Dicaesium silver zinc thio­cyanate, Cs₂[AgZn(SCN)₅]

The title compound, dicaesium(I)-μ-thio­cyanato-κ²N:S-zinc(II)-tetra-μ-thio­cyanato-κ²S:N-argentate(I), crystallizes in the orthorhombic space group Pmn2₁ and contains units of composition AgZn(SCN)₃ lying on a mirror plane and bonded together through Cs⁺ ions and thio­cyanate groups. The crystal studied contained equal numbers of inversion twins.

S1. Comment

The title compound has been known since the beginning of the last century (Wells, 1902, 1922). At that time, most studies were synthetic and analytical. Since those days, many crystal structures of thiocyanates have been solved. The crystal structure of the title compound has not been reported previously. Caesium, silver and zinc all form simple thiocyanates. CsSCN crystallizes in space group Pnma, AgSCN crystallizes in two polymorphic forms in space groups Pmnn and C2/c, and Zn(NCS)₂ crystallizes in space group P1.

There are also other triple thiocyanates similar to the title compound, such as Cs[AgZn(SCN)₄]·H₂O, Cs[Ag₃Zn₂(SCN)₈] and Cs[Ag₄Zn₂(SCN)₉] (Wells, 1902, 1922). We are especially interested in thiocyanates which include an odd number of thiocyanate groups. We presume that an odd number of thiocyanate groups could lead to a non-centrosymmetric crystal structure. Some very interesting optical, electro-optical and electrostrictive properties are related to non-centrosymmetric crystal structures. There are also other thiocyanate complexes of silver, such as Cs₃Sr[Ag₂(SCN)₇] and Cs₃Ba[Ag₂(SCN)₇] (Bohaty & Fröhlich, 1992), which have been found to have these same properties.

Caesium is 8 + 1-coordinated, with three S and five N atoms around it, and moreover, one C atom at a distance of 3.462 (6) Å. In thiocyanate structures, it is unusual to have a bond between a cation and the C atom of the thiocyanate group. The distance between the C and N atoms (about 1.15 Å) of the thiocyanate group indicates a triple bond between them, and the caesium is not actually bonded straight to the C4 atom (Fig. 1) but, by back coordination, forms a dihapto (η²) π bond to the triple bond between the C4 and N4 atoms (see Fig. 1). The average S—C distance is 1.63 Å. The angles of the thiocyanate groups (at C) are all close to 180°. Zinc and silver are both tetrahedrally tetracoordinated, zinc being surrounded by four N atoms and silver being surrounded by four S atoms. The tetrahedra around silver and zinc are both slightly distorted. The coordination of the metal atoms in the title structure is shown in Fig. 1.

The structure of Cs₂[AgZn(SCN)₅] is very interesting; the Flack parameter of 0.53 (3) indicates that the crystal studied contained equal numbers of inversion twins. One unit cell can be thought of as being composed of two basic units, indicated in Fig. 1 with thicker lines. The basic unit contains a mirror plane. Most of the atoms lie on this mirror plane, as can be deduced also from Fig. 1, with only Cs and one of the thiocyanate groups lying out of the mirror plane. In other words, the structure contains units of composition AgZn(SCN)₃ lying on a mirror plane and bonded together through the Cs⁺ ions and thiocyanate groups. The structure can also be described with polyhedra around the metal atoms (Fig. 2). The basic unit described in Fig. 1 is shown in Fig. 2 with hatched polyhedra (bottom left). The thiocyanate groups of the basic unit are not shown fully in Fig. 2, with only the atoms in the coordination spheres being displayed. The vertices of the Cs polyhedra are composed of N and S atoms only, as the Cs—C bond indicated hatched in Fig. 1 has been omitted in Fig. 2. The polyhedra form continuous parallel chains in the direction of the a axis. The polyhedra between two Cs atoms share one common face, the polyhedra between Zn and two Cs atoms share two common edges, and the polyhedra between Ag and two Cs atoms share one common vertex. The chains passing through a unit cell are located at a distance of approximately (0,0,1/2) and are related by both the n-glide (i.e. are shifted x = 1/2, and z = 1/2) and the 2₁ axis. The parallel chains are bonded together with thiocyanate groups, forming a continuous network.

