Systematic FT-Raman spectroscopic study of twelve bis-thiourea complexes, A(tu)2B2 (A = Zn, Cd, Hg; B = Cl, Br, I, SCN)

Article abstract of DOI:10.1016/S1386-1425(99)00028-1

This work undertakes a systematic FT-Raman study of twelve thiourea complexes with stoichiometries A(tu)₂B₂ (A = Zn, Cd, Hg; B = Cl, Br, I, SCN). With the exception of Cd(tu)₂Cl₂, the Raman spectra are reported here for the first time. The main modifications observed in the vibrational dynamics of the organic ligand are basically due to the coordination with the cation, with the influence of the anion less clear. These modifications show an increase with the softness of the metallic ion. The statistical analysis of the band wavenumber data shows that the ligand/cation interactions are stronger than the corresponding cation-anion bonds. This could explain the thermal behaviour of these complexes, which decompose easily at moderately high temperatures (600-700 K), producing the corresponding sulfides. The coordination of the thiocyanate anion in their complexes is accomplished via the sulfur atom (Hg²⁺ complex) or via the nitrogen atom (Zn²⁺ and Cd²⁺ complexes), although in both cases the association is largely ionic.

Full text of DOI:10.1016/S1386-1425(99)00028-1

Spectrochimica Acta Part A 55 (1999) 2423–2435
www.elsevier.nl/locate/saa
A systematic FT-Raman spectroscopic study of twelve
bis-thiourea complexes, A(tu) B (A=Zn, Cd, Hg; B=Cl,
2
2
Br, I, SCN)
a,
a
b
J.M. Al ´ı a *, H.G.M. Edwards , M.D. Stoev
a
Chemical and Forensic Sciences, Uni6ersity of Bradford, Richmond Road, Bradford West Yorkshire BD7 1DP, UK
b
Bulgarian Academy of Sciences, Institute of General and Inorganic Chemistry, BG-1113 Sofia, Bulgaria
Received 7 December 1998; accepted 1 February 1999
Abstract
This work undertakes a systematic FT-Raman study of twelve thiourea complexes with stoichiometries A(tu) B2
2
(
A=Zn, Cd, Hg; B=Cl, Br, I, SCN). With the exception of Cd(tu) Cl , the Raman spectra are reported here for the
2
2
first time. The main modifications observed in the vibrational dynamics of the organic ligand are basically due to the
coordination with the cation, with the influence of the anion less clear. These modifications show an increase with the
softness of the metallic ion. The statistical analysis of the band wavenumber data shows that the ligand/cation
interactions are stronger than the corresponding cation–anion bonds. This could explain the thermal behaviour of
these complexes, which decompose easily at moderately high temperatures (600–700 K), producing the corresponding
2
+
sulfides. The coordination of the thiocyanate anion in their complexes is accomplished via the sulfur atom (Hg
2
+
2+
complex) or via the nitrogen atom (Zn
and Cd
complexes), although in both cases the association is largely
ionic. © 1999 Elsevier Science B.V. All rights reserved.
Keywords: Organometallic compounds; Thiourea; FT-Raman; Cadmium sulfide
1
. Introduction
interest arises for two main reasons: their non-lin-
ear optical properties [1,2] and the convenient
preparation of semiconducting materials based on
CdS through the thermal decomposition of these
complexes [3,4].
Coordination compounds formed by the or-
ganic ligand thiourea (tu) and a metal(II) ion with
d
1
0
2+
2+
2+
electronic configuration (Zn , Cd , Hg
)
have recently received renewed attention. This
Although some of the compounds reported in
the present paper have been previously studied by
vibrational spectroscopy [5–8] (see Table 1), there
has not been a previous systematic study that
emphasises the influence of the different cation–
anion pairs on the vibrational dynamics of the
*
Corresponding author. Present and permanent address:
Universidad de Castilla-La Mancha, E.U.I.T A., Dpto. De
Quimica Fisica, Ronda de Calatrava 5, 13071 Ciudad Real,
Spain.
E-mail address: jmalia@qiifi-cr.uclm.es (J.M. Al ´ı a )
1
386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(99)00028-1

