Chemical effects on the fine structure of L111 absorption edge of rhenium
B. D. Padalia, S. N. Gupta, and V. Krishnan
Citation: J. Chem. Phys. 58, 2084 (1973); doi: 10.1063/1.1679474
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THE JOURNAL OF CHEMICAL PHYSICS
VOLUME 58. NUMBER 5
1 MARCH 1973
Chemical effects on the fine structure of Ll11 absorption edge of rhenium
B. D. Padalia
Depa,.tment of Physics, Indian Institute oj Technology, Bombay-76, India
S. N. Gupta
School of Studies in Physics, Vikram University, Ujjain, India
V. Krishnan
Department of Physics, Indian Institute of Technology, Bombay-76, India
(Received 4 August 1971)
Fine structure of the LllI absorption edge of rhenium in its freshly prepared five complexes has been
investigated with a 1 m curved mica crystal spectrograph. A prominent absorption maximum (white line)
has been detected at the Lm edge. Chemical effects on the fine structure are restricted in the energy region
lying within nearly 100 eV of the main edge. It appears that the EFS is independent of the symmetry of
the absorbing crystal. The shape of L1I1 edge suggests octahedral coordination for rhenium in the complexes
under investigation. Bond lengths are determined employing x-ray fine structure methods. It is found
that the present results favor Lytle's method for the metal and that of Levy's for complexes.
I. INTRODUCTION
Chemical effects on the x-ray K-absorption spectra of
the first- and second-transition group elements have
been discussed by several workers.l-8 In the present
paper, we report the fine structure of the Ll11 absorption
edge of rhenium in its freshly prepared9-12 five complexes.
II. EXPERIMENTAL
The experimental technique was the same as that
described elsewhere. I3-15 The (201) planes of mica gave
a dispersion of about 5 x.u. mm-I . Absorption screens
were prepared from rhenium complexes following the
method discussed by Sandstrom.3 Exposure time was
varied from 20-30 h on Ilford x-ray films. Measurements
were made on the microphotometer tracings with
X 100 magnification.
plexes.5 Chemical effects on the fine structure are restricted in the energy region lying within nearly 100 eV
of the main absorption edge.
A. Effect of Near Neighbors
Of the five complexes selected for the present study,
four have common oxohalide anion, [ReOCI6j2.
This means that the nearest neighbors of rhenium are
maintained the same. If the nearest neighbors were
TABLE I. Chemical shift of the Lm absorption edge of rhenium.
Llll absorption edge
Absorbers
III. RESULTS
Microphotometer tracings of the fine structure of
Ll11 absorption edge of rhenium metal and its complexes
with X25 magnification are shown in Figs. 1 and 2.
The shifts in the Ll11 edge position of the complexes
measured with respect to rhenium metal are given in
Table I. There exists a prominent absorption maximum
usually called a white line14 .16 at the L1l1 absorption
edge. Following the white line, there are distinct fine
structure maxima and minima. The energies of these
maxima and minima measured from the corresponding
L111 edges of the complexes are presented in Table II.
The bond lengths determined employing x-ray fine
structure methods together with available crystallographic data12 .17 are collected in Table III.
IV. DISCUSSION
An inspection of Figs. 1 and 2 reveals the presence of
distinct peaks a' and {3' in the fine structure of the complexes. The appearance of such peaks is regarded to be a
characteristic feature of halide and oxohalide com-
Wavelength
(xu)
±0.05 xu
Energy
(eV)
..:la (eV)
Rhenium metal
1174.8
10 531.4
0
K2 [ReCI 6 ]
1174.7
10 532.1
+0.7
Cs, [ReOC4]
1174.3
10 535.9
+4.5
Rb 2 [ReOCI6]
1174.2
10 536.8
+5.4
Phen H2 [ReOC4]b
1174.2
10 536.6
+5.2
(a-Pic Hh [ReOC4]e
1174.7
10 531. 5
+0.1
• ..:l = E.ornpl•• - Emotal.
1,10 Phenanthrolin.
e a-Picoline.
h
entirely responsible for producing changes in the fine
structure, the absorption spectra of the complexes
would be identical. But the observed fine structure is
not exactly the same. The amplitudes and widths of the
characteristic peaks a' and {3' vary from complex to
complex. Further, it is noticed that light carbon and
nitrogen atoms present in the second coordination
sphere are more efficient for causing high amplitudes
and producing additional peaks in the fine structure
compared to heavy second neighbors, cesium and
rubidium. The present study, therefore, suggests that
2084
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FINE STRUCTURE OF
Llll
2085
OF RHENIUM
TABLE II. Position of the fine structure maxima and minima of Llll absorption edge of rhenium metal and its complexes.
Fine structure features& (eV)
Absorbers
Re metal
K 2[ReC16]
CS2[ReOClo]
R~[ReOC15]
Phen H 2[ReOCls]c
(a-Pic Hh[ReOClo]d
WL
a
B
a'
B'
6.7
6.9
4.5
5.1
5.6
6.6
48.1
45.8
31.9
31.8
24.0
28.1
68.9
60.3
39.1
36.3
40.7
35.9
72.6
52.7
44.7
48.6
50.3
77.7
63.1
58.7
57.6
59.2
{j
C
{j'
C'
92.3 131.6
92.7 100.5
83.3 113.7
79.0 101.6
88.1 99.4 117.5 130.1
73.7 99.4 103.9 115.1
'Y
153.2
152.4
145.4
154.0
148.3
139.9
D
D'
172.2 194.1
187.6
177.3
188.5 232.5
186.2
171.6
0
E
228.7
251. 8
230.2
248.8
226.7
251.8
258.7
293.5
261.6
286.5
&Measured with respect to the position of the corresponding Llll absorption edge.
b WL=white line.
