1
Comments on the paper: Synthesis, growth and characterization
of cadmium manganese thiocyanate (CMTC) crystal
Bikshandarkoil R. Srinivasan,
Department of Chemistry, Goa University, Goa 403 206, India
Email: srini@unigoa.ac.in Telephone: 0091-(0)832-6519316; Fax: 0091-(0)832-2451184
Abstract
The authors of the title paper (Spectrochim. Acta 79A (2011) 340-343) report the crystal growth of
cadmium manganese thiocyanate CdMn(SCN)4 and its characterization by single crystal X-ray
diffraction and infrared spectrum. Many points of criticism, concerning the crystal growth and
reported experimental data are highlighted in this report.
Keywords: cadmium manganese thiocyanate; crystal growth; tetragonal system; X-ray diffraction;
infrared spectrum; dubious data.
Introduction
Rosenheim and Cohn reported on the synthesis of the first examples of bimetallic thiocyanates of
general formula AB(SCN)4 (A = bivalent metal; B = Hg) in 1901 [1]. Since then, these compounds
have been the subject of several investigations [2-14]. This more than century long research has
enriched our understanding of the synthetic aspects, spectral characteristics, structural features and
material applications of the AB(SCN)4 crystals and the related Se analogues. Several of the
structurally characterized AB(SCN)4 compounds are isostructural and crystallize in the noncentrosymmetric tetragonal I4̄ space group (Table 1). The thiocyanate ion functions as a bridge
between the bivalent (A) metal ion and the (B) metal ion in the AB(SCN)4 compounds, resulting in a
three-dimensional polymeric structure consisting of {BS4} and {AN4} tetrahedra (Fig. 1). A recent
paper reported the growth and characterization of a so called cadmium manganese thiocyanate crystal
abbreviated by the authors as CMTC [15]. Unfortunately, the results reported in [15] do not in any
way concern with CdMn(SCN)4 as claimed by the authors and the title paper has many points of
criticism, which are highlighted herein. In order to avoid the non-standard abbreviation CMTC and
also its known use for another well characterized crystal namely cadmium mercury thiocyanate
CdHg(SCN)4 [8], the title compound is referred to by its formula CdMn(SCN)4 in this comment.
2
Table 1 Geometry of metals A and B in thiocyanate bridged AB(SCN)4 crystals
No Compound
Space group Geometry around A Geometry around B
Ref
1
2
3
4
5
6
7
8
9
10
11
Pbcn
C2/c
I4̄
I4̄
I4̄
I4̄
I4̄
I4̄
I4̄
I4̄
I4̄
3
4
5
7
8
6
14
13
10, 12
9
11, 12
CuHg(SCN)4
CuHg(SCN)4
CoHg(SCN)4
ZnHg(SCN)4
CdHg(SCN)4
MnHg(SCN)4
MnHg(SeCN)4
CoHg(SeCN)4
CdHg(SeCN)4
ZnCd(SCN)4
ZnCd(SeCN)4
{CuN4} square
{CuN4} square
{CoN4} tetrahedron
{ZnN4} tetrahedron
{CdN4} tetrahedron
{MnN4} tetrahedron
{MnN4} tetrahedron
{CoN4} tetrahedron
{CdN4} tetrahedron
{ZnN4} tetrahedron
{ZnN4} tetrahedron
{HgS4} tetrahedron
{HgS4} tetrahedron
{HgS4} tetrahedron
{HgS4} tetrahedron
{HgS4} tetrahedron
{HgS4} tetrahedron
{HgSe4} tetrahedron
{HgSe4} tetrahedron
{HgSe4} tetrahedron
{CdS4} tetrahedron
{CdSe4} tetrahedron
Fig. 1. A portion of the polymeric chain of ZnHg(SCN)4 viewed along c axis showing the
tetrahedral {HgS4} and{ZnN4} units (Figure is drawn by using the cif data in [7])
Comment
Structural chemistry of AB(SCN)4 crystals
According to the authors of the title paper, their aim was to attempt to replace Hg in CdHg(SCN)4 by
Mn for growth of CdMn(SCN)4 crystal [15]. Recently it has been shown that structural chemistry
principles are useful to determine beforehand, which atom or ion or molecule can be incorporated into
the known crystal structure of a compound [16]. In view of the availability of a large number of
structurally characterized AB(SCN)4 crystals, their known structural aspects can be used to predict if
substitution of Hg in CdHg(SCN)4 by Mn to grow crystals of CdMn(SCN)4, as mentioned by the
3
authors of the title paper, is actually feasible or not. A scrutiny of the compounds listed in Table 1
reveals that of the two metals A and B, the larger Hg(II) ion (1.10 Å) is always bonded to thiocyanate
at the sulfur site (selenium of the selenocyante), while the smaller bivalent metal ions namely Co
(0.72 Å), Zn (0.74 Å), Mn (0.80 Å) and Cd (0.92 Å) are bonded to the (SCN)- ion at the nitrogen end.
