The alkaline-earth metal cyanamides MgCN* SrCN* and BaCN2 were prepared by the reaction of melamine

with the metal nitrides (M&N2, Sr-jN, Ba3N2 ) at temperatures between 740 e C and 850 °C. MgCN2 was structurally

characterized by X-ray analysis using powder methods (R3m, Ζ - 3 , a=327.34(1) pm, c-1412.82(5) pm). The

crystal structures of SrCN2 and BaCN2 were determined from X-ray single crystal data (SrCN2 : Pnma, Ζ-A,

0-1241.0(2) pm, 6-396.3(2) pm, c-538.9(2) pm, BaCN2 : R3c, Z=18, e-1528.2(2) pm, c-70l.3(2) pm). All

compounds consist of the respective cations and CN2

2~-anions with an increasing degree of distortion of the

coordination polyhedra with increasing radii of the cations. The structural features are correlated with vibrational

spectroscopic data

Berger, Ute

Schnick, Wolfgang

English

Universität München: Elektronischen Publikationen

Journal of Alloys and Compounds, 206 (1994) 179-184 
JALCOM 1031 179 
Syntheses, crystal structures, and vibrational spectroscopic properties 
of MgCN2, SrCN2, and BaCN2 
Ute Berger and Wolfgang Schnick* 
Laboratorium ßr Anorganische Chemie, Universität Bayreuth, D-95440 Bayreuth (Germany) 
(Received October 27, 1993) 
Abstract 
The alkaline-earth metal cyanamides MgCN* SrCN* and BaCN 2 were prepared by the reaction of melamine 
with the metal nitrides (M&N2, Sr-jN, Ba 3 N 2 ) at temperatures between 740 e C and 850 °C. MgCN 2 was structurally 
characterized by X-ray analysis using powder methods (R3m, Ζ - 3 , a=327.34(1) pm, c-1412.82(5) pm). The 
crystal structures of S r C N 2 and BaCN 2 were determined from X-ray single crystal data (SrCN 2 : Pnma, Ζ-A, 
0-1241.0(2) pm, 6-396.3(2) pm, c-538.9(2) pm, BaCN 2 : R3c, Z=18, e-1528.2(2) pm, c-70l.3(2) pm). All 
compounds consist of the respective cations and CN2 2~-anions with an increasing degree of distortion of the 
coordination polyhedra with increasing radii of the cations. The structural features are correlated with vibrational 
spectroscopic data. 
1. Introduction 
Cyanamide (H 2CN 2) and its salts are well known 
compounds with a versatile field of applications. Nu­
merous synthetic routes have been proposed for metal 
cyanamides, however, their preparation in a pure and 
well-defined state has not been unambiguously achieved. 
Additionally, reliable structural data are only available 
for CaCN2 [1]. 
Strid and co-workers obtained SrCN2 [2] and BaCN2 
[3] by the reaction of the respective carbonates with 
hydrogen cyanide at temperatures between 600 °C and 
650 °C, but they were unable to solve the crystal 
structures completely. For MgCN 2 no structural data 
have been reported. 
Here we present a novel simple procedure leading 
to pure and crystalline cyanamides, and we succeeded 
in growing single crystals of SrCN2 and BaCN2. To our 
surprise, the crystallographic data we obtained are not 
in agreement with those reported by Strid. 
2. Experimental details 
27. Syntheses 
MgCN* SrCN2, and BaCN 2 were prepared by the 
reaction of the respective nitrides (M&N* Sr2N, B a ^ ) 
with an excess of melamine [QN 3 (NH 2 ) 3 ] . After ho-
* Author to whom correspondence should be addressed* 
mogenization in a ball mill, the reaction mixture is 
heated in a tungsten crucible under argon atmosphere 
for two days at 220 °C, then for another two days at 
higher temperatures (MgCN2: 750 °C, SrCN2: 850 °C, 
BaCN2: 740 °C). Under these conditions the products 
are yielded as colourless powders. Single crystals of 
SrCN2 were obtained by increasing the temperature to 
980 °Q for the crystal growth of BaCN2 the micro-
crystalline product was annealed at 740 °C for five days. 
The nitrides have been synthesized by reaction of 
the metals (Mg: 99.995% Alfa Ventron, Sr: 99.95% 
ABCR, Ba: 99.95% ABCR) with pure nitrogen (Sr2N, 
Ba3N3) or ammonia (Linde 3.8, MgaN2). Melamine 
(p.a., Janssen) was purified by sublimation. 
The purity of all products was checked by elemental 
analyses (only metals) and X-ray diffraction; the absence 
of amide and imide contamination was proved by in­
frared (IR) spectroscopy. 
To exclude moisture and oxygen, all operations have 
