0.0400	0.0621	0.0575	0.1133	0.0533
Goof		1.012	1.160	1.140	1.109	1.009	1.114
CSD number		2,048,136	2,048,137	2,048,142	2,048,143	2,048,138	2,048,139

.

All crystal structures were initially solved with SHELXS [12] and refined with SHELXL [13]. For the renewed refinement of ZnPO₃F(H₂O)_2.5 and (NH₄)₂Ni(PO₃F)₂(H₂O)₆, the original atom labelling and atomic coordinates (as starting parameters) were resumed from the original structure reports [14,15]. For NH₄Cu₂(H₃O₂)(PO₃F)₂, atom labelling and coordinates were adopted from isotypic KCu₂(H₃O₂)₃(SO₄)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 Pb₂(PO₃F)Cl₂(H₂O), Cr₂(PO₃F)₃(H₂O)_18.8 and (NH₄)₂Zn(PO₃F)₂(H₂O)_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 NH₄Cr(PO₃F)₂(H₂O)₆ and NH₄Cu₂(H₃O₂)(PO₃F)₂ where the N atom of the ammonium cation is situated on a position with site symmetry $\overline{3}$m and 2/m, respectively, which results in a symmetry-restricted disorder of the corresponding ammonium H atoms. Finally, in the crystal structure of (NH₄)₂Zn₃(PO₃F)₄(H₂O), 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. Selected interatomic distances/Å and angles/°.

