Nuclear Quadrupole Resonance of Nitrogen-14 in Potassium
Hexathiocyanatoplatinate(IV) and Tetrathiocyanatomercurate(II)
TETSUO ASAJI a n d
RYUICHI
IKEDA
Department of Chemistry, Nagoya University, Chikusa, Nagoya 4(34, Japan
and
DAIYU
NAKAMURA*
Physikalische Chemie III, Technische Hochschule Darmstadt
(Z. Naturforsch. 31b. 1483-1488 [1976]; received August 2, 1976)
14 N
NQR, Hexathiocyanatoplatinate(IV), Tetrathiocyauatoinercurate(II), Bonding, Libration
The nuclear quadrupole resonance of 14 N was observed in KoPt(SCN)6 and K2Hg(SCN)4
at various temperatures. Both compounds yield large asymmetry parameters indicating the
existence of asymmetric ^-electron distribution about a CN bond axis. The ionic character
of metal-ligand bonds in the platinum and mercury complexes is estimated to be 2 8 % and
58 % , respectively. The temperature dependence of each component of quadrupole
coupling can practically be interpreted in terms of a simple Bayer theory. Both complexes
show the quadrupole coupling constant versus temperature curve faintly convex to the
abscissa. This anomaly is attributable to vibronic interaction in a thiocyanate group.
Introduction
Previously, we have studied the 14N nuclear
quadrupole resonance (NQR) of various alkyl thiocyanates1 and potassium thiocyanate 2 and found
that nitrogen in the alkyl thiocyanates shows
considerably larger asymmetry
parameters**
(0.46-0.47) and also larger quadrupole coupling
constants** (3.5 MHz) than in KSCN (^ = 0.028
and eQq/h = 2.4 MHz). This indicates that the
quadrupole resonance parameters of nitrogen are
highly sensitive to the electronic state of sulfur in
spite of the presence of a carbon atom between
them. Consequently, the study of 14N NQR in thiocyanate compounds provides an interesting possibility to obtain more information about bonding
between thiocyanate groups and corresponding
* Alexander von Humboldt Fellow, 1976/1977.
** The asymmetry parameter and the quadrupole coupling constant in frequency units are usually expressed as -q = (qx^-qyy)lqzz and eQq/h = |eQqzz/h|, respectively, where qxx, qyy, and qzz denote the principal
components of electric field gradient (EFG) holding
a relation, lq.r^-1 < Iq^l < Iqzzl and eQ is a nuclear
quadrupole moment.
Requests for reprints should be sent to Prof. D. NAKA MURA, Physikalische Chemie III der Technischen Hochschule Darmstadt, Petersenstr. 15, D-6100 Darmstadt.
moieties. Since bonding between sulfur and alkyl
carbon can be considered to be almost covalent
without noticeable conjugation, NQR data are now
available for thiocyanate groups in the two extreme
states of bonding, namely, thiocyanate ions, SCN - ,
and thiocyanate groups bound with a covalent
a-bond, \SCN. These data may be helpful for the
elucidation of the electronic structure of thiocyanate
groups in various compounds. In this paper, we
report the 14N NQR in two thiocyanate complexes
and discuss the bonding scheme of thiocyanate
groups in metal complexes.
Experimental
Potassium hexathiocyanatoplatinate(IV) was
prepared according to the method described in the
literature3 and purified by recrystallization from
absolute methanol. Since only one resonance line
attributable to v1 could be observed at liquid nitrogen temperature for the crystals obtained in this
manner, we warmed the sample up to 125 °C in a
sealed glass ampoule and then cooled it gradually to
room temperature over seven days. Then the sample
gave rise to a set of resonance signals, v1 and ? JI
strong enough to observe NQR signals even at room
temperature.
Potassium tetrathiocyanatomercurate(Il) was
synthesized by the method reported by R O S E N H E I M and COHN4, and recrystallized from methanol
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1484
T. Asaji et al. • 14 N NQR in Metal Complexes
containing a small amount of potassium thiocyanate.
No resonance was observed until the sample was
annealed in a similar manner as the former compound. Both compounds were identified by chemical
analysis and also checked by infrared spectra.
Analysis for K 2 Pt(SCN) 6
Calcd
Pt 31.4 S 31.0,
Found Pt 31.4 S 30.5.