been performed using Schlenk techniques under a 
purified argon atmosphere. 
2.2. X-ray structure determination and structure 
calculation 
For intensity data collection MgCN2 powder and 
single crystals of SrCN2 and BaCN2 were sealed under 
dry argon in glass capillaries. The crystal structure of 
MgCN2 was determined from X-ray powder diffraction 
data collected on a Stoe Stadi Ρ diflractometer. The 
°925-8388/94/$07.00© 1994 Elsevier Sequoia. All rights reserved 
SSDI 0925-8388(93)01031-X 
180 U. Berger, W. Schuck I Syntheses and structural data ofMgCN» SrCN2t and BaCN2 
TABLE 1. CrystaJIographic data for MgCN* SrCN 2 and BaCN 2 
Data collected from 
Space group (No.) 
Unit cell dimensions (pm) 
Volume (pmXlO*) 
Formula weight 
Density cala (g cm**3) 
Ζ 
Absorption coefficient (mm"1) 
Diffractometer used 
Radiation (pm) 
Monochromator 
20-range 
Scan type 
Reflections collected 
Independent reflections 
Observed reflections 
Solution and refinement 
Number of parameters refined 
Final R values 
MgCNjt 
powder 
JR3m(166) 
a «327.34(1) 
c « 1412.82(5) 
131.10(1) 
64.3 
2.445 
3 
4.6 
Stoe Stadi Ρ 
CuKa,-154.056 
Germanium 
3-104 
Debye-Scherrcr 
5053 datapoints 
28 
28(>4σ) 
Rietveld method 
Full profile [4] 
/W -4 .5% 
Λρτο-10.8% 
SrCN 2 BaCN 2 
single crystal sin^t crystal 
Pnma(62) R3c(167) 
a -1241.0(2) a -1528.2(2) 
b~ 3963(2) c-701.3(2) 
c~538.9(2) 
265.0(2) 1418.4(5) 
127.7 177.4 
3.199 3.738 
4 18 
20.0 123 
Nonius CAD4 Siemens P4 
MoKa-71.069 MoKa-71.069 
Graphite Graphite 
2-64 2-60 
1881 
61 
2682 
497(A«t: 4.33%) 649(7^,: 234%) 
422(>3σ) 585(>3σ) 
Patterson method Patterson method 
SHELXS-86 [5] SHELX XL-
SHELX-76 [6] PLUS [7] 
26 24 
Λ - 4 . 9 6 % JR-4.40% 
M>*-4.04% M*-3.85% 
structure was refined by using the Rietveld full-profile 
technique. 
Intensity measurements of SrCN2 and BaCN2 were 
performed on four-circle diffractometers, in both cases 
the structures were solved using Patterson's method. 
Further detaus of data collection and the results of 
the X-ray structure determination are shown in Table 
1. 
2.3. Vibrational spectroscopic investigations 
Infrared spectra were recorded on a Bruker IFS 
113 ν vacuum spectrometer equipped with two deu-
terated trigiycin sulfat (DTGS) detectors, measuring a 
range from 150 cm*"1 to 4000 cm" 1. MgCN 2 and SrCN2 
were measured as Csl pellets, BaCN2 was mixed with 
KBr. To avoid broad and extremely tailed bands through 
the Christiansen effect caused by the hardness of the 
cyanamides, the salts have been ground in a micro ball 
mill before mixing with Csl or KBr. 
The Raman spectra were excited by an argon ion 
laser (Amko, Coderg LRT 800) at a wavelength of 
514.5 nm. In this case the pure probes were sealed in 
glass capillaries. 
All preparation procedures have been performed in 
a glove-box under a dry argon atmosphere (H 2O<0.2 
ppm, 0 2 < 1 ppm). 
3. Results 
3.1 The crystal structure of MgCN2 
The crystal structure of magnesium cyanamide tas 
to be considered as an ionic layer structure isostructural 
to the high temperature (or β) phase of NaN3 [8] and 
CaCN2. The cations as well as the anions form close-
packed arrangements in the unit cell with each ion 
coordinated by six counterions. The terminal nitrogen 
atoms surround the Mg 2 +-ions as a regular octahedron 
(Fig. 1), whereas the coordination polyhedron around 
the CN2 2~-ion (C-N: 124.8(2) pm, NCN: 180°) can be 
regarded either as an elongated octahedron or as a 
trigonal antiprism. 
The central C-atom of the anion occupies a centre 
of symmetry, the two nitrogen atoms are generated by 
only one crystallographic site resulting in Dmh symmetry 
of the anion. The positional and isotropic displacement 
parameters for MgCN 2 are shown in Table 2, bond 
lengths, interatomic distances and bond angles are 
summarized in Table 3, 
3.2. The crystal structure of SrCN2 
In strontium cyanamide, Sr^-ions and nearly linear 
CN22~*groups have been found. The N-C-N angle 
(178.8(9)°) may be regarded as 180° with respect to 
the standard deviation, the C-N bond lengths (122.8(10), 