Values in bracket are the Shannon and Prewitt ionic radii of the metal [17]. The compounds
ZnCd(SCN)4 and ZnCd(SeCN)4 are two examples in which Cd(II) (0.92 Å) occupies the B site, while
the smaller Zn(II) (0.74 Å) ion is situated in the A site. However, in CdHg(SCN)4 the smaller Cd(II)
ion occupies the A site and is bonded to N of (SCN)- ligand, while the larger and softer Hg(II) ion
occupies the B site. Thus, the compounds listed in Table 1 serve to demonstrate that the larger metal
ion namely Hg(II) always binds to the soft S of the bridging thiocyanate ligand. In the compounds
MnHg(SCN)4 and MnHg(SeCN)4, the Mn(II) ion occupies the A site and is bonded to N of
thiocyanate. In addition, it has been observed in several structurally characterized examples of related
thiocyanate compounds containing the {MnN4O2} octahedral unit (18-22), that the Mn(II) ion always
binds at the N site of thiocyanate (Supplementary material, Fig. S1). Going by the preference of the A
and B metal ions and the well documented affinity of Mn(II) to bind at the nitrogen site of thiocyanate
in several bimetallic thiocyanates [6, 14, 18-22], structural chemistry principles rule out the
substitution of Hg in CdHg(SCN)4 by Mn to form CdMn(SCN)4 with Mn situated in the B site.
However, the authors claimed that they have successfully grown crystals of CdMn(SCN)4. A perusal
of the details of growth and the characterization of this ‘so called’ CdMn(SCN)4 crystal in the title
paper reveals that the authors’ claim is dubious as shown below.
Growth of a ‘so called’ CdMn(SCN)4 crystal
The authors reported that by taking commercially available AR grade reagents namely KSCN, CdCl2
and MnCl2 in a 1:1:1 mole ratio in water, crystals formulated as CdMn(SCN)4 can be grown as per
their proposed reaction scheme in equation 1.
4 KSCN + CdCl2 + MnCl2
CdMn(SCN)4 + 4 KCl
..(1)
The reason for the disparity in the mole ratio (1:1:1) of reagents taken by authors, in contrast to
equation 1, which requires a 4:1:1 reagent mole ratio, is not explained. Since analytical grade reagents
4
(all are freely water soluble) were used for crystal growth, the statement of the authors, ‘undissolved
materials were filtered out and the clear solution was left undisturbed to slow evaporation of the
solution and gradual growth’ clearly indicates the questionable nature of the entire synthetic protocol.
A tetragonal crystal with unequal a and b unit cell parameters
Based on the unit cell parameters which the authors claim to have determined, they reported that the
crystals obtained from the reaction in equation 1 belong to the tetragonal crystal system and under the
heading single crystal XRD the authors stated, ‘The observed results indicate that the crystal belongs
to tetragonal crystal system and the determined unit cell parameters are a = 12.0835 Å, b = 12.1349
Å, c = 8.5574 Å and α = 89.7128, β = 90.053, γ = 90.0023 and volume = 1254 Å3’. A tetragonal
crystal system is characterized by the unit cell relation a = b ≠ c; α = β = γ = 90o, which is described in
all standard text books [23]. Hence the reported cell in the title paper appears to redefine the
tetragonal system and the assignment of tetragonal system not only contradicts all known laws of
crystal physics but is also not in accordance with the definition of a tetragonal crystal system in the
International Tables for Crystallography [24]. In order to justify such an incorrect assignment, the
authors stated, ‘The family of crystals such as cadmium mercury thiocyanate (CMTC), ferrous
mercury thiocyanate (FMTC), zinc mercury thiocyanate (ZMTC) and zinc cadmium thiocyanate
(ZCTC) also belong to tetragonal crystallographic system’. The reported single crystal study is
dubious as shown below.