CdPO3F(H2O)2				Cr2(PO3F)3(H2O)18.8				Pb2(PO3F)Cl2(H2O)
Cd1	OW1	2.2474(10)		Cr1	O1	1.956(3)	2x	Pb1	O1	2.570(3)	2x
Cd1	O2	2.2505(9)		Cr1	O2	1.960(3)	2x	Pb1	O1W	2.703(7)
Cd1	O1	2.2545(9)		Cr1	O3	1.973(3)	2x	Pb1	Cl1	2.875(2)
Cd1	O3	2.2867(9)		Cr2	O4	1.949(3)	2x	Pb1	Cl2	2.9423(10)	2x
Cd1	O3	2.3310(9)		Cr2	O5	1.959(3)	2x	Pb1	Cl2	3.048(2)
Cd1	OW2	2.3865(10)		Cr2	O6	1.971(3)	2x	Pb2	O2	2.301(5)
P1	O1	1.5058(9)		Cr3	O7	1.939(3)		Pb2	O1	2.503(3)	2x
P1	O2	1.5073(9)		Cr3	O8	1.950(4)		Pb2	Cl1	3.1402(10)	2x
P1	O3	1.5155(9)		Cr3	O9	1.952(3)		Pb2	O2	3.142(3)	2x
P1	F1	1.5747(8)		Cr3	O10	1.969(4)		Pb2	O1	3.167(3)	2x
				Cr3	O11	1.975(4)		P1	O2	1.493(6)
O−P1−O		113.89(5)–114.93(5)		Cr3	O12	1.984(3)		P1	O1	1.517(4)	2x
O−P1−F1		103.37(5)–104.33(5)		Cr4	O13	1.947(3)		P1	F1	1.564(5)
				Cr4	O14	1.950(4)
				Cr4	O15	1.955(3)		O−P1−O		111.92(17)–113.9(2)
ZnPO3F(H2O)2.5				Cr4	O16	1.972(4)		O−P1−F1		105.12(19)–108.2(3)
Zn1	O10	1.9480(7)		Cr4	O17	1.977(3)
Zn1	O8	1.9614(8)		Cr4	O18	1.983(3)
Zn1	O7	1.9647(7)		Cr5	O19	1.942(4)		(NH4)2Co(PO3F)2(H2O)2
Zn1	O5	1.9915(7)		Cr5	O20	1.959(4)		Co1	O2	2.0714(16)	4x
Zn1	P2	2.9235(3)		Cr5	O21	1.960(4)		Co1	O1W	2.161(3)	2x
Zn2	O9	2.0199(8)		Cr5	O22	1.967(4)		P1	O1	1.505(3)
Zn2	O9	2.0199(8)	2x	Cr5	O24	1.974(4)		P1	O2	1.5056(17)	2x
Zn2	O6	2.0941(8)	2x	Cr5	O23	1.976(4)		P1	F1	1.579(2)
Zn2	O4	2.1306(8)	2x	P1	O11P	1.505(3)
Zn3	O3	1.9979(8)	2x	P1	O12P	1.507(3)		O−P1−O		112.54(15)–115.25(8)
Zn3	O11	2.1130(9)	2x	P1	O13P	1.518(3)		O−P1−F1		103.27(9)–105.39(14)
Zn3	O1	2.1308(8)	2x	P1	F1	1.568(3)
P1	O3	1.4866(8)		P2	O21P	1.501(4)
P1	O8	1.5123(8)		P2	O22P	1.503(4)		(NH4)2Mg(PO3F)2(H2O)2
P1	O7	1.5342(7)		P2	O23P	1.521(3)		Mg1	O2	2.0561(18)	4x
P1	F2	1.5604(7)		P2	F2	1.564(3)		Mg1	O1W	2.124(3)	2x
P2	O9	1.4906(8)		P3	O31P	1.498(4)		P1	O1	1.501(3)
P2	O10	1.5167(8)		P3	O32P	1.511(3)		P1	O2	1.5045(19)
P2	O5	1.5302(7)		P3	O33P	1.521(3)		P1	F1	1.586(2)
P2	F1	1.5595(7)		P4	F3	1.563(3)
				P4	O41P	1.497(4)		O−P1−O		113.31(16)–114.74(9)
O−P−O		109.98(5)–115.78(5)		P4	O42P	1.503(4)		O−P1−F1		103.47(9)–105.35(6)
O−P−F		103.61(4)–108.61(4)		P4	O43P	1.523(3)
				P5	F4	1.564(3)
				P5	O51P	1.494(4)		(NH4)2Ni(PO3F)2(H2O)6
NH4Cr(PO3F)2(H2O)6				P5	O52P	1.507(4)		Ni1	O4	2.0295(3)
Cr1	O2W	1.9622(10)	6x	P5	O53P	1.518(3)		Ni1	O6	2.0624(3)
P1	O1	1.5106(10)	3x	P6	F5	1.564(4)		Ni1	O5	2.0747(3)
P1	F1	1.5676(17)		P6	O61P	1.480(4)		P1	O1	1.5082(3)
				P6	O62P	1.483(4)		P1	O2	1.5109(3)
O1-P-O1		113.44(4)		P6	O63P	1.519(3)		P1	O3	1.5113(3)
O1-P-F1		105.13(4)		P6	F6B	1.585(7)		P1	F1	1.6054(3)
				P6	F6A	1.591(9)
								O−P1−O		113.599(17)–115.710(17)
(NH4)2Mn(PO3F)2(H2O)2				O−P−O		113.1(2)–114.6(2)		O−P1−F1		103.168(16)–104.160(18)
Mn1	O3	2.151(2)	2x	O−P−F		90.4(4)–118.9(4)
Mn1	O1	2.1650(19)	2x
Mn1	O1W	2.240(2)	2x					(NH4)2Zn3(PO3F)4(H2O)
P1	O1	1.512(2)		(NH4)2Zn(PO3F)2(H2O)0.2				Zn1A	O1	1.934(3)	4x
P1	O2	1.5140(19)		Zn1	O9	1.911(3)		Zn1B	O1	1.766(11)
P1	O3	1.517(2)		Zn1	O5	1.920(3)		Zn1B	O1	1.830(11)
P1	F1	1.5944(19)		Zn1	O3	1.929(2)		Zn1B	O1	2.199(10)
				Zn1	O2	1.940(2)		Zn1B	O1W	2.204(14)
O-P-O		112.73(12)–114.98(12)		Zn2	O8B	1.835(11)		P1	O1	1.497(3)	3x
O-P-F1		103.15(10)–105.12(10)		Zn2	O8B	1.835(11)		P1	F1	1.554(5)
				Zn2	O4A	1.872(5)
NH4Cu2(H3O2)(PO3F)2				Zn2	O4A	1.872(5)		O−P1−O		113.24(13)
Cu1	O2	1.9493(10)	2x	Zn2	O8A	1.958(6)		O−P1−F1		105.37(16)
Cu1	O1H	2.0217(8)	2x	Zn2	O8A	1.958(6)
Cu1	O1	2.3642(10)	2x	Zn2	O4B	1.988(12)
P1	O1	1.5052(14)		Zn2	O4B	1.988(12)
P1	O2	1.5138(10)	2x	P1	O1	1.484(3)
P1	F1	1.5934(12)		P1	O2	1.512(2)
				P1	O3	1.512(2)
O−P1−O		114.36(8)–114.85(5)		P1	F1	1.575(2)
O−P1−F1		103.15(5)–104.34(7)		P2	O4B	1.469(12)
				P2	O5	1.488(3)
				P2	O6	1.489(3)
				P2	O4A	1.532(7)
				P2	F2	1.556(3)
				P3	O7	1.472(3)
				P3	O8A	1.485(6)
				P3	O9	1.498(3)
				P3	F3	1.556(3)
				P3	O8B	1.597(13)
				O−P−O		101.6(4)–127.3(10)
				O−P−F		86.8(9)–116.2(6)

lists selected bond lengths and angles for all crystal structures, and

Table 3. Details of hydrogen bonding/Å, °.