01,10 Phenanthrolin.
d a-Picoline.
an acceptable fine structure theory must also include
in its mathematical formulation, the effects due to
second neighbors of the absorbing atom.
B. Effect of Crystal Structure
K{ReCl e] and C52[ReOCIs] have cubic close-packed
structures.12 .17 It is found ll that Rlh[ReOCls] crystallizes in tetragonal structure, while the remaining
oxohalide complexes have low-symmetry crystal lattices.
The general features of the extended fine structure
(EFS) in the energy region lying beyond nearly 100 eV
of the Llll absorption edge, are the same for all the
complexes under investigation (Figs. 1 and 2). It
appears that the EFS does not depend on the symmetry
of the crystal lattice.
The energy values of the EFS collected in Table II
have been examined in the light of Kronig relation.U8 •19
According to Kronig, a pair of cubic crystals whose
structures differ only in the value of lattice parameters
al and fl2 should satisfy the relation: EllE,.= (fl21 al) 2,
where El and E,. are the energies of the corresponding
maxima or minima in EFS. The complexes K 2[ReCI6]
and C52[ReOCIs] have fcc lattices with lattice con-
stants 9.86 and 10.25 A, respectively.12.17 The values of
EFS maxima or minima are higher for the former as
compared to the latter. A simple calculation shows that
the given relation holds good for this pair of cubic
crystals in the EFS region, developing confidence in the
proposed relation. Since the lattice types of the remaining complexes are not cubic, the Kronig relation
cannot be applied to such cases.
C. Determination of Coordination Type
An extensive study of the shapes of the principal
absorption edges of a large number of compounds of
various types was made by Van Nordstrand4 who
classified the spectral features into four categories
based mainly on coordination type. Several investigators20--23 have supported Van Nordstrand's classification. A comparison of the shapes of the Llll absorption
edges shown in Figs. 1 and 2 with those reported by
Van Nordstrand, suggests octahedral coordination for
rhenium in the complexes under investigation.
D. Determination of Bond Lengths
We have employed Lytle's method24 •25 for determining
bond lengths. The energy values, E of the EFS maxima
TABLE III. Bond lengths (A) calculated from x-ray fine structure methods for rhenium metal and its complexes.
Absorbers
Re metal
K 2[ReCla]
CS2[ReOClo]
R~[ReOClo]
Phen H 2 [ReOCl.]&
(a-Pic H)2[ReOCls]b
Lytle's
method24
Levy's
method26
2.7
2.6
2.8
2.8
3.0
3.0
2.54
2.15
1.85
1.88
1.85
2.00
Crystallographic
data12 •17
2.74
2.37
Bonds
Re-Re
Re-Cl
Re-O
Re-O
Re-O
Re-Q
1,1O-Phenanthrolin.
b a-Picoline.
a
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2086
PAD ALIA,
GUPTA,
AND KRISHNAN
T.(A)
= (37.6/M)1/ 2• The values of T. calculated by the
given relation are collected in the second column of
Table III.
Bond lengths are also calculated using Levy's
relation26 : T.(A) = (151/ ilE)1/2. The values of the energy
separation, IlE (eV) between second prominent maximum and minimum of the fine structure are taken from
Table II. The mean radius of the first coordination
sphere, T.(A.) around the rhenium metal atom computed by the Levy's relation are presented in the third
column of Table III.
The Re-Re and Re-CI bond lengths calculated by
both the x-ray fine structure methods are compared
with crystallographic data.12 Since crystal structure
data for the oxohalide complexes in question are not
200
WL
o
100
200
300
eV_
FIG. 1. Fine structure of the Lm absorption edge of rhenium
metal and its complexes, K2 [ReCls], CS2[ReOCi:;] and Rb 2
[ReOCI5] represented by the symbols ReI, Re2, and Rea, respectively.
150
t
~
>
.!!,
UJ
100
given in Table II are plotted against Q, the zero root of
the half-order Bessel functions corresponding to 1= 2.
Straight lines obtained for rhenium metal and its
complexes are shown in Fig. 3. The slope, M of the
straight line contains unit sphere radius, T. given by
16
32
48
64
Q-
FIG. 3. Plot of E against Q for rhenium metal and its
five complexes.
>-
....
iii
z
....UJ
Z
WL
o
100
200
300
eV-
FIG. 2. Fine structure of the Lm absorption edge of rhenium
metal and its complexes, Phen H2 [ReOCi:;) and (a-Pic Hh
[ReOCI5] represented by the symbols Re4 and ReG, respectively.
available, the accuracy of the calculated Re-O bond
lengths cannot be estimated. However, the reports12
on crystal data on several other oxohalide complexes
of rhenium indicate that the Re-O bond length is
shorter than that of Re-Cl and varies from 1.6-2 AIt is interesting to note that the values calculated by
Levy's relation lie within this range. A scrutiny of the
data presented in Table III, suggest the use of Lytle's
method for the metal and that of Levy's for complexes.
Finally, it may be mentioned that there are several
crystal chemistry parameters which influence the x-ray
absorption spectra.27 It is rather difficult to isolate the
effect of individual factor on the spectral features.
However, a careful analysis of the experimental results
can provide definite information about the desired
parameter influencing the spectra under investigation.
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FINE STRUCTURE OF
ACKNOWLEDGMENTS
The authors express their sincere thanks to Dr. D. K.
Chakravarty and Mr. R. D. Swarankar for supplying
rhenium complexes and for many fruitful discussion.
One of us (S.N.G.) is thankful to Dr. V. S. Dubey for
his constant interest in the present work. Thanks are
also due to the Council of Scientific and Industrial
Research, New Delhi, India, for financial assistance.
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