Cation distribution in MgxMn1-x(HCOO)2·2H2O mixed crystals. X-ray diffraction and double matrix infrared spectroscopy

Article abstract of DOI:10.1016/j.molstruc.2006.12.043

The isomorphic title series is investigated using single crystal X-ray diffraction and double matrix infrared spectroscopic techniques. The metal ion distribution at the two different metal sites (hexaformate-coordinated Me1 sites and mixed-coordinated Me2 sites) is found to exhibit a slight preference of Mn²⁺ ions to Me1 sites and Mg²⁺ ions to Me2 ones. The spectral regions at 2400-2500 cm^-1 and 1300-1400 cm^-1, corresponding to ν_OD of matrix-isolated HDO molecules and to symmetric COO stretching (ν₂) and bending CH (ν₅) modes, respectively, are mostly sensitive to the environment of the metal ion. When Mg²⁺ and Mn²⁺ are included in the structures of isotopically dilute Mn(HCOO)₂·2H₂O and Mg(HCOO)₂·2H₂O new infrared bands appear, corresponding to ν_OD of HDO molecules linked to the incorporated ions, and implying that new Mg-OH₂···OCHO-Mn and Mn-OH₂···OCHO-Mg interactions build up in the mixed formates. These bands appear at small concentrations of included metal ions (about 13 mol% of magnesium ions and 8 mol% of manganese ions, x = 0.13 and x = 0.92, respectively). The ν₂ and ν₅ modes caused by the incorporated cations bonded to formate ions are detected at x = 0.92 and x = 0.23 (Mn²⁺ ions in Mg(HCOO)₂·2H₂O and Mg²⁺ ions in Mn(HCOO)₂·2H₂O, respectively). Thus, the analysis of the infrared spectra (positions and intensities of the infrared bands) confirms the single crystal X-ray measurements that the metal ions are localized at the two metal positions. A slight preference of the Mg²⁺ ions to Me2 sites is owing to the stronger affinity of these ions to water molecules.

Full text of DOI:10.1016/j.molstruc.2006.12.043

Journal of Molecular Structure 842 (2007) 67–74
www.elsevier.com/locate/molstruc
Cation distribution in MgxMn1¡x(HCOO)2·2H2O mixed crystals.
X-ray diVraction and double matrix infrared spectroscopy
D. Stoilova a,¤, R. Baggio b, M.T. Garland c, D. Marinova a
a Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, bl. 11, 1113 SoWa, Bulgaria
b Depto. de Física, Comisión Nacional de Energía Atómica, Av. Gral Paz 1499, 1650 San Martín, Pcia. de Buenos Aires, Argentina
c Depto. de Fisica, Fac.de Ciencias Físicas y Matemáticas and CIMAT, U.de Chile, Av. Blanco Encalada 2008, Santiago de Chile, Chile
Received 4 December 2006; accepted 13 December 2006
Available online 14 January 2007
Abstract
The isomorphic title series is investigated using single crystal X-ray diVraction and double matrix infrared spectroscopic techniques.
The metal ion distribution at the two diVerent metal sites (hexaformate-coordinated Me1 sites and mixed-coordinated Me2 sites) is found
to exhibit a slight preference of Mn2+ ions to Me1 sites and Mg2+ ions to Me2 ones.
The spectral regions at 2400–2500 cm¡1 and 1300–1400 cm¡1, corresponding to ꢀOD of matrix-isolated HDO molecules and to symmet-
ric COO stretching (ꢀ2) and bending CH (ꢀ5) modes, respectively, are mostly sensitive to the environment of the metal ion. When Mg2+
and Mn2+ are included in the structures of isotopically dilute Mn(HCOO)2·2H2O and Mg(HCOO)2·2H2O new infrared bands appear,
corresponding to ꢀOD of HDO molecules linked to the incorporated ions, and implying that new Mg¡OH2···OCHO¡Mn and
Mn¡OH2···OCHO¡Mg interactions build up in the mixed formates. These bands appear at small concentrations of included metal ions
(about 13 mol% of magnesium ions and 8 mol% of manganese ions, x D 0.13 and x D 0.92, respectively). The ꢀ2 and ꢀ5 modes caused by
the incorporated cations bonded to formate ions are detected at x D 0.92 and x D 0.23 (Mn2+ ions in Mg(HCOO)2·2H2O and Mg2+ ions in
Mn(HCOO)2·2H2O, respectively). Thus, the analysis of the infrared spectra (positions and intensities of the infrared bands) conWrms the
single crystal X-ray measurements that the metal ions are localized at the two metal positions. A slight preference of the Mg2+ ions to Me2
sites is owing to the stronger aYnity of these ions to water molecules.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Magnesium manganese formate dihydrates; Cation distribution; Single crystal X-ray diVraction; Double matrix infrared spectroscopy
1. Introduction
Thus, Me1 has a sixfold coordination through six
oxygen atoms from formate ions (hexaformate-coordi-
nated Me1 site), while Me2 is surrounded by four oxygen
atoms of water molecules and two oxygen atoms belong-
ing to the formate groups (mixed-coordinated Me2 site)
[1–9].