Table 2 Unit cell data of AB(SCN)4 compounds crystallizing in I4̄ space group
No
Compound
1
2
3
4
5
6
7
8
9
10
CoHg(SCN)4
ZnHg(SCN)4
MnHg(SCN)4
CdHg(SCN)4
ZnCd(SCN)4
ZnCd(SeCN)4
CoHg(SeCN)4
MnHg(SeCN)4
CdHg(SeCN)4
CdMn(SCN)4
a (Å)
b (Å)
c (Å)
V (Å3)
11.109
11.109
4.379
540.41
11.0912(4)
11.0912(4)
4.4414(4)
546.36(6)
11.324(3)
11.324(3)
4.270(2)
547.5(3)
11.487(3)
11.487(3)
4.218(1)
556.6(2)
11.135(2)
11.135(2)
4.3760(10)
542.6(2)
11.3420(1)
11.3420(1)
4.6326(1)
595.942(15)
11.281(3)
11.281(3)
4.6207(12)
588.0(3)
11.4545(4)
11.4545(4)
4.5803(4)
600.96(6)
11.6579(7)
11.6579(7)
4.5109(4)
613.06(8)
12.0835
12.1349
8.5574
1254
The odd values in bold are reported in the commented paper
Ref
5
7
6
8
9
12
13
14
12
15
5
A comparison of the unit cell parameters of reported AB(XCN)4 (X = S or Se) compounds (Table 2)
all of which crystallize in the tetragonal space group I4̄ , clearly show that in these compounds, the
cell volumes depend on the size of A, B, S or Se. With increasing size of A ion (0.72 Å for Co+2 to
0.92 Å for Cd+2) the cell volume increases (see entry Nos 1 to 4). Replacement of S by Se results in a
pronounced increase in cell volume. Substitution of Hg in ZnHg(SCN)4 (entry No. 2) by Cd in
ZnCd(SCN)4 (entry No. 5) results in a slight decrease in cell volume. Going by this trend, the
expected cell volume of for a compound having the composition CdMn(SCN)4 should be less than
that for the known compound CdHg(SCN)4 (556.6 Å3) if such a compound indeed crystallizes in the
tetragonal system. In such a compound the Mn will occupy the A site. It is to be noted that the
replacement of a larger ion like Hg(II) (ionic radius 1.10 Å) by Mn(II) (ionic radius 0.80 Å) can never
result in more than double of the unit cell volume as has been claimed by the authors. The only set of
data in table 2 which does not follow any trend and also look very odd / unusual are the unit cell
values reported in the title paper, making one wonder if these parameters were actually determined. It
is also not clear as to why the authors did not determine the structure of their crystal which is
normally done in any single crystal work and substantiate the same in the form of a cif file. One finds
it strange that molecular formula of a crystal is proposed based only on unit cell data.
Intense signal at ~3450 cm- 1in the IR spectrum
The infrared spectrum in the title paper gives a clear indication that the proposed formula for the so
called CdHg(SCN)4 is inappropriate. The spectrum exhibits the most intense signal at ~3450 cm-1 (OH stretching vibration) and another at ~1620 cm-1 (O-H bending vibration) which is never expected to
be observed for any AB(SCN)4 type compound, since it has no oxygen atom in its formula. The signal
at ~3450 cm-1 is so intense that it dwarfs all other signals clearly indicating that this is a characteristic
signal of the crystal under study and not due to atmospheric moisture or due to absorption of moisture
by KBr used for recording spectrum. The authors did neither list this intense band nor made a mention
of it in their discussion but confidently concluded that the presence of functional groups was
confirmed by IR technique. Given the reaction conditions and reagents used for crystal growth, the
intense signal at ~3450 cm-1 can only be assigned for a O-H stretching vibration. Interestingly the
authors attributed the first weight loss in the TG-DTA thermogram for loss of water embedded within
6
the crystal. It is not clear as to how water embedded in a crystal cannot form a part of the molecular
formula of the crystal. The IR characterization would have been meaningful for the proposed formula
CdMn(SCN)4 had the authors identified and assigned the Cd-N and Mn-S vibrations based on
literature precedence [25, 26]. Greenwood and Earnshaw [27] have reported that IR spectrum
provides only a preliminary indication of the coordination mode of thiocyanate ion and IR is not a
reliable criterion for product characterization in view of the mixing of the υ(CN) and υ(CS) group
vibrations, and have instead suggested X-ray diffraction studies as the most reliable data for
crystalline materials. Although the true identity of the crystal described in the title paper cannot be
determined, the intense signal at ~3450 cm-1 in the IR spectrum together with the unusually large unit
cell volume is more than sufficient to convincingly prove that the crystalline product claimed to have
been prepared is certainly not CdMn(SCN)4.