D	H	A	D–H	H···A	D···A	D–H···A	D	H	A	D–H	H···A	D···A	D–H···A
CdPO3F(H2O)2							ZnPO3F(H2O)2.5
OW1	H1	OW2	0.83(2)	1.99(2)	2.7983(15)	164(2)	O1	H1	O10	0.819(9)	1.960(9)	2.7778(11)	177(2)
OW1	H2	O2	0.81(2)	2.34(3)	2.9231(14)	129(2)	O1	H2	O5	0.819(10)	2.004(10)	2.8212(11)	175(3)
OW1	H2	O1	0.81(2)	2.36(3)	2.9635(14)	133(2)	O2	H3	O1	0.818(10)	2.25(2)	2.9854(14)	149(3)
OW2	H3	O2	0.90(2)	2.00(2)	2.8740(15)	162(2)	O2	H4	O4	0.814(10)	2.496(15)	3.2526(14)	155(3)
OW2	H3	F1	0.90(2)	2.57(3)	3.1411(14)	121(2)	O2	H4	O6	0.814(10)	2.52(2)	3.1410(13)	134(2)
OW2	H4	O1	0.84(2)	2.18(2)	3.0032(15)	167(2)	O4	H5	O2	0.823(9)	1.992(11)	2.8053(13)	170(3)
OW2	H4	F1	0.84(2)	2.60(2)	3.0887(13)	118(2)	O4	H6	O5	0.828(9)	2.047(10)	2.8707(11)	173(2)
							O6	H7	O8	0.816(9)	1.900(10)	2.7136(11)	175(2)
(NH4)2Co(PO3F)2(H2O)2							O6	H8	O7	0.818(9)	2.018(10)	2.8336(11)	174(2)
N1H	H1	O1	0.92(4)	1.88(4)	2.800(4)	180(5)	O11	H9	O7	0.819(10)	2.059(11)	2.8641(11)	168(3)
N1H	H2	O2	0.86(4)	2.22(3)	2.924(3)	139.9(8)	O11	H10	O2	0.822(10)	2.059(10)	2.8742(13)	171(3)
N1H	H3	O2	0.85(3)	2.32(3)	3.031(4)	142(3)
O1W	H4	O1	0.81(3)	2.14(3)	2.8702(14)	149(3)	(NH4)2Ni(PO3F)2(H2O)6
							N1	H1N	O3	0.849(12)	2.159(12)	2.9613(5)	157.6(11)
(NH4)2Mg(PO3F)2(H2O)2							N1	H2N	O3	0.917(11)	1.921(11)	2.8283(4)	169.8(10)
N1H	H1	O1	0.90(4)	1.90(4)	2.801(5)	175(5)	N1	H3N	O1	0.896(12)	1.921(12)	2.8134(5)	173.7(10)
N1H	H2	O2	0.88(4)	2.21(3)	2.933(4)	139.7(9)	N1	H4N	O2	0.849(11)	2.115(11)	2.9053(5)	154.7(10)
N1H	H3	O2	0.82(3)	2.34(4)	3.040(4)	143(4)	O4	H5W	O3	0.834(10)	1.866(10)	2.6969(4)	174.0(11)
O1W	H4	O1	0.76(3)	2.15(3)	2.8732(16)	160(4)	O4	H6W	O1	0.826(11)	1.938(11)	2.7489(4)	167.0(10)
							O5	H7W	O2	0.799(11)	1.883(11)	2.6792(4)	173.7(11)
(NH4)2Mn(PO3F)2(H2O)2							O5	H8W	F1	0.804(11)	2.010(11)	2.8146(4)	178.6(12)
O1W	H1W	O2	0.850(10)	1.930(14)	2.748(3)	161(3)	O6	H9W	O1	0.805(12)	1.956(12)	2.7589(4)	175.3(11)
O1W	H2W	O2	0.849(10)	2.44(3)	3.103(3)	136(4)	O6	H10W	O2	0.789(11)	2.001(11)	2.7874(4)	174.1(11)
N1H	H1N	O1	0.899(10)	2.131(19)	2.945(3)	150(3)
N1H	H2N	F1	0.897(10)	2.50(3)	3.041(3)	119(3)	NH4Cu2(H3O2)(PO3F)2
N1H	H2N	O1W	0.897(10)	2.59(3)	3.209(3)	127(3)	O1H	H1O	F1	0.837(10)	2.344(10)	3.1797(18)	177(4)
N1H	H3N	O2	0.902(10)	1.909(11)	2.810(3)	176(3)	O1H	H2O	O1H	0.999(10)	1.50(4)	2.489(3)	169(20)
N1H	H4N	O3	0.900(10)	1.912(12)	2.803(3)	170(3)	N1	H1N	O2	0.8999(10)	2.021(18)	2.8448(10)	152(3)
							N1	H2N	F1	0.9000(10)	2.24(6)	2.9730(13)	139(8)
							N1	H2N	O1	0.9000(10)	2.03(4)	2.8554(14)	152(8)
NH4Cr(PO3F)2(H2O)6
O2W	H1	O1	0.864(16)	1.753(16)	2.6124(9)	173.1(16)	(NH4)2Zn3(PO3F)4(H2O)
N1H	H2	O1	0.95(4)	2.00(4)	2.9493(10)	176(3)	N1H/O1W	H1	F1	1.02(8)	2.31(8)	3.1703(11)	141(6)
							N1H/O1W	H1	O1	1.02(8)	2.40(8)	3.007(3)	117(6)
(NH4)2Zn(PO3F)2(H2O)0.2							N1H/O1W	H1	O1	1.02(8)	2.38(9)	3.190(4)	136(7)
N1	H1	O7	0.897(10)	1.874(16)	2.756(5)	167(5)
N1	H2	O1	0.891(10)	2.17(3)	2.955(5)	147(5)
N1	H3	O3	0.898(10)	2.130(18)	3.000(4)	163(5)
N1	H4	O6	0.894(10)	1.992(14)	2.878(5)	171(5)
N2	H5	O6	0.897(10)	2.015(18)	2.890(4)	164(5)
N2	H6	O2	0.896(10)	2.16(2)	3.000(4)	156(4)
N2	H6	O9	0.896(10)	2.74(4)	3.321(5)	123(4)
N2	H7	O6	0.899(10)	1.925(13)	2.817(5)	172(5)
N2	H8	O8A	0.897(10)	1.96(2)	2.811(8)	158(5)
N2	H8	O8B	0.897(10)	2.49(4)	3.28(3)	148(5)
N3	H11	O1	0.894(10)	2.08(3)	2.869(5)	146(4)
N3	H9	O7	0.899(10)	1.89(2)	2.743(5)	158(5)
N3	H10	O4A	0.897(10)	2.165(16)	3.055(9)	171(4)
N3	H10	O8B	0.897(10)	2.55(5)	3.03(2)	114(4)
N3	H12	O1	0.892(10)	1.949(13)	2.837(5)	173(5)