U. Berger, W. Schnick / Syntheses and structural data of MgCN2, SrCN* and BaCN2 181 
Fig. 1. A view of the layer structure of MgCN 2 with the MgN6-
octahedra along [210]. 
122.2(11) pm) are nearly equal despite two crystallo-
graphically independent sites of nitrogen. 
Anions in two orientations built up layers parallel 
to the a-c plane of the unit cell. The cations also 
belong to these layers, forming distorted octahedra 
around the anions visualized as an Ortep plot in Fig. 
2. Distorted nitrogen octahedra centred by Sr2+-ions 
are connected by corner sharing and by NCN-bridges. 
A view of the structure is depicted in Fig. 3. 
Atomic sites and anisotropic displacement parameters 
are listed in Table 2, important bond lengths and bond 
angles are shown in Table 3. Ν 
3.3. The crystal structure of BaCN2 
In barium cyanamide no layer structure occurs, but 
a three-dimensional arrangement of Ba2*-ions and 
CN22--groups (C-N: 119.2(11) pm) has been found. 
The CN 2 2--anion is crystallographically generated by 
one site of carbon and nitrogen each leading to an 
N-C-N bond angle of 177.2(18)°. However, the sig-
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Η 0 \ Ο Η Η 0 \ ^ 
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Ο ο ο ο ο ο\ σ\ ο 
r-ί *η 
*-< C4 cn Μ Η Η tn 
Ο Γ ) Ο Η <Ν VO 
*Η tH r-i ι-ί <S Η V ) 
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CO CO Ο 
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Η Η Ν Λ 
Η Η Ν Η 
D P S 
0 θ 5 · 0 Ο τ-· CO CO CO « H ^ f l O 
Η Ν CO CN VO t> CN r» ro 
C N C N t N *H rH »H ,-1 CO CS VT> 
<~v / ~ \ "«fr CN CO r - i 
CO 5 Ο Ο C J Η Η 0 \ 
00 Η Η 0 0 Η CN 
ΙΟ Η Η ι lO rH 
1 I 
Ο Ο Ο Η Η Η Η Ο Ο Η 
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a&ss ass 
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CO CO Ό < ί ^ Η Η to 
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182 
tt Berger, W. Schuck / Syntheses and structural data of MgCN» SrCN» and BaCN, 
T A B L E 3. Selected bond distances (pm) and bond angies_£) 
(standard deviations in parentheses) of MgCNj, SrCN* and BaCN, 
MgCN2 SrCN 2 
BaCN 2 
Distances 
C-N 124.8(2) 122.8(10)N1 
122.2(11)N2 
119.2(11) 
M-N 219.0(1) 262.2(4)N1 
265.7(8)N1 
260.1(8)N2 
264.3(5)N2 
277.3(9) 
284.2(9) 
286.7(12) 
M-M 327.34(1) 396.3(1) 
400.0(1) 
446.4(1) 
435.1(1) 
459.9(1) 
Bond angles 
N « C « N 180.00(8) 178.8(9) 177.2(18) 
Fig. 2. Coordination around the CNa2"-ion in SrCN 2 (ellipsoids 
drawn at 50% probability). 
nificance of this deviation from linearity has to be 
analysed by vibrational spectroscopic investigations (c/. 
Section 3.4). 
The Ba2* -cations are coordinated in the crystal struc­
ture by sue different anions in a rather irregular way 
(Fig. 4). The polyhedron formed by the cations around 
the CN 2 2~ -group represents a twisted trigonal prism 
shown as an Ortep plot in Fig. 5. The observed N-C-N 
bond angle of 177.2(18)° suggests a reduction of the 
anion's symmetry from Dmh to C^. 
Table 2 contains the atomic sites and the anisotropic 
displacement parameters of the compound. Selected 
bond lengths, interatomic distances and bond angles 
are summarized in Table 3. 
3Λ Vibrational spectroscopic properties 
The vibrational spectra of alkaline-earth metal cy­
anamides show the typical band sequence of a linear 
Fig. 3. The view of the SrCN 2 structure along [010] with the 
coordination polyhedra around the cations. 
Fig. 4. Coordination sphere of Ba in BaGN 2 (ellipsoids drawn 
at 50% probability). 
three-atomic arrangement with Dmh symmetry (Fig- 6)· 
As expected by the selection principle for molecules 
containing an inversion centre, the asymmetric stretch­
ing and the deformation vibration of the C N ^ - g ^ P 
are allowed in IR spectra, whereas the symmetnc 
stretching mode is Raman active. I f the symmetry ot 
a three atomic ion is lowered from D„h to 8 S 
assumed for the cyanamide anion in BaCN* the sym-
U. Berger, W. Schnick / Syntheses and structural data ofMgCN» SrCN2, and BaCN2 183 
Fig. 5. The twisted trigonal prismatic coordination of the CN 2 2 ~-
group in BaCN 2 by the barium cations (ellipsoids drawn at 50% 
