Solvation structures of manganese(II), iron(II), cobalt(II), nickel(II), copper(II), zinc(II), and gallium(III) ions in methanol, ethanol, dimethyl sulfoxide, and trimethyl phosphate as studied by EXAFS and electronic spectroscopies

Article abstract of DOI:10.1021/jp983799y

Solvation structures of the Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Ga(III) ions in methanol (MeOH), ethanol (EtOH), dimethyl sulfoxide (DMSO), and trimethyl phosphate (TMP) have been determined using extended X-ray absorption fine structure (EXAFS) spectroscopy. In MeOH, EtOH, and DMSO, the solvation structures of all metal(II,III), ions are 6-coordinate octahedral as in water, and the M-O bond lengths are similar to those in water. In the bulky solvent TMP, the 5-coordinate solvation structure is observed for the Zn(II) ion without ligand-field stabilization. The Ga(III) ion has the 6-coordinate solvation structure in TMP despite its smaller ionic radius than the Zn(II) ion because of the higher charge density on the Ga(III) ion. In the cases of the Mn(II), Fe(II), Co(II), Ni(II), and Cu(II) ions, the electronic absorption spectra have been measured in MeOH, EtOH, and DMSO. All solutions for each metal(II) ion show a spectral pattern similar to that in water, which is consistent with the results of the EXAFS measurements.

Full text of DOI:10.1021/jp983799y

J. Phys. Chem. A 1999, 103, 1401-1406
1401
Solvation Structures of Manganese(II), Iron(II), Cobalt(II), Nickel(II), Copper(II), Zinc(II),
and Gallium(III) Ions in Methanol, Ethanol, Dimethyl Sulfoxide, and Trimethyl Phosphate
As Studied by EXAFS and Electronic Spectroscopies
Yasuhiro Inada,‡ Hiroo Hayashi,† Ken-ichi Sugimoto,† and Shigenobu Funahashi*,†
Laboratory of Analytical Chemistry, Faculty of Science, Nagoya UniVersity and Research Center for Materials
Science, Nagoya UniVersity, Chikusa, Nagoya 464-8602, Japan
ReceiVed: September 21, 1998; In Final Form: December 31, 1998
Solvation structures of the Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Ga(III) ions in methanol (MeOH),
ethanol (EtOH), dimethyl sulfoxide (DMSO), and trimethyl phosphate (TMP) have been determined using
extended X-ray absorption fine structure (EXAFS) spectroscopy. In MeOH, EtOH, and DMSO, the solvation
structures of all metal(II,III) ions are 6-coordinate octahedral as in water, and the M-O bond lengths are
similar to those in water. In the bulky solvent TMP, the 5-coordinate solvation structure is observed for the
Zn(II) ion without ligand-field stabilization. The Ga(III) ion has the 6-coordinate solvation structure in TMP
despite its smaller ionic radius than the Zn(II) ion because of the higher charge density on the Ga(III) ion. In
the cases of the Mn(II), Fe(II), Co(II), Ni(II), and Cu(II) ions, the electronic absorption spectra have been
measured in MeOH, EtOH, and DMSO. All solutions for each metal(II) ion show a spectral pattern similar
to that in water, which is consistent with the results of the EXAFS measurements.