Analysis for K 2 Hg(SCN) 4
Calcd
Hg 39.2 SCN 45.5,
Found Hg 37.9 SCN 46.2.
Since 14N has a nuclear spin equal to unity, one
can usually observe two resonance frequencies v1
and v11 for a nitrogen atom having a finite asymmetry parameter
(eQq/h) (3—t])/4. (1)
Potassium hexathiocyanatoplatinate(IV) gave rise
to a pair of resonance lines v1 and vu in the temperature range of 4.2-300 K, while potassium tetrathiocyanatomercurate(II) yielded two sets of the
frequencies at various temperatures between 77 and
300 K. Table I shows the resonance frequencies,
quadrupole coupling constants, and asymmeüy
parameters of both complexes determined at 77 K
along with those of potassium thiocyanate and
methyl thiocyanate for comparison. Fig. 1 indicates
the temperature dependence of resonance frequencies
in the mercury complex.
Table I. NQR frequencies, quadrupole coupling constants, and asymmetry parameters of 14 N in some
thiocyanate compounds at 77 K.
Compound
vl/kHz
v^/kHz
(eQq/h)/kHz 7]
CH 3 SCN
K 2 Pt(SCN) 6
K 2 Hg(SCN) 4
3052.4
2715.0
2382.9
2304.0
1841.3
2220.7
2093.5
2090.1
2046.0
1806.7
3515.4
3205.7
2982.0
2900.4
2432.0
KSCN
0.4732
0.3878
0.1964
0.1783
0.0285
K2Hg(SCN)«
i
V2
2350
•
'I
:
-
2100
2050
• ••
^
1
200
100
Results
= (eQq/h) (3-f-
* ..
2300
The NQR of nitrogen was observed by means of
a modified P O U N D - W A T K I N S type spectrometer
already reported 1 . The Z E E M A N effect of resonance
lines was measured in an attempt to assign the
observed signals to vl and v115. A chromel-alumel
thermocouple calibrated at the standard temperatures was employed for monitoring temperatures
above 77 K. Temperatures below 77 K were determined by a chromel-gold (0.03% iron) thermocouple
calibrated by the 14N NQR signals of hexamethylentetramine6. The accuracy of temperature measurements was estimated to be within ± 1 K.
1
i
i
Fig. 1. Temperature dependence of
quencies in K 2 Hg(SCN)4.
T/K 300
14 N
NQR Fre-
Discussion
1. Structure and bonding
The crystal structure of potassium hexathiocyanatoplatinate(IV) has been reported to form a
hexagonal lattice7 with the space group D| d -P3ml
and Z = 1. All SCN groups in the complex ion are
crystallographically equivalent in agreement with
the results of the present investigation which yields
only one pair of resonance lines v1 and v11 at various
temperatures studied. However, accurate atomic
positions for the light atoms have not been determined as yet.
According to the X-ray analysis carried out by
Z V O N K O V A 8 , the mercury complex has a monoclinic
lattice belonging to the space group Cl h -C2lc with
Z = 4. The complex anion forms a distorted tetrahedron with S - H g - S angles being 102-108°. Although no accurate atomic positions for light atoms
have been determined as yet, a bent Hg-S-C bond
with a sulfur valency angle of about 100° is anticipated from the crystal data of analogous complexes,
[(C6H5)4P]2Hg(SCN)4 9 and Cu(en)2Hg(SCN)4 10.
In the complex anion there are two kinds of
crystallographically nonequivalent SCN groups
1485 T. Asaji et al. •
14 N
because of the presence of the C2 site symmetry at
the central metal atom. Since four complex ions in
a unit cell are crystallographically equivalent 8 , the
two sets of resonance lines observed arise from two
kinds of SCN groups in the complex ion. In order
to determine exact pairing between two v1 and two
v11 frequencies observed, we measured the saturation
recovery rate of resonance signals at liquid nitrogen
temperature11. The saturation recovery of the vl
and v11 lines of higher frequencies is much faster
than that of another pair, indicating that nitrogen
responsible for the former pair has shorter spinlattice relaxation time T\ than the latter. Therefore,
we have assigned the pair of higher frequencies to
one kind of SCN groups (site a), and another pair
to another kind of SCN groups (site b) in the same
complex ion.