2
424
J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
organic ligand. Moreover, as can be observed in
Table 1, despite the relative abundance of the IR
studies, Raman studies have been scarce; this is of
particular importance for the assignment of the low
frequency spectral region.
The present FT-Raman study was carried out for
the purpose of obtaining systematic information
about the effects of a family of cations on the vibra-
tional dynamics of a moderately complex organic
ligand, and how these effects can be modulated by
several simple anions. In addition, the Raman spec-
tra of many of these complexes are reported here
for the first time.
diate dissolution of the inorganic salt. The resulting
solutions were treated as above. The syntheses of
the complexes that contain zinc(II) or cadmium(II)
thiocyanate were carried out in the same way, using
distilled water as the solvent for both thiourea and
the inorganic salt. Mercury(II) thiocyanate was dis-
solved in distilled water at 60°C, adding in slight ex-
cess (8–10%) an aqueous solution of thiourea at the
same temperature. After quick mixing, the resulting
solution was immediately filtered to eliminate a
small quantity of a black precipitate (HgS) and al-
lowed to cool, producing crystals of the white com-
plex.
The absence of unreacted thiourea in the final
product was confirmed by powder X-ray diffrac-
tion carried out using a Philips PW 1710 automatic
diffractometer with Cu Kh radiation. The stoi-
chiometry of the complexes was confirmed by ele-
mental CNS analyses with a NA1500 analyser
2
. Experimental
2.1. Preparation of the complexes
(
Carlo Erba Instrumentazione).
With the exception of the thiocyanates, the rest
of the compounds were prepared following the
method described by Stoev et al. [3,4]. Isoconcen-
trated methanolic solutions of thiourea and the cor-
responding anhydrous salt were mixed at room
temperature in proportion 2:1 and allowed to crys-
tallise by slow evaporation of the solvent. In the
syntheses of the complexes containing mercury(II),
whose halides are only sparingly soluble in
methanol at room temperature, a suspension of the
mercury(II) salt was mixed with the appropriate
quantity of thiourea solution, producing the imme-
2
.2. FT-Raman spectroscopy
FT-Raman spectra were excited at 1064 nm using
a Nd:YAG laser and a Bruker IFS66 optical bench
with a FRA 106 Raman accessory. The laser power
was set at 95–100 mW, and 1000 scans were accu-
mulated with a resolution of 2 cm . The mathe-
matical (curve fitting) treatment of the spectra was
carried out using the commercial software
−
1
®
GRAMS/32 (Galactic Industries). Smoothing
procedures or baseline correction routines were not
applied in this work. Spectra were normalised using
the sum of the integrated areas of the thiourea inter-
Table 1
Previously reported vibrational spectroscopic studies on the
complexes analysed in the present work
nal modes excluding w(NH ) and l(NH ) bands.
2
2
A
B
IR references
Raman references
2.3. Statistics
C1
Br
I
SCN
Cl
Br
I
SCN
Cl
[5–8]
[6–8]
[7,8]
Statistical treatment was carried out using
STATGRAPHICS, Statistical Graphics System’,
v. 5.0., commercial software package from STSC,
Inc. and Statistical Graphics Corporation.
Zn
‘
[6–8]
[7,8]
[7,8]
[2]
Cd
Hg
3. Results and discussion
[6,8]
[8]
Br
I
3.1. Thiourea internal modes
SCN
Fig. 1(a) shows the Raman spectral region that

J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
2425
Fig. 1. FT-Raman spectra of thiourea and the complexes studied. (a) w(NH ) region. (b) l(NH ) region; from bottom to top:
2
2
thiourea, chloride, bromide, iodide and thiocyanate.
contains the w(NH ) fundamentals in the studied
samples. Despite the complexity and considerable
variability from sample to sample, which is possi-
bly the reason for this wavenumber region not
having been studied previously, the envelope can
be conveniently divided into three parts [9–14]: at
2