S2. Experimental

Silver thiocyanate was obtained from the Aldrich Chemical Company Inc. and zinc thiocyanate was obtained from the City Chemical LLC. Caesium thiocyanate was synthesized as follows: NH₄SCN (6.98 g) was dissolved in deionized water (20.0 g) and Cs₂CO₃ (15.0 g) was dissolved separately in deionized water (55.0 g). The solutions were mixed and the mixture heated under magnetic stirring until the smell of ammonia was no longer detected. The resulting solution was evaporated close to dryness on a water bath with continuous stirring. CsSCN was dried for two days using a vacuum pump and stored in an desiccator. Cs₂[AgZn(SCN)₅] was synthesized at room temperature by dissolving CsSCN (2.30 g) in deionized water (3.00 g) and then dissolving AgSCN (0.125 g) in the resulting solution. An aqueous solution of zinc thiocyanate was prepared by dissolving Zn(SCN)₂ (0.270 g) in deionized water (3.25 g). When these two solutions were combined, white transparent crystals of Cs₂[AgZn(SCN)₅] formed immediately.

S3. Refinement

The s.u.'s of the cell constants indicate the internal consistency of the measurements themselves, i.e. the precision of the measurements, not their accuracy. A multi-scan absorption correction (XABS2; Parkin et al., 1995) was performed but not applied. The absorption correction was found to have no significant effect on the refinement results.

Computing details

Data collection: COLLECT (Nonius, 2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2000); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The coordination of the metal atoms in Cs2[AgZn(SCN)5] (the basic structural unit is indicated with thicker lines) displacement ellipsoids at the 50% probability level.
	Fig. 2. The layered structure of the title compound indicated as coordination polyhedra around the metal atoms. The basic unit indicated in Fig. 1 is indicated by hatched polyhedra (bottom left). 2 × 1 × 2 unit cell viewed along [3,10,1]. Shading conventions used: Cs light grey, Ag medium grey and Zn dark grey.

dicaesium(I)-µ-thiocyanate-κ²N,S-zinc(II)-tetra-µ-thiocyanato-κ²S,N- argentate(I)

Crystal data

Cs2[AgZn(SCN)5]	Dx = 2.828 Mg m^−3
Mr = 729.46	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pmn21	Cell parameters from 1721 reflections
a = 10.8990 (4) Å	θ = 1.0–30.5°
b = 5.8759 (2) Å	µ = 7.33 mm^−1
c = 13.3784 (5) Å	T = 293 K
V = 856.77 (5) Å^3	Prism, white
Z = 2	0.1 × 0.1 × 0.1 mm
F(000) = 664

Data collection

Nonius KappaCCD diffractometer	1809 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.060
Horizonally mounted graphite crystal monochromator	θmax = 30.5°, θmin = 2.4°
Detector resolution: 9 pixels mm^-1	h = −15→14
CCD scans	k = −7→8
5859 measured reflections	l = −14→19
2139 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0318P)^2 + 0.9521P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.038	(Δ/σ)max = 0.001
wR(F^2) = 0.087	Δρmax = 0.88 e Å^−3
S = 1.05	Δρmin = −1.28 e Å^−3
2139 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
105 parameters	Extinction coefficient: 0.0039 (5)
1 restraint	Absolute structure: Flack (1983), 714 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.53 (3)

Special details

Experimental. Powder diffraction study of the title compound indicated some minor impurities in the sample. The slight by-precipitation of impurities during the synthesis could not be avoided because the crystallization of the sample occurs instantaneously after mixing the two solutions.