The cation distribution in mixed formates is widely dis-
cussed in the literature [10–18]. For example, the Cu2+
ions are reported to display a clear preference to the Me1
site in the Cu0.5Zn0.5(HCOO)2·2H2O crystal structure [10].
57Fe Mössbauer investigations on the Fe/Me formate
dihydrates (Me D Mn, Co, Ni, Cu, and Zn) show that Fe2+
is equally distributed between the two sites in the Fe–Co,
Fe–Ni, and Fe–Mn mixed crystals, while in the case of Cu-
and Zn-rich mixed formates the highly diluted Fe2+ ions
The diVerent coordination environment of the metal
ions in the structures of the isomorphic series
Me(HCOO)2·2H2O (space group P21/c) (Me: Mg, Mn, Fe,
Co, Ni, Cu, Zn, and Cd), suggests the possibility that
when mixed crystals are formed among the above for-
mates, cations depending on their nature might occupy
preferentially one of the two available positions. The Me
metal ions occupy two sets of non-equivalent centers of
symmetry, which for simplicity we shall refer to as Me1
and Me2, the same as the metal ions occupying them.
*
Corresponding author.
E-mail address: stoilva@svr.igic.bas.bg (D. Stoilova).
0022-2860/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.12.043

68
D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
occupy preferentially the Me2 sites [11]. Single crystal X-
ray diVraction studies of mixed formates CuxMe1¡x
(HCOO)2·2H2O (Me D Mn, x D 0.47; Co, x D 0.35; Ni,
x D 0.37, and Cd, x D 0.47) reveal a clear preference of the
Cu2+ ions to Me1 sites, whereas the Me2+ ions are mainly
localized at the Me2 sites [12].
MgxMn1¡x (HCOO)2·2H2O mixed crystals are reported
and discussed.
2. Experimental
The metal formates, Mg(HCOO)2·2H2O and
Mn(HCOO)2·2H2O, were prepared by neutralization of
dilute formic acid solutions with the corresponding metal
hydroxide carbonate and oxide at 70–80 °C, respectively.
Then the solutions were concentrated and cooled to room
temperature. The mixed magnesium manganese formates
were obtained according to the solubility diagram of the
Mn(HCOO)2¡Mg(HCOO)2¡H2O system at 25 °C [19].
The bulk compositions of the mixed formates were deter-
mined as follows: the sum of the magnesium and manga-
nese concentrations were determined complexometrically
with complexon III at pH 9–9.5 using eriochrome black T
as indicator. Mn2+ ions in the presence of Mg2+ were
determined in the same manner after binding Mg2+ ions
with ammonium Xuoride. The magnesium formate con-
centrations were calculated by diVerence. Samples of
mixed crystals containing matrix-isolated HDO molecules
(isotopically dilute samples, ca. 8–10% D2O) were pre-
pared by the same crystallization procedure in water
partly deuterated. Single crystals were obtained by slow
crystallization from mixed solutions. The reagents used
were “p.a.” quality (Merck).
Single crystal X-ray diVraction data were gathered on a
Bruker AXS SMART APEX CCD diVractometer (radia-
tion: Mo Kꢁ,ꢂ: 0.71069 Å; driving software: SMART [20];
data integration: SAINT [20]; absorption corrections:
SADABS [21]). The main structure was taken from a pre-
vious work on the subject [18], and reWned by least
squares on F2 with anisotropic displacement parameters
for non-H atoms. Hydrogen atoms attached to carbon
were placed at their calculated positions (C–H: 0.93 Å)
and allowed to ride, while those attached to oxygen were
found in the Fourier maps and reWned with no constraints
and free isotropic displacement factors. All relevant calcu-
lations were performed with the computer programs
SHELXS97/SHELXL97 [22] and SHELXTL [23]. Full
use of the CCDC package was also made for searching in
the CSD Database [24]. Crystallographic data (excluding
structure factors) have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication No. CCDC 628699-6286711. Copies of the
data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk).