2
426
J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
higher wavenumbers antisymmetric fundamentals
a and b representations in thiourea) are ob-
modes. These components (numbering 22 in the
present series studied) have been assigned as 2l.
The presence of Fermi resonance, which has been
proposed by some authors [11,15] to justify in
part the complexity of this spectral region in the
vibrational spectra of thiourea, is possibly less
relevant in the spectra of these complexes; other-
wise, the observed frequencies should differ sig-
nificantly from 2l, depending on the magnitude of
the anharmonicity coefficients.
(
1
2
−
1
served; in the central zone (ca. 3230–3300 cm
)
appear the first overtones of the l(NH ) bands;
2
and, in the lower wavenumber zone, the symmet-
ric modes (also a1 and b2 representations in
thiourea). This simple picture can be complicated
further by the presence of more or less intense
hydrogen bonds and possible Fermi resonances.
Figure l (b) shows the bands corresponding to the
l(NH ) normal mode; this fundamental has two
components in the Raman spectrum of thiourea
The complexity of this spectral region has been
pointed out by a number of authors [2,5,9–11]
and is believed to arise from the important hydro-
gen-bonded network, present in the crystal struc-
ture of thiourea, which is much stronger than that
observed in urea itself [12,13,16]. This situation
seems to be even more complicated in some
thiourea complexes [17–19]. There are, however,
some trends in the series of complexes studied
here, which are indicative of a general weakening
of the hydrogen bonds in the complexes. Firstly,
2
(
a and b ), which are preserved in the spectra of
1 2
some complexes. However, only one component is
observable in other spectra and even up to three
components in the spectra corresponding to the
mercury(II) complexes.
Wavenumbers of the observed Raman bands
and their assignments are summarised in Table 2.
In the central part of the stretching band envelope
of the complexes, it is possible to find some
components which are broader than their neigh-
bours, whose wavenumbers are, closely (see Table
the strongest band of the 6(NH ) envelope,
2
assigned to (NH ) symmetric stretching (tu: 3183
2
−
1
3), twice those corresponding to the bending
cm ), shifts toward higher wavenumbers in all
Table 2
−
1
Wavenumbers (cm ) of Raman bands in the w(NH ) and l(NH ) regions and their assignments
2
2
tu
Zn
Cl
Cd
Cl
Hg
Cl
Assignment
Br
I
SCN
Br
I
SCN
Br
I
SCN
3443
3327
3
443
3434
3382
3325
3410
3374
3318
3285
3427
3393
3334
3289
3275
3433
3387
3324
3290
3417
3367
3322
3306
3284
3406
3358
3313
as. w(NH2)
as. w(NH2)
as. w(NH2)
as. w(NH2)
as. w(NH2)
2l
3
3
373
3368
3387
3293
3381
3305
3371
3327
3365
3301
3
335
3316
294
320
3
3
283
3281
3251
3229
3214
3200
3187
3142
3290
3262
3236
3195
3283
3233
3183
3271
3244
3208
3296
3253
3214
3282
3254
3215
3200
3147
3278
3250
3200
3272
3245
3199
2l
2l
3244
3212
3
3174
3131
3241
3198
3261
3234
3206
3257
3231
3205
3244
3214
3199
sym. w(NH2)
sym. w(NH2)
sym. w(NH2)
sym. w(NH2)
199
3
3
160
099
3180
3125
3177
3180
3135
3152
3133
3137
3047
1
1
636
613
1629
1611
1634
1626
1622
1617
1620
1617
1621
1641
1622
1627
1618
1640
1625
1627
1616
1607
1638
1625
1616
l(NH2)
l(NH2)
l(NH2)
1
616