Introduction
tures is observed for the Co(II) ion with d7 electronic config-
uration, in which the steric repulsion of bound PA molecules is
not expected.7 As also seen for the Co(II) (4-coordinate
pseudotetrahedral) and the Ni(II) (5-coordinate square pyrami-
dal) ions in TMU,3 the d electron configration of the metal(II)
ion sometimes plays a key role in the selection of the solvation
structure. Furthermore, as we have recently pointed out,11,12 the
M-N bond length of the solvated metal ion in nitrogen-donating
solvents becomes longer according to the following order:
nitriles with sp hybridizing nitrogen < pyridines with sp2
hybridizing nitrogen < aliphatic amines with sp3 hybridizing
nitrogen. This is because the increase in the p character of the
donating orbital of the solvent molecule causes expansion of
the electron-density distribution. In the case of the Cu(I) and
Ag(I) ions, the solvation structures in some nonaqueous solvents
are 4-coordinate tetrahedral.13-16 Thus, the electrostatic attraction
between the M(I) ion and the solvent dipole does not overcome
either the interligand steric repulsion or the entropic contribution
needed to form a structure with a coordination number greater
than 4, even if the solvent molecules are small in size.15,16 On
the other hand, the solvation structure of the In(III) ion in some
nonaqueous, bulky solvents such as TMU, TMP, triethyl
phosphate, and tributyl phosphate, is 6-coordinate octahedral
as in water because of its high charge.3,12 The solvation structure
is also affected by the charge density on the metal ion. As shown
in the findings previously described, the solvation number of
the metal ion is determined by keeping a balance among the
increasing contribution due to higher charge of the metal ion,
the decreasing contribution due to bulkiness of the solvent
molecule, and the ligand-field stabilization on the metal ion. In
this study, to shed further light on the solvation phenomena of
metal ions, we studied the solvation structures of the Mn(II),
Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Ga(III) ions in MeOH,
EtOH, DMSO, and TMP by measuring the EXAFS and
electronic absorption spectra. The structural characteristics of
these metal(II,III) ions in these nonaqueous solvents of different
A metal ion in solvents is solvated by some solvent molecules,
and knowledge of the solvation structure around the metal ion
is very important for quantitative interpretation of the equilib-
rium and kinetic properties of the chemical reactions concerning
the metal ion. The hydration structures of many mono-, di-,
and trivalent metal ions in water were extensively investigated
using X-ray and neutron diffraction methods and extended X-ray
absorption fine structure (EXAFS) spectroscopy, and the results
were thoroughly reviewed.1,2 For the first-row transition metal-
(II) ions later than Mn(II), it is accepted that the hydration
structure is 6-coordinate octahedral because there is the greatest
electronic stability in the octahedral complex. Recently, solvation
structures of the first-row transition metal(II) ions in some
nonaqueous solvents have been studied, and it has been pointed
out that the bulkiness of the solvent molecule reduces the
coordination number of the metal(II) ion. For example, the
coordination number of the Mn(II), Fe(II), and Ni(II) ions is 5
and that of the Co(II), Cu(II), and Zn(II) ions is 4 in the bulky
solvent 1,1,3,3-tetramethylurea (TMU).3 A similar decrease in
solvation number is observed in the much bulkier solvent
hexamethylphosphoric triamide (HMPA).4 However, such a
decrease is not seen in N,N-dimethylformamide (DMF) and N,N-
dimethylacetamide (DMA) except for the Zn(II) ion in DMA.5
Thus, the reduction of the solvation number is interpreted as
being the result of steric repulsion between the bound solvent
molecules in the first coordination sphere of the metal(II) ion
for these oxygen-donating solvents. The solvation structures in
some nitrogen-donating solvents, such as 1-aminopropane
(PA),6,7 1,2-diaminoethane,8,9 1,3-diaminopropane,6 3- and
4-methylpyridine,10 and nitriles,9,11 were progressively inves-
tigated. Interestingly, in the strong σ-donating solvent PA, the
solvation equilibrium between octahedral and tetrahedral struc-
* E-mail: sfuna@chem4.chem.nagoya-u.ac.jp.
† Faculty of Science.
‡ Research Center for Materials Science.
10.1021/jp983799y CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/23/1999

1402 J. Phys. Chem. A, Vol. 103, No. 10, 1999
Inada et al.
levels of bulkiness will be demonstrated, and the bulkiness of
the solvent molecules will be rationalized on the basis of the
solvation structures. Whether the M-O bond length is affected
by the type of donating oxygen atom will also be discussed.
(II) ion. TMP solutions of the Mn(II) and Ga(III) ions were
prepared by dissolving the TMP solvates in TMP, respectively.
In the case of the Fe(II), Cu(II), and Zn(II) ions, the corre-
sponding anhydrous salts, M(CF3SO3)2, were dissolved in TMP.