Conclusions
It is appalling to note that a crystalline solid can be formulated based only on unit cell data without
determining its structure. The assignment of tetragonal crystal system for an unit cell with differing a
and b parameters is a serious error and unacceptable. Many doubts arise from the reading of the paper,
starting from the research aim of the authors to substitute Hg in CdHg(SCN)4 by Mn for the growth of
CdMn(SCN)4 crystal, which is impractical. The paper is completely erroneous and full of
unsubstantiated claims and the crystal growth details and interpretation of all experimental data are
questionable. It is very unfortunate that such a report was published after a peer-review process. The
importance of scientific publications calls for a more careful scrutiny of submitted papers especially
the ones claiming growth of novel crystals, considering the frequent reporting of improperly
characterized compounds.
7
References
[1] A. Rosenheim, E. Cohn, Über einige Metalldoppelrhodanide und über die Eisenrhodanreaktion,
Z. Anorg. Allg. Chem 27 (1901) 280-303. http://dx.doi.org/10.1002/zaac.19010270124
[2] J. G. Bergman, J.J.H. Mcfee, G.R. Crane, Nonlinear optical properties of CdHg(SCN)4 and
ZnHg(SCN)4, Mater Res Bull 5 (1970) 913-917.
http://www.sciencedirect.com/science/article/pii/0025540870901406
[3] A. Korczynski, The crystal structure of copper tetrathiocyanatomercurate Cu[Hg(SCN)4],
Roczniki Chemii 36 (1962) 1539-1541.
[4] A.A. Khandar, A. Klein, A. Bakhtiari, A.R. Mahjoub, R.W.H. Pohl, Dicarboxylate assisted
synthesis of monoclinic heterometallic tetrathiocyanato bridged copper(II) and mercury(II)
coordination polymer {Cu[Hg(SCN)4]}n: Synthesis, structural, vibration, luminescence, EPR
studies and DFT calculations, J. Solid State Chem. 184 (2011) 379-386.
http://www.sciencedirect.com/science/article/pii/S0022459610005505
[5] J.W. Jefferey, K.M. Rose, The structure of cobalt mercury thiocyanate, Co(NCS)4Hg
Acta Crystallogr. B24 (1968) 653-662.http://dx.doi.org/10.1107/S0567740868002980
[6] Y.X. Yan, Q. Fang, D.R. Yuan, Y.P. Tian, Z. Liu, X.M.Wang, M.H. Jiang, D. Williams, A. Siu,
Z. G. Cai, Synthesis, Structure and Non-linear Optical Properties of Manganese Mercury Tetrathiocyanate, Chin. Chem. Lett. 10 (1999) 257-260.
[7] D. Xu, W-T. Yu, X-Q. Wang, D.R. Yuan, M-K. Lu, P. Yang, S-Y. Guo, F. Q. Meng, M-H. Jiang,
Zinc mercury thiocyanate (ZMTC) Acta Crystallogr. C55 (1999) 1203-1205.
http://dx.doi.org/10.1107/S0108270199005983
[8] D. Yuan, D. Xu, M. Liu, F. Qi, W. Yu, W. Hou, Y.Bing, S. Sun, M. Jiang, Structure and
properties of a complex crystal for laser diode frequency doubling: Cadmium mercury
thiocyanate, Appl. Phys. Lett. 70 (1997) 544-546. http://dx.doi.org/10.1063/1.118335
[9] Y.P. Tian, W.-T. Yu, Q. Fang, X.-Q. Wang, D.R. Yuan, D. Xu, M.-H. Jiang, Zinc cadmium
thiocyanate (ZCTC), Acta Crystallogr. C55 (1999) 1393-1395.
http://dx.doi.org/10.1107/S0108270199007623
[10] H.-Q. Sun, W.-T. Yu, D.-R. Yuan, X.-Q. Wang, G. Xue, Cadmium mercury selenocyanate,
CdHg(SeCN)4Acta Crystallogr. E61(2005) i111-i112. http://dx.doi.org/10.1107/S1600536805015771
[11] H.-Q. Sun, W.-T. Yu, D.-R. Yuan, X.-Q. Wang, L.-Q. Liu, Zinc cadmium selenocyanate Acta
Crystallogr. E62 (2006) i88-i90. http://dx.doi.org/10.1107/S1600536806008610
[12] S-L. Li, J-Y. Wu, Y-P. Tian, H. Ming, P. Wang, M-H. Jiang, H-K. Fun, Design, Crystal Growth,
Characterization, and Second-Order Nonlinear Optical Properties of Two New Three-Dimensional Coordination Polymers Containing Selenocyanate Ligands, Eur. J. Inorg. Chem. (2006)
2900-2907. http://dx.doi.org/10.1002/ejic.200500906
[13] S.-L. Li, H.-K. Fun, S. Chantrapromma, J.-Y. Wu, Y.-P. Tian, Cobalt mercury tetraselenocyanate, CoHg(SeCN)4, Acta Crystallogr. E62 (2006) i47-i49.