J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
2427
Table 3
by its coordination with the central ion, the ability
of the thiocyanate ion to promote or, alterna-
tively, not disturb the hydrogen bonding could be
a consequence of its acidity. The effect of the
metal(II) ion is more complicated, however, and
cadmium(II) ion is the most disrupting ion, fol-
lowed by zinc(II) and mercury(II) ions. We would
have expected a sequence inverse from the corre-
sponding softness (Zn \Cd \Hg ); it has
been demonstrated that the crystal structures of
thiourea complexes with soft cations have strong
hydrogen-bonded networks [17–19].
Statistics of the wavenumbers (cm−1) corresponding to the
central components in the w(NH ) envelope, components of
the l(NH ) vibration and the ratio w(NH )/l(NH )
2
2
2
2
Variable
w(NH2)
l(NH2)
w(NH2)
/
l(NH2)
Sample size
Average
Median
Mode
Geometric
mean
Variance
Standard de-
viation
Standard er-
ror
Minimum
Maximum
Range
Coeff. of vari-
ation
22
22
22
3260.0
3255.8
3243.9
3260.0
1623.6
1622.0
1621.3
1623.6
2.0079
2.0079
2.0076
2.0079
2+
2+
2+
370.56
19.250
84.76
2.9459E-5
5.4276E-5
Fig. 2(a–d) show the Raman spectra in the
9.2065
1.9793
1607.2
−1
region 1500–350 cm
for the twelve complexes
studied. The corresponding assignments of the
bands to several thiourea internal modes are sum-
marised in Table 4. Given the evident multiplicity
of the bands arising from the same fundamental
in the spectra of most of the samples, it is conve-
nient to use the centroids of the band’s groups
that can be assigned to a unique normal mode.
The numerical results are shown in Table 5. It can
be observed that the vibrational modes of the
ligand most affected by the complexation are
those corresponding to a (CS) stretching and
complex mode denoted by w (SCN H ), which can
3.9294
1.1079E-3
3229.0
3295 6
66.6
1.9993
2.0173
0.0180
0.2703
1641.5
34.3
0.5905
0.5671
the complexes studied (see Table 2), with an aver-
−
1
age displacement of 21.397.1 cm . Moreover,
this band is narrower in the spectra of the com-
plexes compared with thiourea. Secondly, bands
located at higher wavenumbers, which have a
maximum frequency of 3373 cm
spectrum of thiourea, appear in the complexes as
high as 3443 cm , with considerable band inten-
sity in some cases. Thirdly, centroid of the envel-
ope (tu: 3221.5 cm ) shifts toward higher
wavenumbers in the spectra of all the complexes
studied, reaching an average value of 3277.69
1
2
4
be described [11] with the following potential en-
ergy distribution (PED): 20% w(CN)+19%
l(NCN)+26% lCNH)+10% w(CS). Other au-
thors [12–14] have calculated similar PED. How-
ever, the sign of the mode modification is different
in both cases: while the vibration w (SCN H )
−
1
in the Raman
−
1
1
2
4
shifts upwards, w(CS) moves downwards. Vibra-
tions w (NCN), w (SCN H ), whose PED can be
−
1
as
2
2
4
divided [11–13] into contributions of approxi-
mately equal amounts of the stretching w (CN)
s
−
1
20.6 cm
(see Table 5). Finally, the important
and rocking z(NH ) modes, and the l(CS) mode
2
intensity increment observed in the l(NH ) Ra-
man bands of the complexes relative to the ligand
2
are relatively less affected by the complexation
and only smaller modifications are observed in
l(NCN). All the reported wavenumber shifts can
be explained on the assumption that the main
effect of the complexation on the thiourea
molecule is the weakening of the C=S double
bond resulting from the transfer to the metal(II)
ion of lone pair from sulfur. The subsequent
charge displacements should strengthen the N–
C–N bonds.
(
see Fig. 1(b)) could be also interpreted as a result
of hydrogen bond weakening.
The value of the w(NH ) band envelope cen-
troid can be correlated with the anion present.
Generally, the centroids of highest value corre-
2
−
spond to the iodides, following the trend I
\
−
−
−
Br \Cl :SCN . As we have observed for
Te(II)-bisthiourea complexes [20], where the
thiourea hydrogen-bonded network is reinforced
One of the features in the Raman spectra of the