In the case of the Co(II) and Ni(II) ion, the sample solutions
were prepared as follows: Excess amounts of o-ethylformate
relative to the containing water were added to the TMP solution
of M(H2O)6‚(CF3SO3)2 (M ) Co and Ni). The solution was
stirred for 1 h at 50 °C and concentrated at 60 °C under reduced
pressure to remove the resulting o-ethylformate, ethanol, and
ethylformate.18 The solution obtained was then used for the
measurements. The concentrations of the metal(II,III) ions in
all sample solutions for the measurements of EXAFS and
electronic absorption spectra were determined by EDTA titration
and summarized in Tables S1 and S2 (S: Supporting Informa-
tion), respectively.
Measurements. The sample solutions for the EXAFS mea-
surements were adsorbed in porous glass disks (radius ) 2 cm
and thickness ) 2 mm) which were sealed in a polyethylene
bag to prevent moisture ingress and evaporation of the solvent.
X-ray absorption spectra were measured at 295 K for all sample
solutions and at various temperatures (281-311 K) for the TMP
solution of the Zn(II) ion using the BL-6B and BL-12C stations
at the Photon Factory of the National Laboratory for High
Energy Physics.19,20 The white synchrotron radiations were
monochromatized by a monochromator with an Si(111) double
crystal, which was detuned to 50%, 60%, 70%, 70%, 80%, 80%,
and 90% of the maximal intensity at the K edge for Mn, Fe,
Co, Ni, Cu, Zn, and Ga, respectively, to remove the higher-
order reflection.21 The incident and transmitted X-ray intensities
(I0 and I) were simultaneously measured by the ionization
chambers with lengths of 17 and 31 cm, respectively. The
ionization chamber for the I0 measurement was filled with N2
gas and that for the I measurement was filled with a 1:3 mixture
of Ar and N2 gas for the measurements at the Mn, Fe, and Co
K edges and with a 3:17 mixture of Ar and N2 gas at the Ni,
Cu, Zn, and Ga K edges. Electronic absorption spectra were
recorded on a V-570 spectrophotometer (JASCO) over the
wavelength range 300-1400 nm at 298 K. Units of M-1 cm-1
(M ≡ mol dm-3) are used for the molar absorption coefficient
(ꢀ), where the mol kg-1 was converted to M, knowing the
density of the solutions.
Experimental Section
Solvents. Reagent-grade methanol (Wako), ethanol (Wako),
dimethyl sulfoxide (Wako), and trimethyl phosphate (Wako)
were dried over 4-Å molecular sieves and distilled before use.
The water content in each solvent was confirmed to be less
than 5 × 10-3 mol kg-1 by the Karl-Fisher method.
M(H2O)6‚(CF3SO3)2 (M ) Mn, Fe, Co, Ni, Cu, and Zn).
MnCO3 (Wako), iron sponge (Wako, 99.99%), CoCO3 (Wako),
NiCO3 (Wako), CuO (Wako, 99.99%), and zinc metal (Wako,
99.995%) were respectively suspended in an aqueous solution
of CF3SO3H (Wako, 98%). Each solution was stirred for a few
days, and the residue was filtered. The filtrate was concentrated
to obtain each hexahydrate salt. The number of water molecules
contained in the salt was 6, which was determined by the
molecular weight of the salt measured by EDTA titration. For
Fe(CF3SO3)2‚6H2O, all procedures were carried out under argon
gas to prevent the oxidation of the Fe(II) ion.
Ga(H2O)n‚(CF3SO3)3 (n ) 9-10). The salt of Ga(H2O)n‚(CF3-
SO3)3 using gallium metal (Wako, 99.99%) was obtained by
the same procedure as described just above, where n was
determined to be 9-10.
Mn(tmp)6‚(CF3SO3)2 and Ga(tmp)6‚(CF3SO3)3. An excess
amount of o-ethylformate relative to water was added to each
TMP solution of Mn(H2O)6‚(CF3SO3)2 and Ga(H2O)n‚(CF3-
SO3)3. The solution was stirred for a few hours at 50 °C, and
the residue was filtered. The filtrate for Mn(II) was concentrated
under reduced pressure to obtain the light red crystals of Mn-
(tmp)6‚(CF3SO3)2. The filtrate for Ga(III) was washed by dry
diethyl ether, and the white crystals of Ga(tmp)6‚(CF3SO3)3 were
obtained. The number of TMP molecules in both salts was
confirmed to be 6 by EDTA titration.