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
Ag	0.0000	0.47661 (10)	−0.56292 (6)	0.0425 (2)
Cs	0.23264 (4)	0.00698 (9)	−0.24246 (5)	0.05860 (19)
Zn	0.0000	−0.01403 (13)	0.01870 (7)	0.0252 (2)
S1	0.0000	0.0396 (3)	−0.58343 (18)	0.0393 (5)
S2	0.0000	0.5910 (4)	−0.7559 (2)	0.0447 (6)
S3	0.20691 (15)	0.5975 (3)	−0.4840 (2)	0.0531 (5)
S4	0.0000	−0.4485 (5)	−0.2292 (3)	0.0598 (8)
C1	0.0000	0.0180 (12)	−0.4587 (8)	0.034 (2)
C2	0.0000	0.3522 (13)	−0.8181 (6)	0.0262 (16)
C3	0.2916 (5)	0.3683 (9)	−0.4842 (5)	0.0289 (11)
C4	0.0000	−0.6535 (14)	−0.1485 (7)	0.0324 (18)
N1	0.0000	−0.0006 (12)	−0.3733 (7)	0.049 (2)
N2	0.0000	0.1811 (12)	−0.8603 (5)	0.0348 (16)
N3	0.3535 (4)	0.2081 (8)	−0.4833 (5)	0.0384 (11)
N4	0.0000	0.1917 (11)	−0.0953 (6)	0.0366 (17)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag	0.0425 (4)	0.0341 (3)	0.0508 (4)	0.000	0.000	0.0023 (4)
Cs	0.0327 (2)	0.0883 (4)	0.0548 (3)	0.0007 (2)	0.0041 (3)	−0.0086 (2)
Zn	0.0252 (5)	0.0234 (4)	0.0270 (5)	0.000	0.000	−0.0003 (5)
S1	0.0500 (13)	0.0324 (9)	0.0355 (13)	0.000	0.000	−0.0056 (10)
S2	0.0794 (17)	0.0262 (9)	0.0285 (12)	0.000	0.000	−0.0039 (9)
S3	0.0328 (8)	0.0328 (7)	0.0937 (14)	0.0102 (7)	−0.0153 (11)	−0.0175 (10)
S4	0.082 (2)	0.0430 (13)	0.0546 (19)	0.000	0.000	0.0220 (14)
C1	0.030 (4)	0.019 (3)	0.053 (7)	0.000	0.000	0.007 (3)
C2	0.027 (4)	0.027 (4)	0.024 (4)	0.000	0.000	−0.002 (3)
C3	0.024 (2)	0.033 (3)	0.030 (3)	−0.001 (2)	−0.001 (3)	−0.002 (3)
C4	0.030 (4)	0.032 (4)	0.035 (5)	0.000	0.000	0.003 (4)
N1	0.067 (6)	0.043 (4)	0.039 (5)	0.000	0.000	0.009 (4)
N2	0.037 (4)	0.034 (4)	0.033 (4)	0.000	0.000	−0.004 (3)
N3	0.032 (2)	0.037 (2)	0.046 (3)	0.006 (2)	−0.001 (3)	0.001 (3)
N4	0.035 (4)	0.029 (3)	0.046 (5)	0.000	0.000	0.008 (3)

Geometric parameters (Å, º)