gives numerical details of hydrogen bonding.

2.3. Vibrational Spectroscopy

The infrared (IR) spectrum of a powdered CdPO₃F(H₂O)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.

2.4. 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.

3. Results

In the current study, only (NH₄)₂Mg(PO₃F)₂(H₂O)₂ and CdPO₃F(H₂O)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.

3.1. 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 cis-aligned water molecules (OW1, OW2) and four O atoms from four PO₃F²⁻ anions. Two [CdO₄(OH₂)₂] octahedra share an edge to form a dimer {Cd₂O₆(OH₂)₄}; adjacent dimers are linked by corner-sharing with PO₃F²⁻ 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 PO₃F²⁻ 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” PO₃F²⁻ anion (C_3v) was correlated with its site symmetry (C₁), as shown in

Table 4. Site symmetry analysis of the PO₃F²⁻ vibrations in the lattice of CdPO₃F(H₂O)₂ and assignment of IR and Raman bands (band positions in cm⁻¹).

	Vibrational Mode	Free Anion (C3v)	Site Symmetry (C1)	IR	Raman
ν1	ν(P–F)	A1	A	825 vs	826 m
ν2	νs(PO3)	A1	A	1006 vs	1022 vs
ν3	δ(FPO3)	A1	A
ν4	νas(PO3)	E	2A	1142 vs, 1106 vs	---
ν5	δ(PO3)	E	2A	560 sh, 541 s	540 w
ν6	ρ(PO3)	E	2A	---	395 w
	ν(OH)			3496 vs, 3393 vs, 3223 sh	---
	δ(H2O)			1648 sh, 1626 m	---

Activity: A₁, E: IR and Raman; A: IR and Raman. Intensity: vs: very strong; s: strong; m: medium; w: weak; sh: shoulder.

. 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 CdPO₃F(H₂O)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(H₂O), shows also splitting signals.

-

The antisymmetric ν(PO₃) 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(PO₃) could be identified, clearly split in the IR spectra as predicted (cf. Table 4), whereas no signals for the δ(FPO₃) mode could be found. In the Raman spectrum of a PO₃F²⁻ solution, both vibrations are reported at the same energy (520 cm⁻¹) [20], although in the case of crystalline Hg₂PO₃F, both vibrations were identified at slightly different wavenumbers, with ν₅ > ν₃ [21].

-

The corresponding ν₆-PO₃-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.

3.2. 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 PO₃F²⁻ 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(OH₂)₆] octahedra, e.g., like those found in alums [25].