M(CF3SO3)2 (M ) Mn, Fe, Co, Cu, and Zn). Hexahydrates
of the Mn(II), Co(II), Cu(II), and Zn(II) ions were dried at 300
°C for a few hours to obtain the corresponding anhydrous salts.
Fe(H2O)6‚(CF3SO3)2 was dried at room temperature in vacuo
to prevent the oxidation of the Fe(II) ion. The amount of the
metal(II) ion was confirmed by the EDTA titration method.
Sample Solutions for Measurements of EXAFS and
Electronic Absorption Spectra. Aqueous sample solutions of
metal trifluoromethanesulfonates were prepared by dissolving
the corresponding hydrates in doubly distilled water. In the case
of the Ga(III) ion, 0.1 mol dm-3 CF3SO3H was added to prevent
hydrolysis. To prepare the aqueous solution of [Ga(ox)3]3- (ox
) C2O42-), which was used as the standard sample for the
EXAFS analysis, 2.1 g of Ga(NO3)3‚mH2O (Wako), 2.2 g of
(NH4)2C2O4 (Wako), and 0.08 g of H2C2O4 (Wako) were
dissolved in water, where the pH was 1.87. Under the present
experimental conditions, [Ga(ox)3]3- was quantitatively formed
in the solution, according to the reported stability constants of
the oxalato complexes of the Ga(III) ion.17 MeOH, EtOH, and
DMSO solutions of the Mn(II), Fe(II), Co(II), Cu(II), and Zn-
(II) ions were prepared by dissolving the corresponding
anhydrous M(CF3SO3)2 in each solvent. On the other hand,
MeOH, EtOH, and DMSO solutions of the Ni(II) ion were
prepared by dissolving Ni(H2O)6‚(CF3SO3)2 in each solvent, and
the water contained in the solution was removed using 4-Å
molecular sieves. It was confirmed by the Karl-Fisher titration
method that the amount of water contained in the sample
solutions was negligible relative to the concentration of the Ni-
EXAFS Data Analysis. The details of the EXAFS data
analysis were the same as previously reported.3,8,9,11,12 The
structure parameters around the metal ion, such as the coordina-
tion number (N), the M-O bond length (R), and the Debye-
Waller factor (σ), were analyzed by the least-squares calculation
using the model function of EXAFS oscillation, øcalc(k)22-28
Nj
2Rj
øcalc(k) )
exp -2σj2k2 -
Fj(π,k) sin{2kRj -
Rj(k)} (1)
∑
2
{
}
(
)
λ
j
kRj
where Fj(π,k) is the backscattering amplitude from each of the
Nj scatterers at distance Rj from the X-ray absorbing atom, σj
is the Debye-Waller factor, λ is the mean free path of a
photoelectron, and Rj(k) is the total phase shift. The values of
Fj(π,k) and Rj(k) were determined using the EXAFS spectra of
the corresponding aqueous solution on the basis of the structure
parameters previously determined1,2 and used during the course
of the structural analysis of the other sample solutions while
optimizing the values of Rj, σj, and Nj as variables. Because
the hydration structure of the Ga(III) ion was not available, we

Solvation Structures of Metal(II,III) Ions
J. Phys. Chem. A, Vol. 103, No. 10, 1999 1403
Figure 1. Fourier filtered k3ø(k) values (dots) and k3øcalc(k) curves (solid lines) for Mn(II) (a), Fe(II) (b), Co(II) (c), Ni(II) (d), Cu(II) (e), and
Zn(II) (f) in MeOH (A), EtOH (B), DMSO (C), and TMP (D).