Both complexes show fairly large asymmetry
parameters which are too large to be ascribable to
the asymmetry of the EFG originating from neighboring ionic charges. Therefore, it is concluded that
an anisotropic 71-electron distribution in thiocyanate
groups produces the large asymmetry parameters
of these complexes. This suggests that Pt-SCN and
Hg-SCN have a nonlinear structure similar to
methyl thiocyanate 12 , and that there exists a
considerable amount of covalency in the metalligand bonds. The latter statement is confirmed by
the fact that both complexes yield larger quadrupole
coupling constants than potassium thiocyanate.
In order to quantitatively determine the electronic
structure of thiocyanate groups, let the principal
axes of the EFG tensor at a nitrogen nucleus be
taken with the x-axis perpendicular to the metalSCN plane and with the z-axis directed along the
SCN group, which is assumed to be linear. In this
coordinate system, one can obtain inequalities
among the principal components of the EFG, i.e.
Iqzzl > \Qyy\ > |qzxl> which have already been
assured for a methyl thiocyanate molecule by
microwave experiments13. The inequalities lead to
new relations, N2 > N^ > N y 14, among the numbers
of p-electrons in the 2p-orbitals of nitrogen along
the respective coordinate axes. By assuming that
these relations are still valid for the present
complexes, one can obtain the following equations
according to the T O W N E S - D A I L E Y procedure 1 .
eQq/h =
[
s + (1—s)i a
1
|eQqp/li|
(2)
NQR in Metal Complexes
eQq/h = — (\nx — i^)|eQqp/h|.
(3)
Here, s and q p denote the extent of sp-hybridization in the cr-bonding orbital of nitrogen and the
field gradient formed by a single 2 p-electron of
nitrogen, respectively. We have also assumed the
cr-bonding orbital, the lone-pair orbital, the n x - and
the 7iy-bonding orbitals of nitrogen to accommodate
1 + i ff , 2, 1 -j- inx, and 1 \ny electrons, respectively.
Since these equations involve many unknown
parameters in spite of only two observables, eQq/h
and r\ obtainable from the present experiment, we
must remove some of these unknown parameters by
making adequate assumptions. The values of
s—
—
j (1—s)i(j are approximately constant for the
compounds discussed here, because the electron
population in both (T-bonding and lone-pair orbitals
of nitrogen might be unaltered by the electronic
state of the distant sulfur. Under this assumption,
the number of excess jr-electrons on the nitrogen
atom, ijr^ + i ^ j depends linearly on the observed
coupling constant. This might be confirmed by the
fact that the CN stretching frequency reported
(2166, 2124, 2115, and 2050 c m - 1 1 . « in the order
of compounds listed in Table I) increases linearly
with increasing quadrupole coupling constant.
From Equation (3), the value of \ n x —i n y can be
calculated by using eQq/h and rj observed as well as
|eQqp/h| estimated to be 11.3 MHz 16 . The results are
shown in Table II. The value of \ nx —\ ny is presumed
to be zero for thiocyanate ions, because of the
essentially linear structure of SCN" in crystals17,
and also because of a negligibly small rj observed.
Accordingly, the 2pa;- and 2pj,-orbitals of nitrogen
in an SCN - ion are occupied by the same amount of
excess ^-electrons, namely \nx = \ny = W011. By using
Equation (2) with the calculated values of \nx—iny,
the quantity of i^1011—\nx and i^10"—\Uy can be
evaluated for the complexes and methyl thiocyanate
as shown in Table II.
Table II. Electronic populations of the jr-bonding
orbitals of nitrogen in some thiocyanate compounds.
Compound
CH3SCN
1 nx 1 Tty
0.098
0.073
K 2 Pt(SCN) 6
K 2 Hg(SCN) 4 a 0.032
KSCN
0
ijiion-ijiv
ljl
ljlj.
0.145
0.105
0.061
0.047
0.032
0.029
-
-
Calculated from averaged values of eQq/h and i] for
the sites a and b.
1486
T. Asaji et al. • 14 N N Q R in Metal Complexes
The electronic structure of an M-SCN group can
be represented by resonance among structure I,
polarized structure II in SCN, and ionic structures
III and IV, consisting of a cation M+ and an anion
SCN - . Here, the local polarization of CN has been
ignored.