2
428
J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
complexes that deserves some comment is the
alteration in the intensity of several bands, shown
in Fig. 2(a, c and d). Changes are particularly
clear in w (NCN), whose intensity is increased
significantly, and w(CS), which behaves inversely.
Both modifications can be ascribed to changes in
as
Fig. 2. FT-Raman spectra of thiourea and the studied complexes. (a) w (NCN) and w (SCN H ) region. (b) w (SCN H ) region. (c)
as
1
2
4
2
2
4
w(CS) region. (d) ~(NH ), l(SCN) and l(NCN) region. Order of the spectra as in Fig. 1.
2

J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
2429
Fig. 2. (Continued)
the CS and N–C–N effective bond orders. The
strong increase observed in the intensity of the
l(CS) vibration is also ascribed to the effect of the
cation coordination and mainly depends on its
polarising power. Similar effects have been re-
ported for the bending C–CꢀN mode in the
coordination of uni- and bivalent cations with
acrylonitrile in solution [21–23].

2
430
J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
3.2. Statistical analysis
the band, or group of bands, assigned to the G
fundamental in the complex and in the thiourea
In order to evaluate quantitatively the effect of
Raman spectra, respectively, and | is the stan-
G
the different cations and anions on internal modes
of the thiourea, a statistical analysis of the data
summarised in Table 5 has been carried out. The
dard deviation of the twelve centroids calculated
for the vibration G. Hence, each complex can be
represented by eight coordinates in an imaginary
8-dimensional space, whose origin is the unper-
turbed thiourea molecule. Once these coordinates
have been obtained, the standard statistical dis-
standard statistical coordinates ( ), correspond-
G
ing to the eight normal modes of vibration stud-
ied, have been calculated for each complex as:
tance of the complex c to the origin (D ) can be
c
wc−wtu
calculated as:
G=
|
G
='%
(
)
2
G
where w and w are the centroid wavenumbers of
D
c
G
c
tu
Table 4
−
1
Wavenumbers (cm ) of Raman bands in the region of the thiourea internal modes for metal complexes, excluding w(NH )and
2
l(NH ), and their assignments
2
tu
Zn
Cl
Cd
Hg
Cl
Assignment
Br
I
SCN
Cl
Br
I
SCN
1522
Br
I
SCN
1
486
1496
1489
1516
1490
1508
1489
1503
1494
1499
1495
1486
1506
1484
1510
1504
1495
1500
1485
1525
1513
1
471
as. w(NCN)
w (SCN H )
1
498
1
1
384
369
1443
1428
1445
1431
1409
1387
1437
1426
1402
1383
1359
1438
1402
1394
1385
1370
1445
1432
1411
1399
1382
1435
1427
1401
1387
1373
1438
1423
1400
1382
1364
1441
1417
1389
1341
1445
1430
1410
1438
1425
1400
1393
1426
1414
1386
1355
1419
1393
1387
1367
1
1
402
383
1
2
4
1
369
1111
093
1093
1069
733
1106
1049
722
1106
1091
1102
1085
1103
1096
1097
1102
1087
1113
1096
1110
1076
1102
1062
1090
1003
10971
1
w (SCN H )
2 2 4
1069
1063
1
1062
1056
1058
980
1054
1053
1030
z(NH2)
006
721
705
733
717
704
6
6
734
725
711
705
720
713
706
715
710
697
717
703
699
683
726
706
722
711
699
715
712
705
712
704
699
684
674
703
698
7
15
6
87
n(CS)
99
81
5
4
74
99
619
513
609
594
609
581
552
520
503
630
598
559
516
604
532
611
591
548
512
488
600
550
527
593
570
527
494
498
5
5
5
63
35
08
~(NH2)
4
78
01
480
473
472
471
475
430
493
425
471
468
459
484
478
479
436
469
461
475
470
4
74
l(NCN)
l(CS)
4
430
431
427
413
429
419
424
409
409
442
426
410
416
406
4
22