TABLE 1: Structure Parameters around the Metal Ionsa
used the aqueous solution of [Ga(ox)3]3- as the structural
H2Ob
MeOH
EtOH
DMSO
TMP
standard. The structure of the complex in a single crystal was
reported.29 A curve-fitting procedure for the refinement of the
structure parameters was applied to the Fourier filtered k3øfilt-
(k) values to minimize the error-squares sum, ∑{k3øfilt(k) -
k3øcalc(k)}2, using the program REX.30
Mn(II)c
Fe(II)d
Co(II)e
Ni(II)f
Cu(II)g
N
6
5.8
217
8.2
5.8
217
8.7
6.1
216
7.3
5.9
216
8.0
R/pm 217
σ/pm
N
6
6.0
5.9
6.0
6.0
R/pm 211
σ/pm
211
9.2
6.0
208
9.7
5.9
211
9.4
6.0
208
9.9
6.0
211
10.1
6.0
207
6.1
6.0
212
9.9
5.6
209
8.7
5.8
205
7.9
3.9
1.7
N
6
Results
R/pm 208
σ/pm
The observed EXAFS oscillations weighted by k3 are shown
in Figure S1 for the Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and
Zn(II) ions and in Figure S2A for the Ga(III) ion. The
symmetrical oscillations are observed for the k3øobsd(k) curves
in water, whereas they are unsymmetrical in MeOH, EtOH,
DMSO, and TMP. The k3øobsd(k) values over the k range of
(2-13) × 10-2 pm-1 were Fourier transformed, and the Fourier
transformations, as shown in Figures S2B and S3, were obtained.
The main peak observed in the Fourier transformations is
attributable to the nearest bonding M-O interaction. In addition
to the main peak, the longer interactions appear over the R range
of 250-300 pm in MeOH and EtOH, 250-350 pm in DMSO,
and 300-350 pm in TMP. They can be assigned to the
nonbonding interactions with the next nearest neighboring
atoms, C for MeOH and EtOH, S for DMSO, and P for TMP.
The peak intensities of these nonbonding interactions in DMSO
and TMP are relatively greater than those in MeOH, EtOH,
DMF,5 DMA,5 and TMU,3 in which the second sphere is
composed of the carbon atom. In this study, because we focused
on the local structure around the metal(II,III) center, only the
main peaks were analyzed. In Figure S2C for the Ga(III) ion
and Figure 1 for the other metal(II) ions, the Fourier filtered
k3øfilt(k) values and the k3øcalc(k) curves calculated using eq 1
and the optimized structure parameters are depicted. The
calculated curves are perfectly in agreement with the values of
k3øfilt(k) in all k ranges used in the least-squares calculation (3
e k/10-2 pm-1 e 12). The obtained structure parameters around
the metal(II,III) centers are summarized in Table 1. In the case
of a TMP solution of the Zn(II) ion, because the N value of 4.8
was obtained at 295 K, we carried out the variable-temperature
EXAFS measurements in the temperature range between 281
and 311 K in order to evaluate whether the solvation structure
N
6
R/pm 205
σ/pm
205
9.0
205
9.5
205
8.6
N
4
2
3.8h
3.9h
4.0h
R/pm 197
229
σ/pm
197h
9.3h
6.0
197h
10.8h
5.9
196h
8.6h
6.0
197
214
7.4
15.7
4.8j
5.0k
4.9l
5.0m
204j
205k
204l
205m
9.5j
9.6k
9.8l
9.6m
6.0
Zn(II)i
N
6
R/pm 207
σ/pm
208
8.6
208
9.1
208
8.8
Ga(III)n
N
5.9
R/pm 197
197
8.6
σ/pm
8.3
a At 295 K. Errors of N, R, and σ are estimated to be ca. 0.1, 1 pm,
and 0.2 pm, respectively. b Reported values by means of X-ray
diffraction technique except for the Ga(III) ion determined in this study.
c E0 ) 6.546 keV and λ ) 607.5 pm. d E0 ) 7.121 keV and λ ) 609.1
f
pm. e E0 ) 7.719 keV and λ ) 653.3 pm. E0 ) 8.343 keV and λ )
703.1 pm. g E0 ) 8.987 keV and λ ) 536.1 pm. The values in the
upper row are those for the equatorial site and the lower are for the
axial site. h Corresponding values for the axial site were not determined.
i E0 ) 9.665 keV and λ ) 564.7 pm. j At 295 K. k At 281 K. l At 302
K. m At 311 K. n E0 ) 10.382 keV and λ ) 1213.0 pm.
of the Zn(II) ion at 295 K is 5-coordinate or there is solvation
equilibrium between the 4- and 6-coordinate structures. The

1404 J. Phys. Chem. A, Vol. 103, No. 10, 1999
Inada et al.