Ag—S1	2.582 (2)	S2—C2	1.631 (8)
Ag—S3	2.5893 (19)	S2—Cs^x	3.7555 (14)
Ag—S3^i	2.5893 (19)	S2—Cs^xi	3.7555 (14)
Ag—S2	2.668 (3)	S3—C3	1.633 (5)
Cs—N1	3.081 (6)	S3—Cs^ii	4.039 (3)
Cs—N4	3.388 (5)	S4—C4	1.617 (9)
Cs—C4^ii	3.462 (6)	S4—Cs^i	3.691 (2)
Cs—N2^iii	3.492 (4)	S4—Cs^xii	4.086 (2)
Cs—S1^iii	3.6184 (16)	S4—Cs^v	4.086 (2)
Cs—N3	3.676 (6)	C1—N1	1.149 (15)
Cs—S4	3.691 (2)	C1—Cs^i	3.848 (8)
Cs—C2^iii	3.738 (5)	C2—N2	1.153 (10)
Cs—S2^iv	3.7555 (14)	C2—Cs^viii	3.738 (5)
Cs—N3^iii	3.808 (6)	C2—Cs^ix	3.738 (5)
Cs—C1	3.848 (8)	C3—N3	1.158 (6)
Cs—S3^v	4.039 (3)	C4—N4^v	1.155 (11)
Zn—N4	1.947 (7)	C4—Cs^xii	3.462 (6)
Zn—N3^vi	1.962 (4)	C4—Cs^v	3.462 (6)
Zn—N3^iii	1.962 (4)	N1—Cs^i	3.081 (6)
Zn—N2^vii	1.984 (7)	N2—Zn^xiii	1.984 (7)
Zn—Cs^i	4.3187 (9)	N2—Cs^ix	3.492 (4)
Zn—Cs^iii	4.3248 (9)	N2—Cs^viii	3.492 (4)
Zn—Cs^vi	4.3248 (9)	N3—Zn^ix	1.962 (4)
S1—C1	1.673 (11)	N3—Cs^ix	3.808 (6)
S1—Cs^viii	3.6184 (16)	N4—C4^ii	1.155 (11)
S1—Cs^ix	3.6184 (16)	N4—Cs^i	3.388 (5)
S1—Ag—S3	108.42 (4)	N3^vi—Zn—Cs	118.90 (19)
S1—Ag—S3^i	108.42 (4)	N3^iii—Zn—Cs	61.83 (18)
S3—Ag—S3^i	121.14 (12)	N2^vii—Zn—Cs	130.07 (13)
S1—Ag—S2	98.49 (8)	Cs^i—Zn—Cs	71.90 (2)
S3—Ag—S2	108.99 (7)	N4—Zn—Cs^iii	124.97 (12)
S3^i—Ag—S2	108.99 (7)	N3^vi—Zn—Cs^iii	124.33 (18)
N1—Cs—N4	73.67 (18)	N3^iii—Zn—Cs^iii	57.81 (18)
N1—Cs—C4^ii	67.16 (19)	N2^vii—Zn—Cs^iii	52.52 (11)
N4—Cs—C4^ii	19.37 (18)	Cs^i—Zn—Cs^iii	173.24 (2)
N1—Cs—N2^iii	115.19 (19)	Cs—Zn—Cs^iii	101.647 (3)
N4—Cs—N2^iii	171.08 (17)	N4—Zn—Cs^vi	124.97 (12)
C4^ii—Cs—N2^iii	163.17 (16)	N3^vi—Zn—Cs^vi	57.81 (18)
N1—Cs—S1^iii	174.61 (15)	N3^iii—Zn—Cs^vi	124.33 (18)
N4—Cs—S1^iii	106.58 (11)	N2^vii—Zn—Cs^vi	52.52 (11)
C4^ii—Cs—S1^iii	114.83 (13)	Cs^i—Zn—Cs^vi	101.647 (3)
N2^iii—Cs—S1^iii	64.50 (11)	Cs—Zn—Cs^vi	173.24 (2)
N1—Cs—N3	78.56 (16)	Cs^iii—Zn—Cs^vi	84.72 (2)
N4—Cs—N3	132.40 (14)	C1—S1—Ag	88.3 (3)
C4^ii—Cs—N3	113.29 (16)	C1—S1—Cs^viii	125.49 (5)
N2^iii—Cs—N3	53.64 (13)	Ag—S1—Cs^viii	97.91 (5)
S1^iii—Cs—N3	104.54 (8)	C1—S1—Cs^ix	125.49 (5)
N1—Cs—S4	56.74 (14)	Ag—S1—Cs^ix	97.91 (5)
N4—Cs—S4	71.96 (11)	Cs^viii—S1—Cs^ix	107.28 (7)
C4^ii—Cs—S4	84.10 (12)	C2—S2—Ag	106.1 (3)
N2^iii—Cs—S4	111.43 (12)	C2—S2—Cs^x	124.43 (11)
S1^iii—Cs—S4	118.04 (5)	Ag—S2—Cs^x	96.44 (6)
N3—Cs—S4	121.43 (10)	C2—S2—Cs^xi	124.43 (10)
N1—Cs—C2^iii	118.66 (19)	Ag—S2—Cs^xi	96.44 (6)
N4—Cs—C2^iii	157.16 (16)	Cs^x—S2—Cs^xi	101.78 (5)
C4^ii—Cs—C2^iii	174.03 (19)	C3—S3—Ag	105.4 (2)
N2^iii—Cs—C2^iii	17.93 (16)	C3—S3—Cs^ii	117.0 (2)