probability). 
Wavenumber cm-1 
Fig. 6. Infrared and Raman spectra of MgCN* SrCN* and BaCN* 
metric stretching vibration is no longer forbidden in 
the IR spectra and has to be observable. Since there 
is no hint of a ^-vibration in the IR spectrum of BaCN* 
we consider the deviation from linearity as determined 
by X-ray structure analysis not to be significant. So the 
symmetry of the anion is Z>„A rather than C^. For 
SrCN2, the i/„-bond is split as a result of the two 
ciystallographically independent nitrogen atoms of the 
anion. Nevertheless, even in this spectrum no sym-
T A B L E 4. Vibrational frequencies and force constants 
Compound 
[cm"" l ] [cm-
δ 
l ] [cm- l ] [Ncm" 
fcNKN 
l ] [Ncm" 
/ N C N 
l ] [Ncm-1] 
MgCN2 1301 2114 681 12.52 1.45 0.57 
SrCN 2 1251 1989 663 11.44 1.48 054 
2023 677 
BaCN 2 1238 1947 662 11.02 1.63 0,54 
673 
fat* / C N / C N 0 valency force constants. 
/NCN"deformation force constant 
MN» /**"•reciprocal masses of carbon and nitrogen. 
c-constant-5.89146X10"7. 
( fcN +/CN/CN) X M N - c X vf. 
<f<x-favad X 0*N+2/i«)-cX V „ 2 . 
/ N C N X 2X (M N +2μ*) « c Χ A 
metrical stretching vibration appears which would in­
dicate the lower C2v symmetry of the CN22--group. 
The bands of the deformation vibration are split in 
the IR spectra of SrCN2 and BaCN2. This may be an 
effect of the asymmetric coordination of the anions by 
the cations which leads to the suppression of the 
degeneracy of the ^vibration. 
The vibrational frequencies and force constants [9] 
are shown in Table 4. 
4. Conclusions 
All three alkaline-earth metal cyanamides investi­
gated contain linear CN22~-anions. Though in all three 
crystal structures the ions are coordinated by six coun-
terions, the crystal structures are not directly related 
to each other. In MgCN2 we found alternating layers 
of cations and anions. In SrGN2 the layers contain both 
ions, and finally in BaCN2 we no longer have a layer 
structure but a three-dimensional arrangement of the 
ions in the solid. 
With increasing radii of the cations, the regularity 
of coordination polyhedra of both, anions and cations, 
decrease. Although the O N bond lengths in all cy­
anamides investigated are of comparable lengths, the 
vibrational frequencies of the i / M and ι/,-bands decrease 
with increasing radii of the corresponding cations in 
the same way as the force constant fen (Table 4). An 
analogous effect has been observed in the spectroscopic 
data of orthonitrates [10] and can be explained by the 
reciprocal relation between the polarization of the 
nitrogen atoms and the size of the cations. This po­
larization reduces the repulsion between the lone pairs 
of the nitrogen atoms. Consequently, the antibonding 
interactions decrease and the G-N bonding gets sta­
bilized. 
184 U. Berger, W. Schuck I Syntheses and structural data ofMgCN2, SrCN2. and BaCN2 
Acknowledgment 
A part of this work was carried out at the Institute 
for Inorganic Chemistry (Bonn University) - the au­
thors would like to thank Professor Dr. M. Jansen. 
Fruitful discussions and help during the single crystal 
structure determinations by Dr. W. Milius are kindly 
acknowledged. Financial support of our work was pro­
vided by the Deutsche Forschungsgemeinschaft and the 
Fonds der Chemischen Industrie. 
References 
1 N.-G. Vannerberg, Acta Chem. Scand., 16 (1962) 2263-
2266. 
2 K.-0 . Strid and N.-G. Vannerberg, Acta Chem. Scand., 20 
(1966) 1064-1073. 
3 K. -G. Strid, Ζ Anorg. Alfg. Chem., 360 (1968) 205-
207. 
4 S T O E Powder Software CSD. 
5 G.M. Sbeldrick, SHELXS-86, Program for Crystal Structure 
Determination, University of Göttingen, 1986. 
6 G.M. Sheldrick, SHELX-76, Program for Crystal Structure De-
termination, University of Göttingen, 1976. 
7 Siemens Software S H E L X T L PLUS. 
8 G. -E . Pringle and D . E . Noakes, Acta Crystallogr., Β 24 (1968) 
262-269. 
9 A. Fadini and F.-M. Scbnepel, Schwingungsspektroskopie: 
Methoden, Anwendungen, Georg Thierne Verlag, Stuttgart, 
1985, p. 117. 
10 Τ. Bremm and Μ. Jansen, Ζ. Anorg, Aug. Chem., 60S (1992) 
49-55. 