The absorbance infrared spectra were recorded on the
Bruker model IFS 25 Fourier transform interferometer
(resolution <2 cm¡1) at ambient temperature using KBr
discs as matrices. Ion exchange or other reactions with
KBr have not been observed (infrared spectra using Nujol
mulls were also measured). The Lorentz Wtting procedure
was used in order to determine the band frequencies and
intensities.
Recently, a variant of double matrix infrared spectros-
copy has been proposed and applied in order to deduce
the cation distribution in copper containing mixed for-
mates CuxMe1¡x(HCOO)2·2H2O (Me D Mg, Mn, Co, and
Ni) (Cu2+ ions were included in host metal formates in
wide concentration ranges (x 6 0.5) additionally to
matrix-isolated HDO molecules) [13–15,17]. The analysis
of the infrared spectra shows that the regions of the O¡H
vibrations are mostly sensitive to the coordination envi-
ronment of the metal ions present (regions of
3200¡3500 cm¡1 (ꢀOH) and 2300–2500 cm¡1 (ꢀOD of
matrix-isolated HDO molecules)) since the water mole-
cules in the isomorphic formate series exhibit diVerent
hydrogen bond strengths due to both the diVerent
metal¡water interactions (i.e., diVerent synergetic eVect)
and the diVerent unit-cell volumes of the formates (i.e.,
repulsion potential of the lattices) [14,15,17,18]. The inclu-
sion of guest ions in host formate matrices will result in
the appearance of new infrared bands for the ꢀOD
modes in the mixed crystals additionally to those due
to the host compounds – Me(g)¡OH2···OCHO¡Me(h),
Me(h)¡OH2···OCHO¡Me(g),
and
Me(g)¡OH2···
OCHO¡Me(g) (g – guest ions; h – host ions; the metal
guest ions are assumed to occupy the two available
positions). The frequencies of the uncoupled ꢀOD modes
for the above hydrogen bond systems depend strongly
on the synergetic eVect of both the guest and the host cat-
ions. If we assume that all Me1 sites are occupied by one
of the ions only, then the Me0.5Me0.5(HCOO)2·2H2O
mixed formate might be considered as a double salt, and
consequently only four bands corresponding to the ꢀOD
modes are expected to appear in its spectrum. The appear-
ance of more than four infrared bands is an indication
that each metal ion is located at the two available
positions.
The water molecules in Mg(HCOO)2·2H2O form the
weakest hydrogen bonds within the isostructural series,
irrespective of the short Mg¡OH2 bond lengths owing to
the ionic character of the respective Mg¡OH2 bonds
[14,17,18]. On the other hand, Mg2+ ions are known to
exhibit a strong aYnity to water molecules. This property
of the Mg2+ ions allows us to assume that when Mg2+ ions
are included in the structure of metal formate dihydrates
they will occupy preferentially the Me2 sites. Our recent
investigations aim at conWrming this assumption. For
example, it has been established in our previous paper [18]
that the Zn2+ ions display a clear preference to Me1 sites,
whereas the Mg2+ ions are localized predominantly at
Me2 ones in MgxZn1¡x(HCOO)2·2H2O mixed crystals.
In the present paper our single crystal X-ray diVraction
and double matrix infrared spectroscopic studies on

D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
69
3. Results and discussion
0.50
0.45
Mg1
3.1. Single crystal X-ray diVraction studies
Mg2
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
The isomorphic series crystallizes in P21/c, and pre-
sents two non-equivalent cationic sites Me1 and Me2 on
the two independent symmetry centers, where the Mg2+
and Mn2+ ions distribute in a mixed fashion, with
well deWned partial occupancies for each bulk composi-
tion which behaved as robust parameters in the reWne-
ment process, with almost no correlation with other
structural parameters. They were reWned subject to the
restraints that in each site both occupations should add
up to 0.5 (the ¡1 site occupation) and share the same
anisotropic displacement factor. These conditions
allowed a smooth convergence of the reWnement proce-
dure in general, and of the occupation factors in particu-
lar (<S.U.> <: 0.01), in accordance to what observed in
the recently reported MgZn formate system [18] but con-
trasting with the problematic reWnement in some other
related structures [12]. The procedure provided, as a
bonus, values for the overall composition, which invari-
ably ended up quite near the bulk value but which
allowed for local Xuctuations impossible to detect other-
wise. The internal distribution of cations, as obtained
from X-ray reWnement is presented in Fig. 1, where for
both species (Mn2+ and Mg2+) the smooth behavior of
each one of the local concentrations (Me1 and Me2) is
plotted as a function of the total concentration. A pref-
erence of Mn2+ ions to site Me1 and Mg2+ ions to
site Me2 is apparent. Other than these variations all
the remaining structural parameters appeared basically
conserved as assessed by the unit-cell parameters in
Table 1 and the Me1 and Me2 coordination distances
shown in Table 2, all of them distributed in a quite nar-
row span.