Figure 2. Electronic absorption spectra in MeOH (solid lines), EtOH (dotted lines), and DMSO (broken lines) for the Mn(II) (A), Fe(II) (B),
Co(II) (C), Ni(II) (D), and Cu(II) (E) ions at 298 K.
TABLE 2: Spectral Data for the Metal(II) Ions at 298 K
tion structure of the Cu(II) ion has been confirmed to be axially-
elongated, distorted-octahedral due to the Jahn-Teller effect.
As in water, the four interactions with the Cu-O bond length
of 197 pm are at the equatorial sites, and the two interactions
of 214 pm are at the axial sites in TMP (see Table 1). Although
the structure parameters for the axial sites of the Cu(II) ion in
MeOH, EtOH, and DMSO could not be determined by EXAFS
spectroscopy, the electronic absorption spectra clearly show the
existence of the axial interactions on the Cu(II) ion in MeOH,
EtOH, and DMSO (see Figure 2E). The solvation structure of
the Cu(II) ion in these solvents is thus the axially-elongated
distorted-octahedral. On the basis of the EXAFS analysis, the
solvation structures of the Zn(II) ion in MeOH, EtOH, and
DMSO are concluded to be 6-coordinate octahedral as in water.
Interestingly, the solvation structure in TMP, however, is
different from that in the other solvents. As shown in Table 1,
the N value of the Zn(II) ion in TMP is ca. 5, and the R value
is shorter than those in the other solvents by ∼4 pm. A similar
decrease in N and R has been observed for the Zn(II) ion in
DMA, and the authors have pointed out that there is solvation
equilibrium between the tetrahedral and octahedral structures.5
Though the decrease in N can be certainly expected due to such
a solvation equilibrium, the N value should be 5 in the case of
the 5-coordinate solvation structure. Because the distinction is
difficult using EXAFS measurement at a single temperature,
we carried out variable-temperature EXAFS measurements
between 281 and 311 K in order to experimentally clarify the
solvation structure. In the case of the Co(II) ion, with an ionic
radius similar to that of the Zn(II) ion, the solvation equilibrium
between tetrahedral and octahedral is familiar due to the d7
electronic configuration, and the enthalpy change, ∆H°, from
the octahedral species (CoS62+, S ) solvent) to the tetrahedral
species (CoS42+) is reported to be 36.1 kJ mol-1 in PA7 and ca.
50 kJ mol-1 in DMA.32 Although the ∆H° value for the
octahedral-tetrahedral equilibrium of the Zn(II) ion may be
different from those of the Co(II) ion, the reported ∆H° values
for the Co(II) ion become a criterion to estimate the component
change in the solution of the Zn(II) ion in TMP for the variable-
temperature EXAFS measurements. According to the observed
M(II)
solvent
λ
max/103 cm-1 (ꢀmax/M-1 cm-1
)
Mn(II)
H2Oa
MeOH
18.7, 23.1, 25.0, 28.0, 29.8
19.1 (0.05), 23.4 (0.06), 25.0 (0.07),
27.8 (0.08), 29.1 (0.08)
19.0 (0.05), 23.3 (0.05), 24.9 (0.06),
27.8 (0.07), 29.6 (0.07)
19.1 (0.03), 23.1 (0.05), 24.6 (0.07),
27.9 (0.17), 28.8 (0.17)
8.3, 10.4
8.6 (1.0), 10.2 (1.8)
8.7 (2.4), 10.1 (3.1)
7.9 (1.8), 9.6 (2.2)
EtOH
DMSO
Fe(II)
Co(II)
Ni(II)
Cu(II)
H2Oa
MeOH
EtOH
DMSO
H2Oa
MeOH
EtOH
DMSO
H2Oa
MeOH
EtOH
DMSO
H2Oa
8.1, 19.4
8.0 (1.6), 19.3 (5.5)
7.8 (2.1), 19.1 (7.0)
7.4 (2.0), 18.6 (11.0)
8.5, 13.8, 25.3
8.3 (1.1), 13.7 (1.3), 25.0 (3.0)
8.0 (1.2), 13.3 (3.1), 24.5 (3.3)
7.8 (3.0), 13.0 (3.1), 23.8 (8.7)
12.4
MeOH
EtOH
DMSO
11.8 (8.9)
12.2 (26.5)
11.8 (33.0)
a Reference 31.
obtained k3øobsd(k) curves, Fourier transform magnitudes, k3øfilt-
(k) values, and k3øcalc(k) curves are shown in Figure S4, and
the structure parameters are given in Table 1.