[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⁻. Interestingly, all F atoms of the monofluorophosphate tetrahedra point away from the intermediate cationic layer. The two remaining [Cr(OH₂)₆] octahedra, associated with the two chromium sites Cr1 and Cr2, lie in-between adjacent {[Cr(OH₂)₆]₃(PO₃F)₆}³⁻ layers. Together with the partly disordered water molecules O1W-O5W they define an own layer with composition {[Cr(OH₂)₆](H₂O)_13.6}³⁺. Both types of layers alternate and stack along [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(OH₂)₆]₃(PO₃F)₆}³⁻ layer the aqua ligands most probably are involved in hydrogen bonding to the monofluorophosphate O atoms and also among each other; in the intermediate {[Cr(OH₂)₆](H₂O)_13.6}³⁺ layer hydrogen bonds might occur between aqua ligands and crystal water molecules. From the same orientation of all monofluorophosphate F atoms towards the {[Cr(OH₂)₆](H₂O)_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.

3.3. Pb₂(PO₃F)Cl₂(H₂O)

The crystal structure of Pb₂(PO₃F)Cl₂(H₂O) 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 [Pb1O₂(H₂O)Cl₄] and [Pb2O₇Cl₂] 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.

3.4. ZnPO₃F(H₂O)_2.5

The crystal structure of ZnPO₃F(H₂O)_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 {Zn1₂(PO₃F)₆} unit. Both Zn2 and Zn3 atoms have an octahedral coordination environment, being bound to two monofluorophosphate O and four water O atoms. The corresponding [ZnO₂(OH₂)₄] octahedra share the non-water O atoms with the {Zn1₂(PO₃F)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).

3.5. (NH₄)₂M(PO₃F)₂(H₂O)₂ (M = Co, Mg)

(NH₄)₂Co(PO₃F)₂(H₂O)₂ and (NH₄)₂Mg(PO₃F)₂(H₂O)₂ crystallize isotypically with the copper member (NH₄)₂Cu(PO₃F)₂(H₂O)₂ in space group C2/m [27]. Instead of the C-centred unit cell for this structure type, PDF entry #00-045-0355 for (NH₄)₂Co(PO₃F)₂(H₂O)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 ų. 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 (NH₄)₂Mg(PO₃F)₂(H₂O)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 ų for [23] versus 484 ų 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 (NH₄)₂Mg(PO₃F)₂(H₂O)₂ unambiguously showed the correctness of the halved cell in space group C2/m (PDF entry #00-059-0045; no structure data given).

The metal cation in the (NH₄)₂M(PO₃F)₂(H₂O)₂ (M = Co, Cu, Mg) structure is situated on Wyckoff position 2c with site symmetry 2/m and thus has four equal M–O bonds to the O2 atoms of four PO₃F²⁻ anions and two equal bonds to two trans-aligned water molecules (O1W). The monofluorophosphate anion is located about a mirror plane, just like the ammonium cation. Adjacent [MO₄(OH₂)₂] octahedra are linked by corner-sharing with the PO₃F²⁻ anions into {Mg(OH₂)(PO₃F)₂)^2-}_n strands running along [010]. Adjacent strands are aligned in parallel and are arranged into layers along (001). Within a strand, medium-strong hydrogen bonds between the water molecules and the non-coordinating O1 atom of the PO₃F²⁻ anion are established. The ammonium cations are situated between the strands and are hydrogen-bonded to the O1 and O2 atoms of the anions into a three-dimensional network (Figure 6).

3.6. (NH₄)₂Mn(PO₃F)₂(H₂O)₂

Although manganese is part of the first-row transition metals, (NH₄)₂Mn(PO₃F)₂(H₂O)₂ does not adopt the (NH₄)₂Cu(PO₃F)₂(H₂O)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 (NH₄)₂Cu(PO₃F)₂(H₂O)₂ structure type, crystallizing in space group P2₁/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 P2₁/n structure. The divalent metal position (Wyckoff position 2c) now shows site symmetry $\overline{1}$, and all other atoms are located on general sites (Wyckoff positions 4e). The general features of the (NH₄)₂Mn(PO₃F)₂(H₂O)₂ crystal structure (Figure 7) are the same as for the (NH₄)₂Cu(PO₃F)₂(H₂O)₂ structure type. {Mn(OH₂)(PO₃F)₂)^2-}_n strands run along [010] with the water molecule hydrogen-bonded to the non-coordinating O atom of the monofluorophosphate anions; adjacent strands are organised into layers parallel (101). The ammonium cations again are hydrogen-bonded to the strands by weak hydrogen bonds. As expected, the Mn–O bond lengths are the longest in all four (NH₄)₂M(PO₃F)₂(H₂O)₂ structures because Mn has the largest ionic radius of all divalent transition metal cations. Most probably, the large ionic radius of Mn²⁺ is the driving force for the symmetry reduction from C2/m to P2₁/n.

3.7. (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^I₂M^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^IO₈ 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 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 (NH₄)₂Ni(PO₃F)₂(H₂O)₆ is depicted in Figure 8.