0.00
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Mn1
Mn2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Total concentration of metal ions
Fig. 1. Fractional cationic concentration vs. total concentration of Me2+
ions (mean standard deviations 0.01).
Table 1
Unit-cell parameters of MgxMn1¡x(HCOO)2·2H2O mixed crystals
Both Me1 and Me2 sites present nearly octahedral
environments, the former being coordinated only to for-
mate oxygens: O1 and O2 from F1, and O3 from F2. The
latter, in turn coordinates to O4 from formate F2, and
the two water molecules O1W and O2W. Both formate
molecules act as bridges, F1 linking only Me1 sites while
F2 mixes both types. This results in F1-linked sheets of
Me1 polyhedra parallel to (0 1 0), at heights y » 0.5, inter-
connected via Me2 polyhedra through the bridging for-
mate F2 (see Fig. 2). Superimposed to this 3D-
coordinated structure there is a complex hydrogen bond-
ing network, with the four hydrogens from the water mol-
ecules taking active part in it. The ones corresponding to
O1W join Me1 polyhedra with each other, while those
bound to O2W link both Me1 and Me2 types. Inspection
of Table 3, which gives the Ow···O bond distances as cal-
culated from the X-ray diVraction data, suggests a trend
in the four hydrogen bond lengths: they appear to exhibit
a slight shortening on going from Mg-rich to Mn-rich
formates, owing to the stronger synergetic eVect of the
Mn2+ ions.
x
a (Å)
b (Å)
c (Å)
ꢀ (°)
0.12
0.16
0.18
0.20
0.23
0.34
0.35
0.44
0.52
0.62
0.66
0.73
0.84
8.8122(14)
8.8045(13)
8.8020(16)
8.7961(12)
8.7907(12)
8.7695(15)
8.7714(12)
8.7520(13)
8.7356(17)
8.7120(2)
7.2658(12)
7.2647(11)
7.2632(13)
7.2585(10)
7.2538(10)
7.2405(12)
7.2451(10)
7.2318(11)
7.2201(14)
7.2064(17)
7.2038(13)
7.1907(12)
7.1713(13)
9.5841(15)
9.5759(14)
9.5757(17)
9.5648(13)
9.5588(13)
9.5357(16)
9.5359(13)
9.5205(14)
9.5023(18)
9.4740(2)
97.704(3)
97.721(2)
97.776(3)
97.741(2)
97.746(2)
97.805(3)
97.805(2)
97.846(2)
97.868(3)
97.892(4)
97.897(3)
97.932(3)
97.953(3)
8.7095(15)
8.6932(14)
8.6638(13)
9.4702(16)
9.4510(16)
9.4190(15)
3.2. Infrared spectroscopic studies
According to the structural data the two crystallograph-
ically diVerent water molecules (W1 and W2, each in C1 site
symmetry) in the structures of the metal formate dihydrates
are asymmetrically hydrogen bonded and consequently
four bands corresponding to four ꢀOD modes are expected

70
D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
Table 2
Bond lengths (in Å) in MgxMn1¡x(HCOO)2·2H2O mixed crystals
x
Me1-O1i/O1ii
Me1-O2/O2iii
Me1-O3/O3iii
Me2-O2W/O2Wiv
Me2-O1W/O1Wiv
Me2-O4/O4iv
0.12
0.16
0.18
0.20
0.23
0.34
0.35
0.44
0.52
0.62
0.66
0.73
0.84
2.1489(12)
2.1472(10)
2.1451(15)
2.1459(13)
2.1426(11)
2.1321(12)
2.1327(11)
2.1230(12)
2.1132(14)
2.1048(16)
2.1012(13)
2.0895(12)
2.0965(11)
2.1707(12)
2.1680(11)
2.1652(15)
2.1638(13)
2.1618(11)
2.1512(12)
2.1513(11)
2.1425(11)
2.1346(14)
2.1217(15)
2.1183(13)
2.1086(11)
2.0965(11)
2.1994(12)
2.1983(10)
2.1958(15)
2.1909(13)
2.1910(11)
2.1807(12)
2.1813(11)
2.1722(11)
2.1656(14)
2.1545(15)
2.1537(13)
2.1432(11)
2.1293(11)
2.1432(15)
2.1360(13)
2.1317(19)
2.1328(15)
2.1283(14)
2.1134(15)
2.1117(13)
2.1034(14)
2.0959(17)
2.0792(19)
2.0785(16)
2.0723(14)
2.0622(13)
2.1861(15)
2.1789(13)
2.1733(18)
2.1710(15)
2.1666(14)
2.1518(15)
2.1499(13)
2.1388(14)
2.1300(16)
2.1182(18)
2.1139(16)
2.1056(14)
2.0928(13)
2.2068(13)
2.2021(11)
2.2003(16)
2.1980(14)
2.1929(12)
2.1787(13)
2.1800(12)
2.1693(13)
2.1609(15)
2.1511(17)
2.1488(14)
2.1404(12)
2.1301(12)
Symmetry codes: (i) x, ¡y + 1/2, z ¡ 1/2; (ii) ¡x + 1, y¡1/2, ¡z + 3/2; (iii) ¡x + 1, ¡y, ¡z + 1; (iv) ¡x, ¡y + 1, ¡z + 1.