Syntheses, crystal structures, and vibrational spectroscopic properties of MgCN2, SrCN2, and BaCN2

The alkaline-earth metal cyanamides MgCN * SrCN * and BaCN 2 were prepared by the reaction of melamine with the metal nitrides (M&amp;N2, Sr-jN, Ba 3 N 2) at temperatures between 740 e C and 850 °C. MgCN 2 was structurally characterized by X-ray analysis using powder methods (R3m, Ζ- 3, a=327.34(1) pm, c-1412.82(5) pm). The crystal structures of S r C N 2 and BaCN 2 were determined from X-ray single crystal data (SrCN 2: Pnma, Ζ-A, 0-1241.0(2) pm, 6-396.3(2) pm, c-538.9(2) pm, BaCN 2: R3c, Z=18, e-1528.2(2) pm, c-70l.3(2) pm). All compounds consist of the respective cations and CN2 2~-anions with an increasing degree of distortion of the coordination polyhedra with increasing radii of the cations. The structural features are correlated with vibrational spectroscopic data. 1

Ute Berger

Wolfgang Schnick

CiteSeerX

Open Access LMU ( Ludwig-Maximilians-Univ. München)

Journal of Alloys and Compounds, 206 (1994) 179-184 JALCOM 1031 179 Syntheses, crystal structures, and vibrational spectroscopic properties of MgCN2, SrCN2, and BaCN2 Ute Berger and Wolfgang Schnick* Laboratorium ßr Anorganische Chemie, Universität Bayreuth, D-95440 Bayreuth (Germany) (Received October 27, 1993) Abstract The alkaline-earth metal cyanamides MgCN* SrCN* and BaCN 2 were prepared by the reaction of melamine with the metal nitrides (M&N2, Sr-jN, Ba 3 N 2 ) at temperatures between 740 e C and 850 °C. MgCN 2 was structurally characterized by X-ray analysis using powder methods (R3m, Ζ - 3 , a=327.34(1) pm, c-1412.82(5) pm). The crystal structures of S r C N 2 and BaCN 2 were determined from X-ray single crystal data (SrCN 2 : Pnma, Ζ-A, 0-1241.0(2) pm, 6-396.3(2) pm, c-538.9(2) pm, BaCN 2 : R3c, Z=18, e-1528.2(2) pm, c-70l.3(2) pm). All compounds consist of the respective cations and CN2 2~-anions with an increasing degree of distortion of the coordination polyhedra with increasing radii of the cations. The structural features are correlated with vibrational spectroscopic data. 1. Introduction Cyanamide (H 2CN 2) and its salts are well known compounds with a versatile field of applications. Nu­merous synthetic routes have been proposed for metal cyanamides, however, their preparation in a pure and well-defined state has not been unambiguously achieved. Additionally, reliable structural data are only available for CaCN2 [1]. Strid and co-workers obtained SrCN2 [2] and BaCN2 [3] by the reaction of the respective carbonates with hydrogen cyanide at temperatures between 600 °C and 650 °C, but they were unable to solve the crystal structures completely. For MgCN 2 no structural data have been reported. Here we present a novel simple procedure leading to pure and crystalline cyanamides, and we succeeded in growing single crystals of SrCN2 and BaCN2. To our surprise, the crystallographic data we obtained are not in agreement with those reported by Strid. 2. Experimental details 27. Syntheses MgCN* SrCN2, and BaCN 2 were prepared by the reaction of the respective nitrides (M&N* Sr2N, B a ^ ) with an excess of melamine [QN 3 (NH 2 ) 3 ] . After ho-* Author to whom correspondence should be addressed* mogenization in a ball mill, the reaction mixture is heated in a tungsten crucible under argon atmosphere for two days at 220 °C, then for another two days at higher temperatures (MgCN2: 750 °C, SrCN2: 850 °C, BaCN2: 740 °C). Under these conditions the products are yielded as colourless powders. Single crystals of SrCN2 were obtained by increasing the temperature to 980 °Q for the crystal growth of BaCN2 the micro-crystalline product was annealed at 740 °C for five days. The nitrides have been synthesized by reaction of the metals (Mg: 99.995% Alfa Ventron, Sr: 99.95% ABCR, Ba: 99.95% ABCR) with pure nitrogen (Sr2N, Ba3N3) or ammonia (Linde 3.8, MgaN2). Melamine (p.a., Janssen) was purified by sublimation. The purity of all products was checked by elemental analyses (only metals) and X-ray diffraction; the absence of amide and imide contamination was proved by in­frared (IR) spectroscopy. To exclude moisture and oxygen, all operations have been performed using Schlenk techniques under a purified argon atmosphere. 