The alkali hypophosphites KH₂PO₂, RbH₂PO₂ and CsH₂PO₂

The structures of the hypophosphites KH₂PO₂ (potassium hypophosphite), RbH₂PO₂ (rubidium hypophosphite) and CsH₂PO₂ (caesium hypophosphite) have been determined by single-crystal X-ray diffraction. The structures consist of layers of alkali cations and hypophosphite anions, with the latter bridging four cations within the same layer. The Rb and Cs hypophosphites are isomorphous.

S1. Comment

The hypophosphite H₂PO₂²⁻ anion is a distorted tetrahedron, with the P atom at the centre and two O and two H atoms at the vertices. This anion may coordinate a metal cation via the O atoms in different ways, viz. as a mono- or bidentate ligand, or as a bridging ligand between two cations. The goal of the present work was to investigate further the functional role of the hypophosphite anion in the crystals of the title compounds.

Previous crystal-structure investigations of anhydrous hypophosphites include Ca(H₂PO₂)₂ (Goedkoop & Loopstra, 1959), CaNa(H₂PO₂)₂ (Matsuzaki & Iitaka, 1969), Cu(H₂PO₂)₂ (Naumov et al., 2002), Zn(H₂PO₂)₂ (Weakley, 1979; Tanner et al., 1997), GeCl(H₂PO₂) and SnCl(H₂PO₂) (Weakley & Watt, 1979), La(H₂PO₂)₃ (Tanner et al., 1999), Er(H₂PO₂)₃ (Aslanov et al., 1975), and U(H₂PO₂)₄ (Tanner et al., 1992). The limited number of studies is probably due to the difficulty of preparation of these compounds, resulting from their highly hygroscopic nature and their low stability. The title compounds are rather easy to synthesize but crystals are difficult to grow.

The present crystal-structure determinations show that the unit-cell of KH₂PO₂ differs in size and symmetry from those of the isostructural Rb and Cs analogues. The monoclinic unit cell of KH₂PO₂ is related to the orthorhombic unit cell of the Rb and Cs hypophosphites by the matrix (1/2 − 1 0 / 1/2 1 0 / 0 0 1).

All three compounds adopt layer structures, in which the metal cations are coordinated by hypophosphite anions (Figs. 1 and 2). The packing of the cations and P atoms is identical in all three compounds, and the cations and P positions are nearly coplanar, forming a layer similar to a (100) NaCl layer.

The full coordination environments of the K⁺ and Rb⁺ cations are shown in Figs. 3 and 4, respectively. The Rb and Cs environments are identical to those found in KF₂PO₂, RbF₂PO₂ and CsF₂PO₂ (Harrison et al., 1966; Granier et al., 1975; Trotter & Withlow, 1967). The H-atom positions in the present hypophosphites correspond to the F positions in the difluorophosphates.

S2. Experimental

It was established during crystal growth experiments that the preparation of anhydrous hypophosphites of K, Rb and Cs from aqueous solutions depends on the precursor and on the presence of impurities in that precursor at levels of less than 3–5%. Crystals of the title K, Rb, Cs hypophosphites were grown from aqueous solutions prepared by the reaction of hypophosphorous acid with the corresponding alkali carbonates. Crystal growth was carried out at 313 K in a dry box under dry nitrogen. Crystal growth is not possible at room temperature in air because of the strong hygroscopic nature of the hypophosphites and the tendency of the crystals to dissolve as a result of the fast absorption of water vapour from the atmosphere (increasing in the order K, Rb, Cs). The crystals were protected from moisture by coating them with epoxy resin. Powder diffraction analysis shows agreement between the bulk products and the single crystals. However, in the case of the rubidium hypophosphite, the presence of additional peaks in the powder pattern shows the presence of an additional phase similar to potassium hypophosphite.

S3. Refinement

The H atoms were located from difference electron-density maps and their positions were refined without any constraint.