The observed absorption spectra for the Mn(II), Fe(II), Co-
(II), Ni(II), and Cu(II) ions in MeOH, EtOH, and DMSO are
depicted in Figure 2. The spectral data are summarized in Table
2, in which the corresponding values in water previously
compiled are included.31 These values are assigned to the d-d
transitions.
Discussion
In all solvents used in this study, the coordination numbers
of the Mn(II), Fe(II), Co(II), and Ni(II) ions are close to 6, and
the M-O bond lengths are similar to the corresponding values
in water (see Table 1). Furthermore, all solutions for each metal-
(II) ion show a spectral pattern of d-d electronic transitions
similar to that in water as shown in Table 2. Therefore, it is
concluded that the solvation structures of the Mn(II), Fe(II),
Co(II), and Ni(II) ions are 6-coordinate octahedral as in water.
These results are reasonable because the solvent molecules of
MeOH, EtOH, DMSO, and TMP are not so bulky. The hydra-
2+
N value of 4.8 at 295 K, the mole ratio of ZnS62+: ZnS4 is
estimated to be 4:6, and the conditional equilibrium constant,
K, for the reaction of ZnS62+ h ZnS42+ is ca. 1.5. If the value
of ∆H° is approximated to be 15 kJ mol-1 as a lower limit,
which is still smaller than half of the smaller value of 36.1 kJ
mol-1 in the case of the Co(II) ion, for the solvation equilibrium

Solvation Structures of Metal(II,III) Ions
J. Phys. Chem. A, Vol. 103, No. 10, 1999 1405
of the Zn(II) ion in TMP, the entropy change, ∆S°, is calculated
to be 54 J mol-1 K-1. The mole ratio of ZnS62+/ZnS42+ should
be varied from 5:5 at 280 K to 3:7 at 310 K, and the N and R
values should be changed in response to the change in the
component ratio. However, the finding that there is no change
in the obtained structure parameters for the Zn(II) ion in TMP
at 281, 295, 302, and 311 K indicates that there is no equilibrium
between octahedral and tetrahedral. Furthermore, as pointed out
in the previous study on the octahedral-tetrahedral solvation
equilibrium for the Co(II) ion in PA,7 the Fourier transformation
function in the solvation equlibrium shows a distorted Co-N
interaction peak because of the large difference (ca. 15 pm) in
the Co-N bond lengths between [Co(pa)6]2+ (Co-N ) 217
pm) and [Co(pa)4]2+ (Co-N ) 201 pm). This difference in the
Co-N bond lengths is consistent with the difference in the ionic
radii of the Co(II) ions with the coordination numbers of 6 and
4, and the value of the ionic radius of the Zn(II) ion is similarly
different by ca. 14 pm between the 4- and 6-coordinate species.33
Thus, the distorted Fourier transformation should be observed
for the Zn(II) ion in TMP, if there is solvation equilibrium. As
is apparent from Figures S3 and S4B, any distortion of the main
peak assigned to the Zn-O interactions is not observed. We
have concluded, then, that the Zn(II) ion has the 5-coordinate
solvation structure in TMP with a Zn-O bond length of 204
pm. Considering no ligand-field stabilization of the Zn(II) ion,
the 5-coordinate solvation structure is completely acceptable,
while the Co(II) ion, with an ionic radius similar to the Zn(II)
ion, is 6-coordinate octahedral. As given in Table 1, the N values
of the Ga(III) ion in water and in TMP are 5.9 and 6.0,
respectively. The Ga(III) ion, thus, has the 6-coordinate
octahedral structure with a Ga-O bond length of 197 pm in
both solvents. Interestingly, in TMP the smaller Ga(III) ion is
6-coordinate, while the larger Zn(II) ion is 5-coordinate. This
finding indicates that, as observed for the In(III) ion in TMU,3
the higher charge of the metal ion makes the solvation number
large, because the electrostatic attraction between the metal ion
and solvent molecules exceeds the energetic loss due to the steric
repulsion between the coordinating solvent molecules. On the
other hand, we have recently reported that the Ga(III) ion in
TMU, which is still bulkier than TMP, is 4-coordinate tetrahe-