3.8. 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$\overline{3}$m determined from the present single crystal X-ray data. In the crystal structure, isolated [Cr(OH₂)₆] octahedra (point group symmetry $\overline{3}$m) are organised in layers parallel (001) and are sandwiched by double layers of PO₃F²⁻ anions (point group symmetry 3m) along the [001] stacking direction. The disordered ammonium cations (site symmetry $\overline{3}$m) are situated between the PO₃F²⁻ 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).

3.9. 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.

3.10. (NH₄)₂Zn(PO₃F)₂(H₂O)_0.2

Three ammonium cations, two zinc cations, three monofluorophosphate anions and one positionally and occupationally disordered water molecule are present in the asymmetric unit. Except one Zn site (Zn2) located on a twofold rotation axis (Wyckoff position 4e), all other atoms in this structure are located on general positions (Wyckoff position 8f). 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$\overline{1}$) 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.

3.11. (NH₄)₂Zn₃(PO₃F)₄(H₂O)

(NH₄)₂Zn₃(PO₃F)₄(H₂O) has a higher Zn and PO₃F content than (NH₄)₂Zn(PO₃F)₂(H₂O)_0.2 (ratio NH₄:Zn:PO₃F = 2:3:4 versus 2:1:2) but together with ZnPO₃F(H₂O)_2.5 also crystallized from the same solution. A very similar I-centred cubic unit cell (without further assignments of possible space groups) was reported for anhydrous (NH₄)₂Zn₃(PO₃F)₄ at room temperature (PDF entry #00-044-0539; a = 11.4769(5) Å). In the crystal structure of (NH₄)₂Zn₃(PO₃F)₄(H₂O) 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 $\overline{4}$ 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 $\overline{4}$ axis) in a ratio of 0.67:0.33. The site symmetry of the PO₃F 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.

3.12. Survey on the PO₃F²⁻ Group

• Mean bond lengths and angles in the PO₃F tetrahedron

For the statistical analysis of bond lengths and angles within a PO₃F 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 PO₃F groups were considered as criteria, disregarding different measurement temperatures or redeterminations. HPO₃F⁻ tetrahedra present in hydrogenmonofluorophosphates were not taken into account. In summary, 88 independent PO₃F tetrahedra from 63 different monofluorophosphate phases (including the examples of the current study) were used (

Table 5. Structural details of monofluorophosphates, site symmetries of corresponding PO₃F tetrahedra, relation to the sulfate analogues and details of D–H···P–F hydrogen bonding.