J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
2431
Table 5
Band centroids (cm−1) of the thiourea internal modes in the different complexes. |=standard deviation; the parameter ‘difference
|)’ (last row) is the difference between the centroid of the corresponding thiourea Raman band and the average, expressed in
(
standard deviations.
Complex
w(NH2)
l(NH2)
was(NCN)
w1(tu)
w2(tu)
w(CS)
l(NCN)
l(CS)
Zn(tu) Cl2
3288.3
3288.3
3285.4
3267.1
1627.8
1630.7
1623.0
1617.2
1491.1
1500.9
1495.6
1498.9
1410.7
1404.8
1398.8
1403.0
1105.6
1097.3
1094.9
1102.1
719.7
710.6
708.9
712.5
475.2
473.2
471.7
473.7
429.8
427.7
421.2
428.8
2
Zn(tu) Br2
2
Zn(tu)2I2
Zn(tu) (SCt)2
2
Cd(tu) C12
3305.8
3314.5
3308.7
3243.9
1620.4
1617.5
1621.3
1635.5
1499.5
1493.4
1492.7
1522.2
1412.4
1404.9
1398.3
1404.8
1103.3
1096.6
1094.5
1110.9
718.2
713.1
706.8
707.1
475.0
473.1
471.3
470.8
430.5
425.3
419.9
409.3
2
Cd(tu) Br2
2
Cd(tu)2I2
Cd(tu) (SCN)2
2
Hg(tu) C12
3248.8
3260.3
3288.3
3232.4
1625.9
1622.0
1615.3
1624.0
1509.7
1501.0
1492.7
1512.8
1421.0
1410.4
1393.1
1407.3
1110.2
1102.5
1094.6
1097.5
718.3
712.7
700.3
702.2
479.4
478.0
466.1
468.4
442.2
436.1
421.8
408.2
2
Hg(tu) Br2
2
Hg(tu)2I2
Hg(tu) (SCN)2
2
Average
|
3277.7
26.9
1623.4
5.9
1500.9
9.5
1405.8
7.4
1100.8
5.9
710.9
6.2
473.0
3.7
425.1
9.9
Thiourea
Difference (|)
3221.5
\2
1631.4
\1
1479.2
\2
1377.9
\3
1093.5
\2
732.0
\3
478.3
\1
401.3
\2
Table 6
Normalised statistical distances (D ) of the metal complexes from thiourea
c
Cl
Br
I
SCN
Average
|
Zn
Cd
Hp
Average
|
6.6965
7.5362
8.7320
7.6549
1.0229
6.7779
7.1019
7.3162
7.0654
0.2710
6.3059
6.8375
7.8198
6.9878
0.7680
6.7626
8.0315
7.4476
7.4139
0.6351
6.6357
7.3768
7.8289
0.2227
0.5229
0.6387
The numerical results obtained after this statisti-
cal treatment of the Raman spectra of the com-
plexes are given in Table 6. It is seen that the
principal effect arises from the metal(II) ion, as
seems logical since the coordination of thiourea
molecules is accomplished with the cation. How-
ever, the different anions also play an important
role. Mercury(II) most strongly affects the vibra-
tional behaviour of thiourea, followed by cadmiu-
m(II) and zinc(II), as expected from their
cluding thiocyanates, whose coordination is not
systematic, as will be discussed below, and which
2
+
includes bonding as a thiocyanate in the Hg
2
+
complex and as isothiocyanate in the Zn
Cd
and
2
+
complexes, the observed trend reveals that
the effect of the anion decreases as its size in-
creases. Reported crystal structures of some
bisthiourea complexes with soft metal ions, clearly
demonstrate that the S -cation bond lengths are
tu
very close to the sum of the covalent radii, while
the cation-anion distances are in the range of
ionic bonds [18,25–29]. For this reason, it is likely
that the cation–anion interactions, which could
modulate the effect of the cation on the thiourea
2
+
2+
corresponding softness (Zn : 0.35; Cd : 0.58;
2
+
Hg : 1.27) [24]. The effect of the anionic coun-
terpart is not so evident from these results, mainly
because of their great standard deviations. Ex-