Computing details

For all compounds, data collection: CD4CA0 (Enraf-Nonius, 1989); cell refinement: CD4CA0; data reduction: CADDAT (Enraf-Nonius, 1989); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The (010) layer in KH2PO2. Displacement ellipsoids are plotted at the 50% probability level and H atoms are drawn as small spheres of arbitrary radii.
	Fig. 2. The (001) layer in RbH2PO2. Displacement ellipsoids are plotted at the 50% probability level and H atoms are drawn as small spheres of arbitrary radii.
	Fig. 3. The coordination environment of the K^+ cation in KH2PO2.
	Fig. 4. The coordination environment of the Rb^+ cation in RbH2PO2.

(I) Potassium hypophosphite

Crystal data

KH2PO2	F(000) = 208
Mr = 104.09	Dx = 2.011 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 22 reflections
a = 7.3131 (10) Å	θ = 9.0–14.7°
b = 7.2952 (8) Å	µ = 1.78 mm^−1
c = 7.1814 (10) Å	T = 296 K
β = 116.205 (10)°	Plate, colourless
V = 343.75 (8) Å^3	0.59 × 0.46 × 0.06 mm
Z = 4

Data collection

Enraf-Nonius CAD4 diffractometer	458 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.020
Graphite monochromator	θmax = 29.9°, θmin = 4.2°
2θ/θ scans	h = 0→10
Absorption correction: empirical (using intensity measurements) (CADDAT; Enraf-Nonius, 1989)	k = −1→10
Tmin = 0.421, Tmax = 0.903	l = −10→9
606 measured reflections	3 standard reflections every 60 min
500 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.031	All H-atom parameters refined
wR(F^2) = 0.079	w = 1/[σ^2(Fo^2) + (0.0549P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.14	(Δ/σ)max < 0.001
500 reflections	Δρmax = 0.50 e Å^−3
25 parameters	Δρmin = −0.59 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.075 (8)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
K	0.5000	0.14284 (7)	0.2500	0.0267 (2)
P	0.0000	0.17744 (8)	0.2500	0.0257 (2)
H	−0.119 (4)	0.066 (3)	0.112 (4)	0.048 (7)*
O	0.1173 (2)	0.28267 (19)	0.16055 (19)	0.0337 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K	0.0290 (3)	0.0334 (3)	0.0221 (3)	0.000	0.0153 (2)	0.000
P	0.0310 (4)	0.0282 (3)	0.0240 (3)	0.000	0.0178 (3)	0.000
O	0.0364 (7)	0.0429 (7)	0.0331 (6)	0.0012 (5)	0.0256 (6)	0.0044 (5)

Geometric parameters (Å, º)

K—O	2.7747 (14)	P—O	1.4914 (12)
K—O^i	2.7368 (13)	P—O^vi	1.4914 (12)
K—O^ii	2.7368 (13)	P—H	1.28 (3)
K—O^iii	2.7747 (14)	O—K^i	2.7368 (13)
K—O^iv	2.9220 (15)	O—K^vii	2.9220 (15)
K—O^v	2.9220 (15)
O—K—O^i	82.73 (4)	O—K—O^v	91.00 (2)
O—K—O^ii	88.89 (3)	O^iv—K—O^v	51.90 (5)
O—K—O^iii	136.86 (6)	O—P—O^vi	118.04 (11)
O^i—K—O^ii	157.10 (6)	O—P—H	108.3 (10)
O^i—K—O^iii	88.89 (3)	O^vi—P—H	109.8 (11)
O^ii—K—O^iii	82.73 (4)	H—P—H^vi	101 (2)
O^i—K—O^iv	85.67 (4)	P—O—K^i	126.87 (7)
O^ii—K—O^iv	115.63 (4)	P—O—K	115.10 (7)
O^iii—K—O^iv	91.00 (2)	K^i—O—K	97.27 (4)
O—K—O^iv	130.07 (5)	P—O—K^vii	95.03 (6)
O^i—K—O^v	115.63 (4)	K^i—O—K^vii	94.33 (4)
O^ii—K—O^v	85.67 (4)	K—O—K^vii	130.07 (5)
O^iii—K—O^v	130.07 (5)

Symmetry codes: (i) −x+1/2, −y+1/2, −z; (ii) x+1/2, −y+1/2, z+1/2; (iii) −x+1, y, −z+1/2; (iv) x+1/2, y−1/2, z; (v) −x+1/2, y−1/2, −z+1/2; (vi) −x, y, −z+1/2; (vii) x−1/2, y+1/2, z.