http://dx.doi.org/10.1107/S1600536806004259
[14] X.-Q. Wang, W.-T. Yu, D. Xu, H.-Q. Sun, W.-L. Liu, Manganese mercury
selenocyanate, MnHg(SeCN)4, Acta Crystallogr. E63 (2007) i45-i46.
http://dx.doi.org/10.1107/S1600536807001808
8
[15] P. Paramasivam, C. Ramachandra Raja, Synthesis, growth and characterization of
cadmium manganese thiocyanate (CMTC) crystal Spectrochim. Acta A79 (2011)
1109-1111. http://dx.doi.org/10.1016/j.saa.2011.04.028
[16] B.R. Srinivasan, Can zinc(II) ions be doped into the crystal structure of L-proline cadmium
chloride monohydrate?, Spectrochim Acta (2013). http://dx.doi.org/10.1016/j.saa.2013.07.025
[17] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in
halides and chalcogenides, Acta Crystallogr. A32 (1976) 751-767.
http://dx.doi.org/10.1107/S0567739476001551
[18] X. Q. Wang, W. T. Yu, D. Xu, M. K. Lu, D. R. Yuan, G. T. Lu, Manganese mercury
thiocyanate (MMTC) glycol monomethyl etherActa Crystallogr. C56 (2000) 647- 648.
http://dx.doi.org/10.1107/S0108270100003188
[19] X. Q. Wang, W. T. Yu, D. Xu, M. K. Lu, D. R. Yuan, Poly[[bis(dimethyl sulfoxide- O)tris(thiocyanato- N)manganese(II)]- -thiocyanato- 2N:S-mercury(II)] Acta Crystallogr. C56 (2000)
418-420. http://dx.doi.org/10.1107/S0108270199016765
[20] X.-Q. Wang, W.-T. Yu, D. Xu, M.-K. Lu, D.-R. Yuan, J.-R. Liu, G.-T. Lu, Polymeric Diaquatetra- -thiocyanato-manganese(II)mercury(II) bis(N,N-dimethylacetamide) solvate, Acta
Crystallogr. C56 (2000) 1305-1307. http://dx.doi.org/10.1107/S010827010001043X
[21] X.-Q. Wang, W.-T. Yu, D. Xu, Y.-L. Geng, Poly[[ -aqua-tetra- -thiocyanato- manganese(II)mercury(II)] sulfolane solvate], Acta Crystallogr. E62 (2006) m2249-m2251.
http://dx.doi.org/10.1107/S160053680603176X
[22] X.-Q. Wang, W.-T. Yu, D. Xu, G.-H. Zhang, Y.-L. Geng, Poly[bis(N-methylformamide)tetra- -thiocyanato-manganese(II)mercury(II)] Acta Crystallogr. C61 (2005), m278-m280.
http://dx.doi.org/10.1107/S0108270105010814
[23] N.W. Ashcroft, N.D. Mermin, Solid State Physics, Harcourt Asia Pvt Ltd. (Indian edition)
(2001).
[24] International Tables for Crystallography (2006). Vol. A, Chapter 7.1, pp.112-717.
[25] D. Forster, D.M.L. Goodgame, Infrared Spectra (400-200 Cm.-l) of Some Thiocyanate and
Isothiocyanate Complexes, Inorg. Chem. 4 (1965) 715-718.
http://pubs.acs.org/doi/pdf/10.1021/ic50027a025
[26] R.A. Bailey, S.L. Kozak, T.W. Michelsen, W.N. Mills, Infrared spectra of complexes of the
thiocyanate and related ions, Coord. Chem. Rev. 6 (1971) 407-445.
http://www.sciencedirect.com/science/article/pii/S0010854500800156
[27] N.N. Greenwood, A. Earnshaw, Chemistry of Elements, First Ed., Pergamon Press,
(1986) Chapter 8, pp. 343-345.