assigned to hydrogen bonds formed by W2 and W1, respec-
tively [2]. The shorter Mg¡OH2 bond lengths are expected
to favor the formation of stronger hydrogen bonds, while
the smaller unit-cell volume of the magnesium compound,
i.e., the larger repulsion potential of the magnesium formate
lattice is expected to favor the formation of weaker hydro-
gen bonds in the magnesium compound as compared
to those formed in the respective manganese compound
(the unit-cell volumes of Mg(HCOO)2·2H2O and
Mn(HCOO)2·2H2O have values of 573.8 and 596.9Å3,
respectively). Indeed, as Fig. 3 shows the respective OD
modes in Mg(HCOO)2·2H2O are shifted to higher frequen-
cies than those in Mn(HCOO)2·2H2O, thus indicating the
formation of comparatively stronger hydrogen bonds in the
latter compound, irrespective of the close hydrogen bond
distances in both compounds. This Wnding is due to both
the increasing covalency of the Mn¡OH2 bonds as com-
pared to that of the Mg¡OH2 bonds and the larger unit-
cell volume of the manganese compound.
When Mg2+ ions are incorporated in the
Mn(HCOO)2·2H2O crystal structure at low concentration
(xD0.13) new bands at 2501, 2456, and 2430 cm¡1 appear
in the spectrum of the sample Mg0.13Mn0.87(HCOO)2·2H2O,
which are attributed to new hydrogen bonds of the type
MgOH2···OCHOMn (see Fig. 3a). The intensity of these
bands increases with the increasing in the concentrations of
the magnesium ions. The sample Mg0.5Mn0.5(HCOO)2
·2H2O exhibits seven bands at 2497, 2484, 2465, 2454, 2443,
2430, and 2415 cm¡1. The bands at 2497, 2465, 2454, and
2430 cm¡1 are due to hydrogen bonds formed by water
molecules coordinated to Mg2+ ions (Mg¡OH2···OCHO¡
Mn and Mg¡OH2···OCHO¡Mg hydrogen bond types)
and the bands at 2484, 2443, and 2415 cm¡1 to those formed
by water molecules linked to Mn2+ ions (Mn¡OH2···
OCHO¡Mn and Mn¡OH2··· OCHO¡Mg hydrogen bond
types) (see Fig. 5). Similarly, the inclusion of the manganese
ions in the structure of Mg(HCOO)2·2H2O at low concen-
tration (x D0.92, Fig. 3b) leads to the appearance of two
new bands at 2483 and 2444 owing to hydrogen bonds
formed by water molecules attached to Mn2+ ions (i.e.,
hydrogen bonds of the type Mn¡OH2···OCHO¡Mg).