dral,12 because the effect of the steric repulsion between the
coordinating TMU molecules around the Ga(III) center overrides
the effect of the charge on the center. The N value of 5.6 for
the Co(II) ion in TMP at 295 K determined in this study is
somewhat smaller than 6. A similar trend in N for the Co(II)
ion has been observed in DMA.5 Thus, we have measured the
electronic absorption spectra of the Co(II) ion in TMP and DMA
at a variety of temperatures to clarify the existence of the
solvation equilibrium. As is apparent from the spectra shown
in Figure S5, the absorption maxima are shifted to longer
wavelengths and the molar absorption coefficients are signifi-
cantly increased with increasing temperature. This characteristic
clearly indicates that there is solvation equilibrium between the
octahedral and tetrahedral structures. Because the 4-coordinate
species is favored to exist at extremely high temperatures, the
main species at room temperature is the octahedral one, and
the solvation structure of the Co(II) ion in TMP and DMA at
295 K can be concluded to be 6-coordinate octahedral. In this
study, we determined the solvation structures of a series of metal
ions from Mn to Ga, belonging to the fourth row of the periodic
table, in four oxygen-donating solvents. Considering the sol-
vation structures already determined in other nonaqueous
solvents, the bulkiness of the solvent molecules becomes clear.
In MeOH, EtOH, DMSO, and DMF,5 all of the metal ions take
6-coordinate octahedral solvation structures, while the decrease
in the solvation number of the Zn(II) ion is observed in DMA5
and TMP. A gradual decrease in the solvation number, from 5
to 4, with increasing atomic number of the metal ion is seen in
TMU,3 and the 4-coordinate tetrahedral structure is dominant
in HMPA with the exception that the Mn(II) ion has a
5-coordinate structure.4 In conclusion, the order in bulkiness
of the solvent molecules is MeOH ∼ EtOH ∼ DMSO ∼ DMF
< DMA ∼ TMP < TMU < HMPA, which was determined on
the basis of the solvation number of the metal ions. It has been
noticed that the M-N bond length of solvated metal ions varies
with the kind of nitrogen atoms in the nitrogen-donating
solvents.11,12 There is, however, no difference in the M-O bond
length of the hexasolvated metal ions in the solvents with sp3
hybridizing oxygen, such as water, MeOH, and EtOH, and those
with sp2 hybridizing oxygen, such as DMSO, TMP, DMF, and
DMA. This suggests that the electron-distribution area on
different kinds of oxygen atoms in these oxygen-donating
solvents is almost the same, which is consistent with the harder
character of the oxygen atom relative to the nitrogen atom.
Acknowledgment. This work was financially supported by
the Grants-in-Aid for Scientific Research (Nos. 10440221,
10740305, and 10874081) from the Ministry of Education,
Science, Sports, and Culture of Japan and the Kurata Research
Grant from the Kurata Foundation. The EXAFS measurements
were performed under the approval of the Photon Factory
Program Advisory Committee (Proposal Nos. 96G004 and
97G054).
Supporting Information Available: The concentrations of
the metal(II,III) ions for the measurements of EXAFS (Table
S1) and electronic absorption specta (Table S2), the observed
EXAFS oscillations for the Mn(II), Fe(II), Co(II), Ni(II), Cu-
(II), and Zn(II) ions (Figure S1), the results of the EXAFS
measurements for the Ga(III) ion (Figure S2), Fourier transform
magnitudes for the Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and
Zn(II) ions (Figure S3), the results of the variable-temperature
EXAFS measurements for the Zn(II) ion in TMP (Figure S4),
and the electronic absorption spectra of the Co(II) ion in TMP
and DMA (Figure S5). This material is available free of charge
via the Internet at http://pubs.acs.org.
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