S1^iii—Cs—C2^iii	59.25 (11)	Ag—S3—Cs^ii	123.37 (7)
N3—Cs—C2^iii	70.41 (14)	C4—S4—Cs	124.9 (2)
S4—Cs—C2^iii	98.00 (10)	C4—S4—Cs^i	124.9 (2)
N1—Cs—S2^iv	128.34 (15)	Cs—S4—Cs^i	86.78 (6)
N4—Cs—S2^iv	114.01 (11)	C4—S4—Cs^xii	56.3 (2)
C4^ii—Cs—S2^iv	102.90 (11)	Cs—S4—Cs^xii	172.71 (9)
N2^iii—Cs—S2^iv	61.97 (11)	Cs^i—S4—Cs^xii	98.014 (14)
S1^iii—Cs—S2^iv	56.69 (4)	C4—S4—Cs^v	56.3 (2)
N3—Cs—S2^iv	58.52 (9)	Cs—S4—Cs^v	98.014 (14)
S4—Cs—S2^iv	172.49 (4)	Cs^i—S4—Cs^v	172.71 (9)
C2^iii—Cs—S2^iv	74.77 (10)	Cs^xii—S4—Cs^v	76.71 (5)
N1—Cs—N3^iii	108.03 (16)	N1—C1—S1	178.9 (7)
N4—Cs—N3^iii	52.62 (13)	N1—C1—Cs	41.39 (12)
C4^ii—Cs—N3^iii	71.35 (16)	S1—C1—Cs	138.69 (11)
N2^iii—Cs—N3^iii	120.77 (13)	N1—C1—Cs^i	41.39 (12)
S1^iii—Cs—N3^iii	68.78 (8)	S1—C1—Cs^i	138.69 (11)
N3—Cs—N3^iii	173.27 (6)	Cs—C1—Cs^i	82.4 (2)
S4—Cs—N3^iii	63.01 (9)	N2—C2—S2	178.6 (8)
C2^iii—Cs—N3^iii	104.56 (14)	N2—C2—Cs^viii	68.9 (3)
S2^iv—Cs—N3^iii	116.34 (9)	S2—C2—Cs^viii	110.3 (2)
N1—Cs—C1	14.27 (18)	N2—C2—Cs^ix	68.9 (3)
N4—Cs—C1	86.46 (16)	S2—C2—Cs^ix	110.3 (2)
C4^ii—Cs—C1	77.33 (17)	Cs^viii—C2—Cs^ix	102.45 (18)
N2^iii—Cs—C1	102.46 (15)	N3—C3—S3	178.6 (5)
S1^iii—Cs—C1	166.93 (12)	N4^v—C4—S4	176.2 (9)
N3—Cs—C1	64.63 (13)	N4^v—C4—Cs^xii	76.7 (4)
S4—Cs—C1	66.16 (11)	S4—C4—Cs^xii	100.8 (3)
C2^iii—Cs—C1	108.64 (16)	N4^v—C4—Cs^v	76.7 (4)
S2^iv—Cs—C1	117.69 (11)	S4—C4—Cs^v	100.8 (3)
N3^iii—Cs—C1	121.86 (13)	Cs^xii—C4—Cs^v	94.2 (2)
N1—Cs—S3^v	58.65 (15)	C1—N1—Cs	124.34 (15)
N4—Cs—S3^v	127.14 (10)	C1—N1—Cs^i	124.34 (15)
C4^ii—Cs—S3^v	125.65 (12)	Cs—N1—Cs^i	110.8 (3)
N2^iii—Cs—S3^v	60.55 (10)	C2—N2—Zn^xiii	154.6 (7)
S1^iii—Cs—S3^v	118.77 (4)	C2—N2—Cs^ix	93.2 (3)
N3—Cs—S3^v	60.99 (9)	Zn^xiii—N2—Cs^ix	100.68 (16)
S4—Cs—S3^v	63.82 (6)	C2—N2—Cs^viii	93.2 (3)
C2^iii—Cs—S3^v	60.07 (11)	Zn^xiii—N2—Cs^viii	100.68 (16)
S2^iv—Cs—S3^v	112.97 (6)	Cs^ix—N2—Cs^viii	113.1 (2)
N3^iii—Cs—S3^v	120.91 (8)	C3—N3—Zn^ix	161.2 (4)
C1—Cs—S3^v	50.40 (11)	C3—N3—Cs	93.5 (5)
N4—Zn—N3^vi	110.5 (2)	Zn^ix—N3—Cs	95.3 (2)
N4—Zn—N3^iii	110.5 (2)	C3—N3—Cs^ix	96.7 (5)
N3^vi—Zn—N3^iii	108.9 (3)	Zn^ix—N3—Cs^ix	91.2 (2)
N4—Zn—N2^vii	106.3 (3)	Cs—N3—Cs^ix	127.09 (12)
N3^vi—Zn—N2^vii	110.3 (2)	C4^ii—N4—Zn	166.5 (8)
N3^iii—Zn—N2^vii	110.3 (2)	C4^ii—N4—Cs^i	83.9 (4)
N4—Zn—Cs^i	49.32 (14)	Zn—N4—Cs^i	104.85 (18)
N3^vi—Zn—Cs^i	61.83 (18)	C4^ii—N4—Cs	83.9 (4)
N3^iii—Zn—Cs^i	118.90 (19)	Zn—N4—Cs	104.85 (18)
N2^vii—Zn—Cs^i	130.07 (13)	Cs^i—N4—Cs	96.9 (2)
N4—Zn—Cs	49.32 (14)

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