2.2. X-ray structure determination and structure calculation For intensity data collection MgCN2 powder and single crystals of SrCN2 and BaCN2 were sealed under dry argon in glass capillaries. The crystal structure of MgCN2 was determined from X-ray powder diffraction data collected on a Stoe Stadi Ρ diflractometer. The °925-8388/94/$07.00© 1994 Elsevier Sequoia. All rights reserved SSDI 0925-8388(93)01031-X 180 U. Berger, W. Schuck I Syntheses and structural data ofMgCN» SrCN2t and BaCN2 TABLE 1. CrystaJIographic data for MgCN* SrCN 2 and BaCN 2 Data collected from Space group (No.) Unit cell dimensions (pm) Volume (pmXlO*) Formula weight Density cala (g cm**3) Ζ Absorption coefficient (mm"1) Diffractometer used Radiation (pm) Monochromator 20-range Scan type Reflections collected Independent reflections Observed reflections Solution and refinement Number of parameters refined Final R values MgCNjt powder JR3m(166) a «327.34(1) c « 1412.82(5) 131.10(1) 64.3 2.445 3 4.6 Stoe Stadi Ρ CuKa,-154.056 Germanium 3-104 Debye-Scherrcr 5053 datapoints 28 28(>4σ) Rietveld method Full profile [4] /W -4 .5% Λρτο-10.8% SrCN 2 BaCN 2 single crystal sin^t crystal Pnma(62) R3c(167) a -1241.0(2) a -1528.2(2) b~ 3963(2) c-701.3(2) c~538.9(2) 265.0(2) 1418.4(5) 127.7 177.4 3.199 3.738 4 18 20.0 123 Nonius CAD4 Siemens P4 MoKa-71.069 MoKa-71.069 Graphite Graphite 2-64 2-60 1881 61 2682 497(A«t: 4.33%) 649(7^,: 234%) 422(>3σ) 585(>3σ) Patterson method Patterson method SHELXS-86 [5] SHELX XL-SHELX-76 [6] PLUS [7] 26 24 Λ - 4 . 9 6 % JR-4.40% M>*-4.04% M*-3.85% structure was refined by using the Rietveld full-profile technique. Intensity measurements of SrCN2 and BaCN2 were performed on four-circle diffractometers, in both cases the structures were solved using Patterson's method. Further detaus of data collection and the results of the X-ray structure determination are shown in Table 1. 2.3. Vibrational spectroscopic investigations Infrared spectra were recorded on a Bruker IFS 113 ν vacuum spectrometer equipped with two deu-terated trigiycin sulfat (DTGS) detectors, measuring a range from 150 cm*"1 to 4000 cm" 1. MgCN 2 and SrCN2 were measured as Csl pellets, BaCN2 was mixed with KBr. To avoid broad and extremely tailed bands through the Christiansen effect caused by the hardness of the cyanamides, the salts have been ground in a micro ball mill before mixing with Csl or KBr. The Raman spectra were excited by an argon ion laser (Amko, Coderg LRT 800) at a wavelength of 514.5 nm. In this case the pure probes were sealed in glass capillaries. All preparation procedures have been performed in a glove-box under a dry argon atmosphere (H 2O<0.2 ppm, 0 2 < 1 ppm). 3. Results 3.1 The crystal structure of MgCN2 The crystal structure of magnesium cyanamide tas to be considered as an ionic layer structure isostructural to the high temperature (or β) phase of NaN3 [8] and CaCN2. The cations as well as the anions form close-packed arrangements in the unit cell with each ion coordinated by six counterions. The terminal nitrogen atoms surround the Mg 2 +-ions as a regular octahedron (Fig. 1), whereas the coordination polyhedron around the CN2 2~-ion (C-N: 124.8(2) pm, NCN: 180°) can be regarded either as an elongated octahedron or as a trigonal antiprism. The central C-atom of the anion occupies a centre of symmetry, the two nitrogen atoms are generated by only one crystallographic site resulting in Dmh symmetry of the anion. The positional and isotropic displacement parameters for MgCN 2 are shown in Table 2, bond lengths, interatomic distances and bond angles are summarized in Table 3, 3.2. The crystal structure of SrCN2 In strontium cyanamide, Sr^-ions and nearly linear CN22~*groups have been found. The N-C-N angle (178.8(9)°) may be regarded as 180° with respect to the standard deviation, the C-N bond lengths (122.8(10), U. Berger, W. Schnick / Syntheses and structural data of MgCN2, SrCN* and BaCN2 181 Fig. 1. A view of the layer structure of MgCN 2 with the MgN6-octahedra along [210]. 