2
432
J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
Table 7
2
Band centroid wavenumber of the thiourea internal modes in the different complexes: correlation (r ) matrix. P: significance level.
In italics, statistically significant correlations
l(NH2)
was(NCN)
w(SCN H )
w (SCN H )
w(CS)
0.2091
l(NCN)
l(CS)
0.2003
2
4
2
2
4
w(NH )−
0.4315
0.1614
−0.8172
0.0012
−0.3452
0.2717
−0.5014
0.0968
−0.0427
0.8951
2
P
0.5143
0.5325
l(NH2)
P
0.6354
0.0264
0.3345
0.2880
0.5853
0.0456
0.1297
0.6880
0.1648
0.6088
−0.2120
0.5083
was(NCN)
P
0.3761
0.2283
0.6109
0.0348
−0.1608
0.6175
0.0171
0.9579
−0.3459
0.2707
w (SCN H )
P
0.7621
0.0040
0.7489
0.0051
0.8128
0.0013
0.5651
0.0556
1
2
4
w (SCN H )
P
0.5935
0.0419
0.6166
0.0327
0.2854
0.3686
2
2
4
w(CS)
P
0.8775
0.0002
0.7508
0.0049
l(NCN)
P
0.8323
0.0008
Table 8
7 shows the corresponding correlation matrix.
The normal modes of vibration whose centroids
are significantly correlated with more elements are
w (SCN H ) and w (SCN H ), which are the com-
Thiocyanate anion normal modes (cm−1) in the thiourea
complexes
1
2
4
2
2
4
Zinc
Cadmium
Mercury
Assignment
plex modes that involve the molecular kernel of
the ligand. Furthermore, about 80% of the total
data variance can be statistically justified with
only three fundamentals: w(CS) and the two com-
plex modes.
2
2
2
090
085 (sh)
070
2063
2080
2087 (sh)
n1
w1
w1
2076 (sh)
9
9
68
50
968
953
935
917
2w2
2w2
2w2
911
3.3. Thiocyanate anion 6ibrational modes
8
8
28
21
797
790
737
731
w3
w3
Wavenumbers of Raman bands attributable to
thiocyanate anion and their assignments are given
in Table 8. Fig. 3 shows the w(CN) region in the
Raman spectra of the complexes. The w1
wavenumbers are below 2090 cm , which proba-
bly means that the interaction with the cation is
largely ionic [30–32], as can be inferred from the
insignificant effect of the anions on the thiourea
vibrational dynamics. The relatively high
4
80
484
463
w2
w2
456 (sh)
−
1
vibrations, would be weak. Another possible ef-
fect of anionic influence is the hydrogen bond
formation. However, from previous discussion,
this possibility can be ruled out in the complexes
studied here.
wavenumbers corresponding to w are justified in
2
Another interesting feature that emerges from
these analyses is the statistical correlation between
the data that belong to the same spectrum. Table
the same way. The orientation of the thiocyanate
group with respect to the metal(II) ion can be
clearly deduced from the w3 wavenumbers [30]