M
M
\
M+
M+
\
S-C=N
S+=C=N-
S--C=N
S=C=N-
I
II
III
IV
Since the structures I and III have a CN triple
bond, jr-electrons distribute cylindrically symmetric
about the bond axis. The same is true for the
structure IV, because there exists an isolated SCN ion. However, the formation of jr-bonding in the
SCN group of the structure II is considerably
restricted by the presence of an M-S a-bond. The
Spy-orbital of sulfur being concentrated in the
M-SCN plane should mainly be employed in
a-bonding with M, and might not form a jr-bond
with the 2p2/-orbital of carbon, which, therefore,
forms a Ti-bond with the 2p2/-orbital of nitrogen.
Accordingly, an excess charge of the nitrogen atom
in the structure II must occupy the 2pa;-orbital of
nitrogen. This means that the 2 p,,. and 2p y orbitals
of nitrogen are occupied by 2 and 1 electron,
respectively, for the structure II, whereas they are
equally occupied by 3/2 electrons for the structure IV. Let the contribution of the resonance
structures I, II, III, and IV, be denoted by a, b, c,
and d, respectively, where a - f - b - f c - j - d = l . The
weighted number of electrons in the 2pa;-orbital
of nitrogen can be given by a, 2 b, c, and 3d/2
for the respective resonance structures, while those
in the 2p2/-orbital are a, b, c, and 3d/2 in the same
order. It is self-evident within the approximation of
this discussion that one has a = b = 0 for potassium
thiocyanate containing nearty isolated SCN - ions.
To estimate the ionicity of metal-ligand bonds
in both complexes, some further assumptions must
be made. First, we assume c = d = 0 for methyl
thiocyanate, because ^-bonding between methyl
carbon and sulfur can be regarded to be almost
purely covalent by electronegativity considerations.
Second, all compounds in question, except methyl
thiocyanate, may have the same ratios of contributions from the structures III and IV, namely c/d.
This is because both III and IV involve the
resonance structure of an isolated SCN - ion. Under
these assumptions, contributions from each resonance structure can be calculated as shown in
Table III by using the values, i^1™—i„x and
j ^ i o n — g i v e n in Table II. Here, iMs (equal to c -f- d)
denotes the ionic character of metal-ligand bonds
in the platinum and mercury complexes. J O N E S 1 8
has reported the same value of 0.71 as the present
results for the contribution from III in potassium
thiocyanate by the analysis of infrared spectra. For
methyl thiocyanate, the present value of 0.10 for b
agrees well with that estimated by L E T T and
FLYGARE 13 from the results of microwave measurements on gaseous methyl thiocyanate. These facts
confirm the adequacy of the present analysis.
Table III. Contributions of the various resonance
structures in some thiocyanate compounds.
Compound
a
b
c
d
CH3SCN
K 2 Pt(SCN) 6
K 2 Hg(SCN) 4
KSCN
0.90
0.65
0.39
0
0.10
0.07
0.03
0
0
0.20
0.41
0.71
0
0.08
0.17
0.29
IMS
-
0.28
0.58
-
2. Temperature dependence of quadrupole coupling
constants
f
A 14N nucleus is known to have a positive
quadrupole moment 19 and the principal field
gradient, qzz takes negative values for the compounds
discussed owing to concentration of lone-pair
electrons in the z-axis. Consequently, the principal
values of a quadrupole coupling tensor in energy
units can be expressed by,
1
— eQq„ = eQq, eQq^ = — eQq(l + ??),
(4)
eQq** = — eQq(l—??).
Figs. 2 and 3 show the temperature dependence of
eQq/h, eQq^/h, and eQq^/h in both complexes. All
of these values decrease almost linearly with
increasing temperature in a higher temperature
region than 77 K. By applying the B A Y E R theory 20
to the present systems, the principal values of the
EFG are given by the following equations 14 - 21 ,
where the amplitudes of libration about the x-, y-,
and z-axes are denoted by 6X, 6y, and 6Z, respectively.
q2z = qo[l —(3-h>7o)<^ 2 >/2—(3—^o)<V>/2],
<\vv=—qo[(l
(5a)
+ i?o)/2— (3 + i?o)<0,a>/2—i?o<flf2>],
(5 b)
1487
T. Asaji et al. • 14N NQR in Metal Complexes
3210
'•..