Fig. 2. Schematic view of the structure showing both types of coordination
polyhedra (symmetry codes as in Table 2).
to appear in the spectra of isotopically dilute metal for-
mates. Infrared spectra of the neat formates as well as those
of mixed formates in the region of 2300¡2600 cm¡1 are
shown in Fig. 3a and b. In Fig. 5 the wavenumbers of
the infrared bands are plotted as a function of the mixed
crystal compositions. The assignments of ꢀOD in
Mg(HCOO)2·2H2O are made according to the hydrogen
bond distances Ow···O (X-ray diVraction study), while those
in Mn(HCOO)2·2H2O according to both the H···O and the
Ow···O bond lengths (neutron diVraction study). For
Mg(HCOO)2·2H2O – the bands at 2496 and 2465 cm¡1
(bond distances 2.814 and 2.794 Å) and the bands at 2453
and 2433 cm¡1 (bond distances 2.80 and 2.781 Å) are attrib-
uted to hydrogen bonds formed by W1 and W2, respec-
tively [1]. For Mn(HCOO)2·2H2O – the bands at 2486 and
2442cm¡1 (bond distances 2.80 and 2.75 Å) and the bands
at 2442 and 2408cm¡1 (bond distances 2.79 and 2.75 Å) are

D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
71
Table 3
Hydrogen bonding distances in MgxMn1¡x(HCOO)2·2H2O mixed crystals (Å)
x
Ow···O distances
O1W-H1WA···O3
O1W-H1WB···O1v
O2W-H2WA···O2v
O2W-H2WB···O4vi
0.12
0.16
0.18
0.20
0.23
0.34
0.35
0.44
0.52
0.62
0.66
0.73
0.84
2.765(2)
2.764(2)
2.766(3)
2.764(2)
2.762(2)
2.771(2)
2.770(2)
2.771(2)
2.770(2)
2.771(3)
2.776(2)
2.775(2)
2.777(2)
2.783(2)
2.786(2)
2.786(2)
2.785(2)
2.788(2)
2.780(2)
2.787(2)
2.785(2)
2.784(2)
2.787(3)
2.788(2)
2.789(2)
2.786(2)
2.790(2)
2.791(2)
2.791(3)
2.784(2)
2.787(2)
2.790(2)
2.795(2)
2.795(2)
2.792(2)
2.794(3)
2.797(2)
2.796(2)
2.796(2)
2.764(2)
2.767(2)
2.771(3)
2.766(2)
2.768(2)
2.772(2)
2.775(2)
2.776(2)
2.774(2)
2.780(3)
2.785(2)
2.782(2)
2.781(2)
Symmetry codes: (v) ¡x + 1, y + 1/2, ¡z + 3/2; (vi) ¡x, y + 1/2, ¡z + 3/2.
Thus, the analysis of the infrared spectra in the region of
2400¡2600cm¡1 evidences that the both ions are distrib-
uted over the two crystallographically diVerent sites. How-
ever, the comparison of the intensities of the bands in the
spectrum of the sample Mg0.5Mn0.5(HCOO)2·2H2O reveals
that the Mg2+ ions show a slight preference to the Me2 sites
in accordance with the crystal structure data.
tence of two crystallographically diVerent formate groups in
the formate structures. The bands at higher frequencies (1404,
1394, and 1382cm¡1 in the Mg(HCOO)2·2H2O spectrum, and
at 1395, 1389, 1385, and 1373 in the Mn(HCOO)2·2H2O spec-
trum, respectively) are attributed to the bending modes ꢀ5.
The appearance of three components for ꢀ5 in Mg(HCOO)2·
2H2O and four components for ꢀ5 in Mn(HCOO)2·2H2O is
due to the crystal Weld splitting (each HCOO mode can theo-
retically split into two components (Au +Bu)¡C1 site symme-
try of the formate ion under C2h factor group symmetry). As
was commented in Ref. [13] the stronger Cu¡OCHO bonds
in Cu(HCOO)2·2H2O weaken the intraionic C¡O bonds,
thus leading to red shifts of ꢀ2 in the copper formate as com-
pared to those in the other metal formates. Thus, the lower
frequencies of the bands corresponding to ꢀ2 in Mn(HCOO)2·
2H2O as compared to those in Mg(HCOO)2·2H2O could be
attributed to the stronger Me¡OCHO interactions in the
former compound.
At this stage it might be worth comparing the quality
and richness of the hydrogen bonding picture aVordable
from the two diVerent techniques, X-ray diVraction and
infrared spectroscopy, as a consequence of the diVerent
physical principles they relay on. Due to the intrinsic
nature of the diVraction process, X-ray analysis can only
be expected to ‘‘see’’ a multiply disordered hydrogen
bonding network on an average fashion, ‘‘collapsed’’ into
the four basic hydrogen bonds present in a (virtual)
ordered system with each substituted site occupied by an
illusory, atomic species of an average nature weighted by
the individual concentrations: the richness of local diVer-
ences is deWnitely lost in this averaging process. Infrared
spectroscopy, on the other hand, can probe and detect
these diVerences in detail, thus being able to ‘‘see’’ a much
larger number of hydrogen bond types, possible spread
over a wide wavenumbered region. Comparison of Table
3 and Fig. 3 exempliWes how infrared spectroscopy might
reXect in a much Wner degree the interactions arising
between diVerent entities in crystal lattices, in particular
hydrogen bonds.