122.2(11) pm) are nearly equal despite two crystallo-graphically independent sites of nitrogen. Anions in two orientations built up layers parallel to the a-c plane of the unit cell. The cations also belong to these layers, forming distorted octahedra around the anions visualized as an Ortep plot in Fig. 2. Distorted nitrogen octahedra centred by Sr2+-ions are connected by corner sharing and by NCN-bridges. A view of the structure is depicted in Fig. 3. Atomic sites and anisotropic displacement parameters are listed in Table 2, important bond lengths and bond angles are shown in Table 3. Ν 3.3. The crystal structure of BaCN2 In barium cyanamide no layer structure occurs, but a three-dimensional arrangement of Ba2*-ions and CN22--groups (C-N: 119.2(11) pm) has been found. The CN 2 2--anion is crystallographically generated by one site of carbon and nitrogen each leading to an N-C-N bond angle of 177.2(18)°. However, the sig-ο ο ο ο fO οο ON Η 5f Ν ( S τΗ ^ ν-/ w w v ^ O o Η 0 \ Ο Η Η 0 \ ^ I I I Ο ο ο ο ο ο\ σ\ ο r-ί *η *-< C4 cn Μ Η Η tn Ο Γ ) Ο Η <Ν VO *Η tH r-i ι-ί <S Η V ) Γ> τΗ CO CO Ο x^s ' Η Η Ν Λ Η Η Ν Η D P S 0 θ 5 · 0 Ο τ-· CO CO CO « H ^ f l O Η Ν CO CN VO t> CN r» ro C N C N t N *H rH »H ,-1 CO CS VT> <~v / ~ \ "«fr CN CO r - i CO 5 Ο Ο C J Η Η 0 \ 00 Η Η 0 0 Η CN ΙΟ Η Η ι lO rH 1 I Ο Ο Ο Η Η Η Η Ο Ο Η I a&ss ass d 43 υ p o o p ο8 οο ν|? CO CO Ό < ί ^ Η Η to CS 1 * 2 CO 182 tt Berger, W. Schuck / Syntheses and structural data of MgCN» SrCN» and BaCN, T A B L E 3. Selected bond distances (pm) and bond angies_£) (standard deviations in parentheses) of MgCNj, SrCN* and BaCN, MgCN2 SrCN 2 BaCN 2 Distances C-N 124.8(2) 122.8(10)N1 122.2(11)N2 119.2(11) M-N 219.0(1) 262.2(4)N1 265.7(8)N1 260.1(8)N2 264.3(5)N2 277.3(9) 284.2(9) 286.7(12) M-M 327.34(1) 396.3(1) 400.0(1) 446.4(1) 435.1(1) 459.9(1) Bond angles N « C « N 180.00(8) 178.8(9) 177.2(18) Fig. 2. Coordination around the CNa2"-ion in SrCN 2 (ellipsoids drawn at 50% probability). nificance of this deviation from linearity has to be analysed by vibrational spectroscopic investigations (c/. Section 3.4). The Ba2* -cations are coordinated in the crystal struc­ture by sue different anions in a rather irregular way (Fig. 4). The polyhedron formed by the cations around the CN 2 2~ -group represents a twisted trigonal prism shown as an Ortep plot in Fig. 5. The observed N-C-N bond angle of 177.2(18)° suggests a reduction of the anion's symmetry from Dmh to C^. Table 2 contains the atomic sites and the anisotropic displacement parameters of the compound. Selected bond lengths, interatomic distances and bond angles are summarized in Table 3. 3Λ Vibrational spectroscopic properties The vibrational spectra of alkaline-earth metal cy­anamides show the typical band sequence of a linear Fig. 3. The view of the SrCN 2 structure along [010] with the coordination polyhedra around the cations. Fig. 4. Coordination sphere of Ba in BaGN 2 (ellipsoids drawn at 50% probability). three-atomic arrangement with Dmh symmetry (Fig- 6)· As expected by the selection principle for molecules containing an inversion centre, the asymmetric stretch­ing and the deformation vibration of the C N ^ - g ^ P are allowed in IR spectra, whereas the symmetnc stretching mode is Raman active. I f the symmetry ot a three atomic ion is lowered from D„h to 8 S assumed for the cyanamide anion in BaCN* the sym-U. Berger, W. Schnick / Syntheses and structural data ofMgCN» SrCN2, and BaCN2 183 Fig. 5. The twisted trigonal prismatic coordination of the CN 2 2 ~-group in BaCN 2 by the barium cations (ellipsoids drawn at 50% probability). Wavenumber cm-1 Fig. 6. Infrared and Raman spectra of MgCN* SrCN* and BaCN* metric stretching vibration is no longer forbidden in the IR spectra and has to be observable. Since there is no