Monofluorophosphate (Reference)	Space Group, Z	Site Symmetry PO3F Group(s)	D–H···F–P Hydrogen Bonding with D···F/Å and D–H···F/°	Corresponding Sulfate	Relationship	Remark
LiKPO3F(H2O) [36]	P21/c, 4	1	3.15; 120	-	-	-
Li(NH4)PO3F [37]	P21/c, 4	1	2.98; 147	Li(NH4)(SO4) P21cn, Z = 4; P21/c Z = 8; Pmcn, Z = 8.	-	Sulfate shows poly-morphism.
Na2PO3F [38]	P212121, 8	1, 1	-	Na2SO4 Fddd, Z = 8; P63/mmc, Z = 2.	-	Sulfate shows dimorphism.
Na2PO3F(H2O)10 [39]	P21/c, 4	1	2.83, 149 3.01, 178	Na2SO4(H2O)10 (Glauber salt)	Isotypic	-
NaK3(PO3F)2 [40]	P$\overline{3}$m1, 1	3m	-	NaK3(SO4)2 (glaserite)	Isotypic	-
(NH4)Na(PO3F)(H2O) [41]	Pn, 2	1	-	-	-	-
K2PO3F [6,42]	Pnam, 4	m	-	K2SO4 (arcanite); high-temperature form: P63/mmc; Z = 2.	Isotypic	Isotypic with low-temperature form;
K3(PO3F)F [34]	I4/mcm, 4	$\overline{4}$2m	-	K3(PO3F)F	Isotypic,	PO3F disordered. Phase transition reported for the sulfate.
Rb2PO3F [43]	Pnma, 4	m	-	Rb2SO4	Isotypic	K2SO4 structure type.
Cs2PO3F [43]	Pnma, 4	m	-	Cs2SO4	Isotypic	K2SO4 structure type.
Cs3(NH4)2(HPO3F)3(PO3F) [44]	P21/c, 8	1,1,1,1	-	-	-	-
(NH4)2(PO3F) [45]	Pna21, 4	1	-	(NH4)2(SO4)	Isotypic	Ferroelectric phase; K2SO4 structutre type.
(NH4)2(PO3F)(H2O) [46,47,48]	P21/c, 4	1	-	-	-	-
(NH4)2Mg(PO3F)2(H2O)2 [this work]	C2/m, 2	m	-	-	-	-
CaPO3F(H2O)2 [46]	P$\overline{1}$, 2	1	3.12, 147	CaPO4(H2O)2 (gypsum) C2/m, Z = 4.	-	-
SrPO3F [49]	P21/c, 4	1	-	Barite-type SrSO4; Pnma, Z = 4;	-	SrPO3F adopts the monazite structure type.
SrPO3F(H2O) [49]	P21/c, 4	1	2.77, 108 2.95, 110	-	-	X-ray powder data.
BaPO3F [50]	P21/c, 8	1, 1	-	BaSO4 (barite), Pnma, Z = 4; F$\overline{4}$3m, Z = 4 (HT)	-	Sulfate shows dimorphism.
CsTi2F2(PO4)(PO3F)2 [51]	P2/c, 2	1	-	-	-
Cr2(PO3F)3(H2O)18.8 [this work]	P$\overline{1}$, 4	1, 1, 1, 1, 1, 1	?	-	-	H atoms not determined.
NH4Cr(PO3F)2(H2O)6 [this work]	R$\overline{3}$m, 3	3m	-	-	-	-
MnPO3F(H2O)2 [52]	P$\overline{1}$, 2	1	2.91, 111 3.11, 128 3.18, 127	-	-	-
Li3Mn(PO3F)2F2 [53]	P21/c, 2	1	-	-	-	-
KMnF2(PO3F) [53]	P21/n, 4	1	-	-	-	-
K2Mn3(HPO4)2(PO3F)F [54]	P21/c, 4	1	3.09, 142 2.93, 122	-	-	-
Rb3Mn3(PO4)(PO3F)2F5 [53]	Cc, 4	1, 1	-	-	-	-
Cs2Mn2F4(PO3F)2 [53]	P21, 2	1, 1	-	-	-	-
(NH4)2Mn(PO3F)2(H2O)2 [this work]	P21/n, 2	1	-	-	-	-
(NH4)Mn(PO3F)F2 [53,55]	P21/n, 4	1	-	NH4Mn(SO4)F2, Pnna, Z = 8.	-	-
(NH4)2Mn3(HPO4)2(PO3F)F2 [54]	P21/c, 4	1	N–H 3.11, 160 O–H 2.87, 104	-	-	-
(NH4)Mn3(PO3F)2(H2PO4)F2 [54]	C2/c, 4	1	O–H 2.97, 128	-	-	Same unit cell as (NH4)Mn3(PO3F)2(PO2F2)F2 *.
(NH4)Mn3(PO3F)2(PO2F2)F2 [56]	C2/c, 4	1	-	-	-	Same unit cell as (NH4)Mn3(PO3F)2(H2PO4)F2 *.
Ba2Mn2(PO3F)F6 [57]	P21/c, 4	1	-	-	-	-
Fe2(PO3F)3 [33]	P63/m, 6	m, m, m	-	Fe2(SO4)3, P21/n, Z = 4; R$\overline{3}$, Z = 6.	-	Sulfate shows dimorphism.
NaFe(PO3F)2 [33]	P21/c, 4	1, 1	-	NaFe(SO4)2, C2/m, Z = 2	-	-
KFe(PO3F)F2 [54]	P21/c, 4	1	-	-	-	-
KFe2(PO2F2)(PO3F)2F2 [33]	P$\overline{1}$, 2	1, 1	-	-	-	-
RbFe3(PO3F)((PO2)2(F1.5(OH)0.5)2)F2 [33]	C2/c, 4	1	-	-	-	-
Cs2Fe2F3(PO3F)2(PO2F2) [33]	Aea2, 4	1	-	-	-	-
(NH4)2Fe2(PO3F)2FCl2 [33]	Pca21, 4	1, 1	-	-	-	H atoms not determined.
(NH4)3Fe(PO3F)2F2 [33]	P21/m, 4	m, m	-	-	-	F and O wrongly assigned for P2.
CoPO3F(H2O)3 [58]	P$\overline{1}$, 2	1	2.98, 118	-	-	-
(NH4)2Co(PO3F)2(H2O)2 [this work]	C2/m, 2	m	-	-	-	-
(NH4)Co3(PO3F)2(PO2F2)F2 [56]	C2/c, 4	1	2.96, 115	-	-	-
Ba2Co2(PO3F)F6 [55]	P21/c, 4	1	-	-	-	-
(NH4)2(Ni(H2O)6)(PO3F)2 [15]	P21/c, 2	1	2.84, 176	(NH4)2(Ni(H2O)6(SO4)2	Isotypic	Picromerite structure type.
(NH4)2(Ni(H2O)6)(PO3F)2 [this work]	P21/c, 2	1	2.81, 179	(NH4)2(Ni(H2O)6 (SO4)2	Isotypic	-
Ba2Ni2(PO3F)F6 [57]	P21/c, 4	1	-	-	-	-
CuPO3F(H2O)2 [59]	P21/c, 4	1	-	-	-	-
Cu2K(OH)(PO3F)2(H2O) [30]	C2/m, 2	m	3.00, 174	Cu2K(H3O2) (SO4)2	Isotypic
KCu3(PO2F2)(PO3F)2F2 [55]	C2/c, 4	1	-	-	-	-
RbCu3(PO2F2)(PO3F)2F2 [55]	C2/c, 4	1	-	-	-	-
NH4Cu2(H3O2)(PO3F)2 [this work]	C2/m. 2	m	O–H 3.18, 177 N–H 2.97, 139	-KCu2(H3O2)(SO4)2		Natrochalcite structure type.
(NH4)2(Cu(H2O)2(PO3F)2) [27]	C2/m, 2	m	-	-	-	-
Ba2Cu2(PO3F)F6 [57]	P21/c, 4	1	-	-	-
ZnPO3F(H2O)2.5 [14]	P$\overline{1}$, 4	1, 1	-	-		H atoms not reliably determined.
ZnPO3F(H2O)2.5 [this work]	P$\overline{1}$, 4	1, 1	-	-	-	-
(NH4)2Zn(PO3F)2(H2O)0.2 [this work]	C2/c, 12	1, 1, 1	?	-	-	H atoms of water not determined.
(NH4)2Zn3(PO3F)4(H2O) [this work]	I$\overline{4}$3d, 4	3	(N,O)– 3.17, 141	-		-
SnPO3F [60]	P21/c, 4	1	-	Barite-type SnSO4,Pnma, Z = 4	-	-
Ag2PO3F [4]	C2/c, 8	1	-	Ag2SO4, Fddd, Z = 8; P63/mmc, Z = 2	-	Sulfate shows dimorphism.
(NH4)Ag3(PO3F)2 [61]	I2, 8	1, 1, 1, 1	?	-		H atoms not determined
CdPO3F(H2O)2 [this work]	P-$\overline{1}$, 2	1	3.14, 121 3.09, 118	-		-
Hg2PO3F [21]	Ibam, 8	m	-	Hg2SO4, P2/c, Z = 2	-	-
Pb2PO3FCl2(H2O) [this work]	Pnma, 4	m	?	-		H atoms not determined.