(II) Rubidium hypophosphite

Crystal data

RbH2PO2	F(000) = 280
Mr = 150.46	Dx = 2.595 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 24 reflections
a = 7.9835 (9) Å	θ = 9.9–14.2°
b = 6.3678 (7) Å	µ = 13.06 mm^−1
c = 7.5755 (11) Å	T = 298 K
V = 385.12 (8) Å^3	Plate, colourless
Z = 4	0.32 × 0.16 × 0.04 mm

Data collection

Enraf-Nonius CAD4 diffractometer	289 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.032
Graphite monochromator	θmax = 24.9°, θmin = 3.7°
2θ/θ scans	h = −9→9
Absorption correction: empirical (using intensity measurements) (CADDAT; Enraf-Nonius, 1989)	k = 0→7
Tmin = 0.095, Tmax = 0.593	l = 0→9
732 measured reflections	3 standard reflections every 60 min
366 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.023	Hydrogen site location: difference Fourier map
wR(F^2) = 0.057	All H-atom parameters refined
S = 0.96	w = 1/[σ^2(Fo^2) + (0.026P)^2] where P = (Fo^2 + 2Fc^2)/3
366 reflections	(Δ/σ)max < 0.001
27 parameters	Δρmax = 0.58 e Å^−3
0 restraints	Δρmin = −0.58 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Rb	0.63134 (9)	0.2500	0.13795 (8)	0.0322 (3)
P	0.3847 (3)	0.7500	0.3289 (2)	0.0343 (4)
H1	0.491 (10)	0.7500	0.437 (8)	0.041*
H2	0.255 (9)	0.7500	0.439 (9)	0.041*
O	0.3797 (5)	0.5504 (4)	0.2297 (4)	0.0434 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb	0.0286 (4)	0.0313 (4)	0.0368 (4)	0.000	−0.0015 (4)	0.000
P	0.0356 (10)	0.0335 (8)	0.0336 (9)	0.000	0.0024 (10)	0.000
O	0.0471 (19)	0.0297 (15)	0.0533 (19)	0.000 (2)	0.0100 (19)	−0.0003 (15)

Geometric parameters (Å, º)

Rb—O	2.859 (3)	P—O	1.478 (3)
Rb—O^i	2.932 (3)	P—O^vi	1.478 (3)
Rb—O^ii	2.859 (3)	P—H1	1.18 (7)
Rb—O^iii	2.932 (3)	P—H2	1.33 (7)
Rb—O^iv	3.063 (3)	O—Rb^vii	2.932 (3)
Rb—O^v	3.063 (3)	O—Rb^v	3.063 (3)
O—Rb—O^ii	83.96 (13)	O^iv—Rb—O^v	49.05 (10)
O—Rb—O^iii	87.46 (7)	O—P—O^vi	118.7 (3)
O—Rb—O^i	145.93 (3)	O—P—H1	111.9 (13)
O^ii—Rb—O^iii	145.93 (3)	O^vi—P—H1	111.9 (13)
O^ii—Rb—O^i	87.46 (7)	O—P—H2	107.4 (13)
O^iii—Rb—O^i	81.44 (12)	O^vi—P—H2	107.4 (13)
O^ii—Rb—O^iv	85.58 (9)	H1—P—H2	97 (4)
O—Rb—O^iv	118.58 (6)	P—O—Rb	132.9 (2)
O^iii—Rb—O^iv	126.97 (7)	P—O—Rb^vii	113.91 (18)
O^i—Rb—O^iv	93.43 (6)	Rb—O—Rb^vii	97.00 (8)
O^ii—Rb—O^v	118.58 (6)	P—O—Rb^v	96.11 (13)
O—Rb—O^v	85.58 (9)	Rb—O—Rb^v	94.42 (9)
O^iii—Rb—O^v	93.43 (6)	Rb^vii—O—Rb^v	124.22 (12)
O^i—Rb—O^v	126.97 (7)

Symmetry codes: (i) x+1/2, −y+1/2, −z+1/2; (ii) x, −y+1/2, z; (iii) x+1/2, y, −z+1/2; (iv) −x+1, y−1/2, −z; (v) −x+1, −y+1, −z; (vi) x, −y+3/2, z; (vii) x−1/2, y, −z+1/2.