hint of a ^-vibration in the IR spectrum of BaCN* we consider the deviation from linearity as determined by X-ray structure analysis not to be significant. So the symmetry of the anion is Z>„A rather than C^. For SrCN2, the i/„-bond is split as a result of the two ciystallographically independent nitrogen atoms of the anion. Nevertheless, even in this spectrum no sym-T A B L E 4. Vibrational frequencies and force constants Compound [cm"" l ] [cm-δ l ] [cm- l ] [Ncm" fcNKN l ] [Ncm" / N C N l ] [Ncm-1] MgCN2 1301 2114 681 12.52 1.45 0.57 SrCN 2 1251 1989 663 11.44 1.48 054 2023 677 BaCN 2 1238 1947 662 11.02 1.63 0,54 673 fat* / C N / C N 0 valency force constants. /NCN"deformation force constant MN» /**"•reciprocal masses of carbon and nitrogen. c-constant-5.89146X10"7. ( fcN +/CN/CN) X M N - c X vf. <f<x-favad X 0*N+2/i«)-cX V „ 2 . / N C N X 2X (M N +2μ*) « c Χ A metrical stretching vibration appears which would in­dicate the lower C2v symmetry of the CN22--group. The bands of the deformation vibration are split in the IR spectra of SrCN2 and BaCN2. This may be an effect of the asymmetric coordination of the anions by the cations which leads to the suppression of the degeneracy of the ^vibration. The vibrational frequencies and force constants [9] are shown in Table 4. 4. Conclusions All three alkaline-earth metal cyanamides investi­gated contain linear CN22~-anions. Though in all three crystal structures the ions are coordinated by six coun-terions, the crystal structures are not directly related to each other. In MgCN2 we found alternating layers of cations and anions. In SrGN2 the layers contain both ions, and finally in BaCN2 we no longer have a layer structure but a three-dimensional arrangement of the ions in the solid. With increasing radii of the cations, the regularity of coordination polyhedra of both, anions and cations, decrease. Although the O N bond lengths in all cy­anamides investigated are of comparable lengths, the vibrational frequencies of the i / M and ι/,-bands decrease with increasing radii of the corresponding cations in the same way as the force constant fen (Table 4). An analogous effect has been observed in the spectroscopic data of orthonitrates [10] and can be explained by the reciprocal relation between the polarization of the nitrogen atoms and the size of the cations. This po­larization reduces the repulsion between the lone pairs of the nitrogen atoms. Consequently, the antibonding interactions decrease and the G-N bonding gets sta­bilized. 184 U. Berger, W. Schuck I Syntheses and structural data ofMgCN2, SrCN2. and BaCN2 Acknowledgment A part of this work was carried out at the Institute for Inorganic Chemistry (Bonn University) - the au­thors would like to thank Professor Dr. M. Jansen. Fruitful discussions and help during the single crystal structure determinations by Dr. W. Milius are kindly acknowledged. Financial support of our work was pro­vided by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. References 1 N.-G. Vannerberg, Acta Chem. Scand., 16 (1962) 2263-2266. 2 K.-0 . Strid and N.-G. Vannerberg, Acta Chem. Scand., 20 (1966) 1064-1073. 3 K. -G. Strid, Ζ Anorg. Alfg. Chem., 360 (1968) 205-207. 4 S T O E Powder Software CSD. 5 G.M. Sbeldrick, SHELXS-86, Program for Crystal Structure Determination, University of Göttingen, 1986. 6 G.M. Sheldrick, SHELX-76, Program for Crystal Structure De-termination, University of Göttingen, 1976. 7 Siemens Software S H E L X T L PLUS. 8 G. -E . Pringle and D . E . Noakes, Acta Crystallogr., Β 24 (1968) 262-269. 9 A. Fadini and F.-M. Scbnepel, Schwingungsspektroskopie: Methoden, Anwendungen, Georg Thierne Verlag, Stuttgart, 1985, p. 117. 10 Τ. Bremm and Μ. Jansen, Ζ. Anorg, Aug. Chem., 60S (1992) 49-55. 

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W. SCHNICK

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Syntheses, crystal structures, and vibrational spectroscopic properties of MgCN2, SrCN2, and BaCN2

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