The normal vibrations of the formate ions (two crystallo-
graphically diVerent formate ions, each in C1 site symmetry)
are likewise sensitive to the coordination environment of the
metal ions. Infrared spectra of the title mixed crystals in the
region of 1450–1300cm¡1 (ꢀ2(ꢀsCOO) and ꢀ5(ꢀꢃCH) modes,
(A1) and (B1) symmetry, respectively) are shown in Fig. 4a
and b (see also Fig. 5). According to Raman spectroscopic
studies the bands at 1375 and 1364cm¡1 (Mg(HCOO)2·2H2O
[14]), and at 1368 and 1357cm¡1 (Mn(HCOO)2·2H2O [13])
are assigned to symmetric stretching modes of the formate
groups. The appearance of two bands for ꢀ2 reXects the exis-
Fig. 4a shows that Mg2+ ions included in
Mn(HCOO)2·2H2O up to concentrations of 23 mol% do
not reXect on the spectra shapes and the number of the
bands (the samples with x D 0 and 0.13 display identical
spectra). The mixed formate containing 23 mol% of Mg2+
ions exhibits a new band at 1402 cm¡1. A new band at
1361 cm¡1 is seen and in the spectrum of the sample
Mg0.5Mn0.5(HCOO)2·2H2O (see Fig. 5b). The intensities of
these bands increase with the increasing in the magnesium
ion concentration. Consequently, these bands could be
attributed to ꢀ5 and ꢀ2 of formate ions bonded to Mg2+
ions. However, a comparatively smaller concentration of
Mn2+ ions incorporated in Mg(HCOO)2·2H2O (8 mol% of
Mn2+ ions, x D 0.92) causes the appearance of new bands
(1391 and 1358 cm¡1) owing to ꢀ5 and ꢀ2 of formate ions
bonded to the included Mn2+ ions, respectively (Figs. 4b
and 5b). The lower concentration of the included manga-
nese ions in the mixed crystals at which ꢀ5 and ꢀ2 of for-
mate ions bonded to them are detectable in the infrared
spectra as compared to that of the included magnesium
ions conWrms the results of the single crystal X-ray study

72
D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
2453
2442
x = 0
x = 1
2486
2496
2433
2408
2511*
2518*
2465
2442
x = 0. 92
2497
x = 0. 13
2454
2485
2501
2444
2433
2430
2409
2516*
2515*
2442
2430
2410
2454
2444
2433
x = 0. 23
x = 0. 80
2483
2485
2497
2500
2511*
2516*
x = 0. 70
x = 0. 37
2443
2430
2413
2483
2454
2444
2431
2484
2498
2513*
2497
2511*
x = 0. 61
x = 0. 50
2454
2443
2454
2443
2484
2497
2484
2497
2430
2415
2431
2508*
2511*
2600
2500
2400
2600
2500
2400
Wavenumbers, cm–1
Fig. 3. Infrared spectra of isotopically dilute MgxMn1¡x(HCOO)2·2H2O mixed crystals in the OD stretching mode region (¤, ꢀ2 + ꢀR).
that the Mn2+ ions exhibit a slight preference to the Me1
sites.