J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
2433
state and dissolved in various solvents [33–37].
From the observed results, two conclusions may
be made. The relatively low wavenumbers of
those bands assigned to cation–anion symmetric
stretching, w(A–B), reveals the mainly ionic char-
acter of these bonds in the studied complexes.
This could be a direct consequence of the covalent
character of the S –A bonds, which are spectro-
tu
scopically characterised by the high wavenumbers
of their stretching vibrations. It has already been
discussed that the strongest interactions in these
complexes occur in the system metal(II) ion/
thiourea. The second interesting feature of this
spectral region is the position of the bands at-
tributable to the cation–anion stretching in the
thiocyanate complexes. Assuming that the coordi-
nation in the mercury(II) complex is through the
−
1
sulfur atom (w=164 cm ), it is clear that the
remaining coordination is accomplished through
the nitrogen atom since their observed wavenum-
−
1
bers (zinc(II) complex: 162 cm ; cadmium(II)
−
1
complex: 159+145 cm ) should be substantially
−
1
higher (ca. +20 cm ) if the anion orientation
were of the thiocyanate type. This means that the
coordination observed in the simple thiocyanates
[33] is preserved in the complexes.
Fig. 3. Thiocyanate anion w (CN stretching) in the complexes
1
Zn(tu) (NCS) , Cd(tu) (NCS) and Hg(tu) (SCN) .
2
2
2
2
2
2
4
. Conclusions
From the experimental results of this work, the
following five conclusions can be made:
which demonstrate that the thiocyanate coordina-
2
+
−
tion (M …SCW ) applies in the mercury(II)
complex and the isothiocyanate coordination
1
. In comparison with the crystal structure of
thiourea, the series of complexes studied ex-
hibit a general weakening of the hydrogen-
bonded network that can be correlated with
the type of anion. Arranged from higher to
lower weakening ability, the anions follow the
2
+
−
(
M
…S ) occurs in the zinc(II) and cadmiu-
m(II) complexes. The analysis of the low fre-
quency spectral region confirms this conclusion.
3.4. Low frequency spectral region
−
−
−
−
trend I \Br \Cl :SCN , as expected
from their corresponding acidities [24].
Fig. 4 shows the Raman spectra of the com-
2
. The internal modes of vibration of thiourea
which are most affected by the complexation
are the complex mode w (SCN H ) and the
plexes in the low frequency region, for which the
corresponding assignments are given in Table 9.
These assignments have been made taking into
account previous IR results [7,8] and assignments
proposed for complexes of the cations studied
with S- and N-donor ligands, both in the solid
1
2
4
w(CS) mode, which shift upwards and down-
wards in wavenumber, respectively. These ef-
fects and the associated significant intensity
changes observed in some fundamentals can be

2
434
J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
Fig. 4. FT-Raman spectra of thiourea and the complexes studied in the low frequency region. Order of the spectra as in Fig. 1.
Table 9
Wavenumbers (cm−1) of Raman bands in the low frequency region and their assignments
tu
Zn
Cl
Cd
Cl
Hg
Cl
Assignment
Br
I
SCN
237
Br
I
SCN
207
Br
I
SCN
256
2
50
258
241
255
233
223
203
233
210
231
214
257
239
253
231
220
205
w(S −A)
tu
2
38
2
74
198
163
148
162
127
171
158
135
159
145
208
157
130
164
w(A−B)
1
84
1
41
61
145
136
147
136
144
125
125
176
136
140
127
150
149
137
l(S −A)
tu
1
l(B−A)
1
1
8
17
01
4
113
99
83
112
100
84
109
92
114
101
86
119
101
86
120
105
95
113
98
81
114
91
84
116
96
78
106
90
119
100
119
102
87
Lattice modes
87
interpreted as the direct result of cation coor-
dination, which weakens the C=S bond and,
consequently, strengthens the N–C–N bonds.
parallels their softness. The principal interac-
tions in the complex structure are those of the
system metal(II) ion/ligand. The cation–anion
interaction is largely ionic and secondary. This
could explain the thermal behaviour of these
complexes, which are transformed in the corre-
sponding sulfides at moderate temperatures,
irrespective of the anion present.
3
. The statistical analysis carried out on the vi-
brational wavenumbers demonstrates that the
main influence on the vibrational dynamics of
thiourea arises from the metal(II) ion, accord-
2
+
2+
2+
.
ing to the trend Hg \Cd \Zn
, which

J.M. Al ´ı a et al. / Spectrochimica Acta Part A 55 (1999) 2423–2435
2435
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