(III) Caesium hypophosphite

Crystal data

CsH2PO2	F(000) = 352
Mr = 197.90	Dx = 2.991 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 24 reflections
a = 8.3776 (9) Å	θ = 9.5–13.7°
b = 6.6271 (6) Å	µ = 8.61 mm^−1
c = 7.9165 (10) Å	T = 298 K
V = 439.52 (8) Å^3	Plate, colourless
Z = 4	0.45 × 0.30 × 0.11 mm

Data collection

Enraf-Nonius CAD4 diffractometer	554 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.058
Graphite monochromator	θmax = 29.9°, θmin = 3.5°
2θ/θ scans	h = −1→11
Absorption correction: empirical (using intensity measurements) (CADDAT; Enraf-Nonius, 1989)	k = 0→9
Tmin = 0.056, Tmax = 0.388	l = 0→11
769 measured reflections	3 standard reflections every 60 min
687 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.037	All H-atom parameters refined
wR(F^2) = 0.114	w = 1/[σ^2(Fo^2) + (0.07P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max < 0.001
687 reflections	Δρmax = 1.41 e Å^−3
27 parameters	Δρmin = −1.06 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.050 (4)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cs	0.63434 (6)	0.2500	0.13628 (8)	0.0404 (3)
P	0.3901 (3)	0.7500	0.3333 (4)	0.0461 (6)
H1	0.528 (13)	0.7500	0.425 (15)	0.055*
H2	0.255 (13)	0.7500	0.453 (16)	0.055*
O	0.3849 (5)	0.5577 (9)	0.2368 (9)	0.0584 (15)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.0410 (4)	0.0360 (4)	0.0443 (4)	0.000	−0.0005 (2)	0.000
P	0.0511 (14)	0.0427 (14)	0.0445 (14)	0.000	0.0017 (11)	0.000
O	0.065 (3)	0.036 (3)	0.074 (4)	0.003 (2)	0.012 (3)	0.000 (3)

Geometric parameters (Å, º)

Cs—O^i	3.026 (5)	P—O	1.487 (6)
Cs—O	3.026 (5)	P—O^vi	1.487 (6)
Cs—O^ii	3.094 (5)	P—H1	1.37 (11)
Cs—O^iii	3.094 (5)	P—H2	1.48 (12)
Cs—O^iv	3.221 (7)	O—Cs^vii	3.094 (5)
Cs—O^v	3.221 (7)	O—Cs^v	3.221 (7)
O—Cs—O^i	84.7 (2)	O^iv—Cs—O^v	46.6 (2)
O—Cs—O^ii	86.51 (14)	O—P—O^vi	118.1 (5)
O—Cs—O^iii	145.80 (5)	O—P—H1	107 (2)
O^i—Cs—O^ii	145.80 (5)	O^vi—P—H1	107 (2)
O^i—Cs—O^iii	86.51 (14)	O—P—H2	108 (2)
O^ii—Cs—O^iii	82.45 (19)	O^vi—P—H2	108 (2)
O^i—Cs—O^iv	86.57 (17)	H1—P—H2	108 (7)
O—Cs—O^iv	118.25 (11)	P—O—Cs	133.8 (3)
O^ii—Cs—O^iv	126.33 (16)	P—O—Cs^vii	114.7 (3)
O^iii—Cs—O^iv	94.06 (9)	Cs—O—Cs^vii	96.31 (16)
O^i—Cs—O^v	118.25 (11)	P—O—Cs^v	97.7 (3)
O—Cs—O^v	86.57 (17)	Cs—O—Cs^v	93.43 (17)
O^ii—Cs—O^v	94.06 (9)	Cs^vii—O—Cs^v	121.64 (18)
O^iii—Cs—O^v	126.33 (16)

Symmetry codes: (i) x, −y+1/2, z; (ii) x+1/2, y, −z+1/2; (iii) x+1/2, −y+1/2, −z+1/2; (iv) −x+1, y−1/2, −z; (v) −x+1, −y+1, −z; (vi) x, −y+3/2, z; (vii) x−1/2, y, −z+1/2.