of the lattice, which cause small blue shifts, and (iii) inter-
molecular coupling of normal modes, which lead to both
red and blue shifts, etc. [25]. For example, the stronger
Mn¡O bond interactions will favor a weakening of the
C¡O bond interactions in the formate ions bonded to
Mn2+ ions in the mixed formates, thus leading to red shifts
of the respective ꢀ5 and ꢀ2 modes with a increasing in the
manganese concentration. The same trend is expected if
the unit-cell volumes of the both compounds are consid-
ered, i.e., the smaller unit-cell volume of the magnesium
compound will lead to blue shifts of the bands correspond-
The analysis of the Fig. 5b reveals that some bands cor-
responding to the normal vibrations ꢀ5 and ꢀ2 in the mixed
crystals shift to lower or higher frequencies as compared to
the respective modes in the neat compounds. The fre-
quency shifts of the normal modes to lower or higher fre-
quencies in solids depend on diVerent factors such as: (i)
weakening of the intramolecular (or intraionic) bonds
owing to intermolecular (or interionic) bonding features,
which result in red shifts of the bands, (ii) repulsive forces

D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
73
1389
1395
1373
1382
x = 0
1357
x = 1
1368
1375
1404
1364
1381
x = 0. 92
x = 0. 13
1368
1374
1374
1376
1394
1404
1390
1390
1391
1374
1363
1396
1357
1358
x = 0. 8
1379
x = 0. 23
1373
1363
1394
1403
1368
1396
1402
1357
1358
1378
x = 0. 7
x = 0. 37
1371
1362
1393
1369
1399
1403
1397
1357
1402
1358
1377
1377
x = 0. 61
x = 0. 5
1393
1370
1371
1392
1398
1402
1398
1402
1361
1362
1357
1357
1440
1400
1360
1320
1440
1400
1360
1320
Wavenumbers, cm–1
Fig. 4. Infrared spectra of MgxMn1¡x(HCOO)2·2H2O mixed crystals in the ꢀ2 and ꢀ5 regions of the formate ions.
ing to ꢀ5 and ꢀ2 with the increasing of the magnesium con-
4. Conclusions
centration in the mixed crystals. Indeed, the bands 1394,
1382, and 1375 cm¡1 shift to 1389, 1373, and 1368 cm¡1 on
going from the magnesium compound to the manganese
one. However, the bands corresponding to ꢀOD of the
water molecules in the mixed crystals do not display con-
siderable band shifts with the change in the metal ion con-
centrations (only the band at 2408 cm¡1 shifts slightly to
higher frequencies with the increasing in the magnesium
concentration –2415 cm¡1 in Mg0.5Mn0.5(HCOO)2·2H2O).
Single crystal X-ray diVraction measurements of the title
mixed formates evidence that the Mn2+ ions exhibit a slight
preference to Me1 sites and the Mg2+ ions to Me2 sites.
The analysis of the infrared spectra reveals: (i) the spectral
regions 2400–2500cm¡1 (ꢀOD of matrix-isolated HDO mole-
cules) and 1300¡1400cm¡1 (ꢀ2 and ꢀ5 of the formate ions)
are mostly sensitive to the metal ion environment. (ii) The
stronger synergetic eVect of Mn2+ ions (i.e., the stronger

74
D. Stoilova et al. / Journal of Molecular Structure 842 (2007) 67–74
1375
1382
1391
1394
1364
2465
2483
1404
2496
2453
2444
2433
1.0
0.8
0.6
0.4
0.2
0.0
1358
1399
2415
1361
2465
1402
2456
2430
2501
1395
1357
2408
2486
1368
2442
1385
1389
1373
2520
2480
2440
2400
2360
1400
1380
1360
1340
Wavenumbers, cm–1
Fig. 5. Infrared wavenumbers of ꢀOD ꢀ5 and ꢀ2 vs. the compositions of MgxMn1¡x(HCOO)2·2H2O mixed crystals (for band assignments see Figs. 3 and 4).
[6] K. Krogmann, R. Mattes, Z. Kristallogr. 118 (1963) 291.
[7] M. Bukowska-Strzyzewska, Acta Crystallogr. 19 (1965) 357.
[8] N. Burger, H. Fuess, Z. Kristallogr. 145 (1977) 346.
[9] M.L. Post, J. Trotter, Acta Crystallogr. B30 (1974) 1880.
Mn¡OH2 and Mn¡OCHO bond interactions due to the
increasing covalency of the respective Mn¡O bonds) weaken
the respective O¡H and C¡O bond interactions and as a
consequence the normal modes ꢀOD, ꢀ5 and ꢀ2 in the manga-
nese compound appear at lower frequencies as compared to
those in the magnesium compound. (iii) The small amounts
of cations included in the neat formates lead to an appear-
ance of new bands corresponding to the normal vibrations
ꢀOD, ꢀ2 and ꢀ5 characteristic for the incorporated ions, thus
indicating that the both cations are localized over the two
metal positions in the mixed formates. (v) The slight prefer-
ence of the Mg2+ ions to Me2 sites is owing to the strong
aYnity of these ions to water molecules.
[10] T. Ogata, T. Taga, K. Osaki, Bull. Chem. Soc. Jpn. 50 (1977) 1680.
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[14] D. Stoilova, V. Koleva, J. Mol. Struct. 553 (2000) 131.
[15] D. Stoilova, V. Koleva, Spectrochim. Acta A57 (2001) 2629.
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