(kHz)
(kHz)
K2Pt(SCN)6
3190 -
300
200
100
(kHz)
K2Hg(SCN)4
eQq/h
2950
2890
(eQq/h)a
(eQq/h)b*'»..#
2900
2230 eQq„/h
2220
2870
1780
(e Q q y y / h )a
(
990*
1710
1750
(eQq„/h)b
1700
970 -
\
eQqx»/h
- 1200
1190
(eQqxx/h>a
(eQqu/h)b
950 -
100
200
T/tt
Fig. 2. Temperature dependence of eQq.tj/h, eQq^/h,
and eQq/h of 14 N in K 2 Pt(SCN) 6 .
1170
100
q * * = — qo[(l — ^ o ) / 2 — ( 3 — 7 ? o ) < 0 2 / 2 > / 2 +
k T
l
fi/Ico,
200
300
^o<^ 2 >1.
(5 c)
Here, |eQqo/h| and ?]o are the quadrupole coupling
constant and the asymmetry parameter, respectively, in a fictitious lattice performing no vibration.
The mean square amplitude, (0 2 ) of harmonic
oscillations with a frequency co can be calculated
under a high temperature approximation by,
<02> = ( j +
1150
300
(6)
where I is the moment of inertia.
For the calculation of the temperature variation
of the EFG, both of the structural and the vibrational data of crystals are indispensable. Unfortunately, no accurate structural analysis has been
performed for the platinum and mercury complexes
as yet, and moreover, no vibrational data can be
available for the former. Therefore, we have
estimated contributions only from librational
motion of thiocyanate groups for the latter complex.
Contributions from the other vibrational modes,
except libration of a complex ion as a whole, usually
give small vibrational amplitudes and can be
ignored.
Fig. 3. Temperature dependence of eQq^.r/h, eQq?/y/h,
and eQq/h of 14 N in K 2 Hg(SCN) 4 .
T R A M E R 2 2 has carried out the experiments of
Raman scattering on a single crystal of K2Hg(SCN)4
and has assigned the bands at 78 and 138 cm - 1 to
bending modes of Hg-SCN perpendicular and
parallel, respectively, to the Hg-SCN plane. These
modes can be approximated as libration of SCN
groups. We have evaluated |eQqo/h| and r/o as
3.010 MHz and 0.194 for nitrogen in the site a, and
as 2.910 MHz and 0.177 for that in the site b by
extrapolating observed values in Fig. 3 to 0 K. The
moment of inertia of an SCN group is estimated to
be 2.44 X IO-38 g • cm 2 from the bond distances
reported for KSCN 17 . One can calculate the temperature coefficients of eQq^/h, eQqyy/h, and eQq/h
from Equations (5) and (6) with the values estimated.
The results are given in Table IV.
The agreement between the observed and calculated temperature coefficients is extremely well for
nitrogen at the site b, indicating that the temperature variation can be interpreted mostly in terms
of the bending motion of Hg-SCN rather than the
1488
T. Asaji et al. •
14 N
N Q R in Metal Complexes
Table IV. Observed and calculated temperature coefficients of eQq^/h, eQqyy/h, and eQq/h in KoHg(SCN)4 at
high temperatures.
Site
ld(eQq^/h)/dTl/Hz K - 1
Obsd.
Calcd.
ld(eQqra/h)/dTl/Hz K " 1
Obsd.
Calcd.
ld(eQq/h)/dTI/Hz K " 1
Obsd.
Calcd.
a
b
215
100
155
40
370
140
111
108
40
39
libration of complex ions. On the other hand, each
of the calculated values for the site a is less than
half of the corresponding observed one. This
suggests that the libration of complex ions contributes considerably to averaging of the EFG of
nitrogen at the site a.
The temperature variation curves of eQq^/h and
eQq/h of the platinum complex and those of eQq^/h
and eQq/h for the site b of the mercury complex
are slightly convex to the abscissa in a high temperature region. This is anomalous because the
observed coupling constant versus temperature
curve normally has downward curvature owing to
1
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IKEDA,
D.
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We are grateful to Prof. A L A R I C H W E I S S for
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authors, D. N., thanks the Alexander-von-HumboldtStiftung for a research fellowship.
13
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15
16
[1901].
5
7
the anharmonicity of thermal vibrations in addition to the B A Y E R term 21 . The anomaly might be
interpreted in terms of a partial scission of S = C
jr-bonds taking place at high temperatures due to
the excitation of thermal motion. In other words,
contributions from the resonance structures I and/or
III slightly increase with increasing temperature,
whereas those from II and/or IV decrease in compensation. This is due to a vibronic interaction.
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146
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21
22
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