* It is most unlikely that (NH₄)Mn₃(PO₃F)₂(PO₂F₂)F₂ and (NH₄)Mn₃(PO₃F)₂(H₂PO₄)F₂ 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., PO₂F₂- and PO₂(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.

). 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 PO₃F tetrahedron differ markedly from those of the difluorophosphate PO₂F₂ 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 PO₂F₂ 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 (NH₄)₃Fe(PO₃F)₂F₂ [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–F bond, and with O–P–O and O–P–F angles unexpected: P2–O1 = 1.562(5) Å, P2–F4 = 1.498(5) Å, P2–O4 = 1.485(6) Å (2x); O1–P2–F4 = 106.7(3)°, O1–P2–O4 = 102.7(2)° (2x), F4–P2–O4 = 115.0(2)° (2x), O4–P2–O4 = 112.8(3)°. Based on the current averaged data for a PO₃F tetrahedron, it is clear that atoms O1 and F4 were wrongly assigned and must be interchanged.

• Symmetry of the PO₃F group in crystal structures

Possible point group symmetries of a PO₃F 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 $\overline{4}$2m for the PO₃F group in K₃(PO₃F)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, only nine show isotypism with the corresponding sulfate, viz. Na₂PO₃F(H₂O)₁₀, NaK₃(PO₃F)₂, K₃(PO₃F)F, M₂PO₃F (M = K, Rb, Cs, NH₄), (NH₄)₂Ni(PO₃F)(H₂O)₆ and CuK(OH)(PO₃F)(H₂O). Eleven monofluorophosphate phases have equivalent sulfate phases but with different crystal structures, viz. Li(NH₄)PO₃F, Na₂PO₃F, CaPO₃F(H₂O)₂, SrPO₃F, BaPO₃F, (NH₄)Mn(PO₃F)F₂, Fe₂(PO₃F)₃, NaFe(PO₃F)₂, SnPO₃F, Ag₂PO₃F, and Hg₂PO₃F, 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(NH₄)PO₃F, Na₂PO₃F(H₂O)₁₀, CaPO₃F(H₂O)₂, Cu₂K(OH)(H₂O)(PO₃F)₂, K₂Mn₃(HPO₄)₂(PO₃F)F, and (NH₄)₂Ni(PO₃F)₂(H₂O)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.

4. 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 PO₃F²⁻ 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 PO₃F 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” PO₃F 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 C₁ (1) followed by point group symmetry C_s (m) with 15 examples and C₃ (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.