The Mixed-Valence Rhenium(IV, V) Complexes

by B. Banaœ, M. Nahorska, A. Tomkiewicz, J. K³ak, M. Cyfert and J. Mroziñski*

Faculty of Chemistry, University of Wroc³aw, F. Joliot-Curie 14, 50-383 Wroc³aw, Poland

(Received March 13th, 2006; revised manuscript June 12th, 2006)

Reaction of K4[Re2OCl10] with oxidizing agents in hydrochloric acid produced binuclear

compounds: Cs3[Re2OCl10] 1, (Ph4As)3[Re2OCl10] 2 , (ChinH)3[Re2OCl10] 3. These

complexes had been characterized by kinetic and magnetic investigations. The low-tem-

perature magnetic susceptibility measurements have revealed, that Cs3[Re2OCl10] com-plex is antiferromagnets, with Néel temperature at 10 K. The temperature dependence of

the magnetic moment for 2 and 3 complexes indicates the existence of a magnetically iso-

lated exchange-coupled dimer. In the electronic spectra, the intensive band at 20150 cm–1

is associated with the presence of the two different oxidation states. In agreement with the

evidence from the oxidation of oxochlororhenate ion and the disproportionation in solu-

tion, it is proposed that the compound should be considered as a Re(IV)–Re(V)

mixed-valence system.

Key words: bimetallic complexes, rhenium(IV), rhenium(V), mixed-valence system

The binuclear rhenium(IV) complex of K4Re2OCl10 type was prepared for first

time by Je¿owska-Trzebiatowska [1]. Morrow [2] suggested that this compound

crystallizes as a salt like K4Re2OCl10·H2O and consists of potassium ions, Re2OCl104�

ions, and water molecules. The crystallographic investigation of Lis et al . [3] have

shown that this compound forms non-hydrated crystals. This complex forms a

blood-red compound in diluted hydrochloric acid solution when oxidizing agents are

added. The effect of the addition of oxidizers to �-oxochlororhenate was discovered

by Je¿owska-Trzebiatowska et al. [4–6]. On the basis of additional experimental re-

sults it was suggested that the blood-red compound has the formula Cs3Re2O2HCl10

and that the –O– or –O–O– groups form a bridge between two rhenium atoms [3–5].

O

However, the further investigations of Lis et al. [7] have shown that the oxidized

form of the complex is Re2OCl103� ion with a linear Cl–Re–O–Re–Cl group. Another

binuclear rhenium(IV) complexes were obtained in the form Cs3H[Re2O(SCN)10] [8]

and K4[Re2O(OH)6(SCN)10] [9]. Both compounds similar to K4Re2OCl10 exhibit

diamagnetism as a result of total coupling between magnetic centers ReIV in linear

bridge Re–O–Re system. The linear group Re–O–Re with both rhenium centres on

plus five oxidation state was found in complexes Re2 O3en2 Cl4 [10] and

Re2O3(S2CNEt2)4 [11]. Both types of systems with linear bridges of the type

Polish J. Chem., 80, 1663–1676 (2006)

*Corresponding author: Tel./fax +48-71-3757307, E-mail address: jmroz@wchuwr.chem.uni.wroc.pl

ReIV–O–ReIV and ReV–O–ReV reveal totally extinguished magnetic moment. Only

the complexes containing [Re2OCl10]3– anion show paramagnetic properties at the

higher temperature region [9]. On the basis of the X-ray studies it was suggested that

in the [Re2OCl 10]3– ion both rhenium atoms are in the +4 oxidation state, and that the

unpaired electron is most probably located on the O atom and therefore oxygen should

be in the –1 oxidation state [7]. For the oxidation reaction of Re2OCl104� hydrogen peroxide

or some other oxidizing agents take up one electron from the Re2OCl104� ion and the exis-

tence of different oxidation state of the rhenium atoms is also possible. Because this

problem has not been clarified up to now we have decided to solve it.

EXPERIMENTAL

Synthesis of the complex. The rhenium(IV) complex K4[Re2OCl10] was prepared following the

procedure of Je¿owska-Trzebiatowska [1]. The potassium perrhenate was reduced by the stoichiometric

quantity of iodine anions in hydrochloric acid solution. The reduced solution after the reflux process was

evaporated under reduced pressure at 18�C in the stream of nitrogen. The dark-red substance created in

the process was washed several times with carbon tetrachloride, ethanol and finally with diethyl ether. Af-

ter dissolving in hydrochloric acid solution, the substance, in the presence of hydrogen peroxide, or other

strong oxidizing agents, turns into blood-red complex.

This compound was transformed into the caesium, tetraphenylarsonium and quinolineH+

salts,

Cs3[Re2OCl10], (Ph4As)3[Re2OCl10], (ChinH)3[Re2OCl10] by methatetic reaction.

Anal. Calcd. for Cs3Re2OCl10 1: Cl, 31.1. Found: Cl, 31.3. The rhenium content was determined by

ICP analysis: Re (calcd.), 32.6; Re (found), 32.7. Anal. Calcd. for As3Re2C72H60OCl10 2: C, 45.7; H,

3.20; Cl, 18.7. Found: C, 45.2; H, 3.10; Cl, 18.2. Re (calcd.), 19.7; Re (found), 19.2. Anal. Calcd. for

Re2C27H24N3OCl10 3: C, 28.6; H, 2.13; N, 3.69; Cl, 31.3. Found: C, 28.5; H, 2.10; N, 3.82; Cl, 31.1. Re

(calcd.), 33.1; Re (found), 32.7.

Physical measurements. The rate of oxidation of [Re2OCl10]4–

with hydrogen peroxide has been

measured spectrophotometrically in a water-HClO4 solution at ionic strength 0.5 mol dm–3

(HClO4/NaClO4). An increase in the absorbance at 490 nm was followed during the course of the reac-

tion. The excess of hydrogen peroxide and perchloric acid over the rhenium complex concentration was

used for all kinetic experiments. The observed zero order reaction rate was determined from linear slope

of absorbance A versus time t. The molar extinction coefficient at 490 nm, �0 = 7500 [4], was used to cal-

culate the concentration of [Re2OCl10]3–

.

The kinetic measurements of oxidation reaction of [Re2OCl10]4–

with Cr(VI) were performed in the

0.05 mol dm–3

acetate buffer or in a water-HClO4 solution at an ionic strength 0.1 mol dm–3

(HClO4/NaClO4), under pseudo-first order conditions with Cr(VI) concentration in an excess over the

rhenium complex concentration. The hydrogen ion concentration was in the range 4�10–5

– 0.1 mol dm–3

.

The reaction was monitored at the absorbance maximum of Re2OCl103� (� = 490 nm) and the observed first

order rate constants were calculated by linear least-squares fitting of ln(A� – At) to time t. At and A� are

the absorbance of the solution at time t and after 8–10 half lives.

The electronic absorption spectra as well as the kinetic measurements were made on Carry 5

UV-Vis-NIR spectrophotometrer. Fast processes with Cr(VI) at [H+] > 0.01 mol dm

–3were measured by

the stopped-flow technique on the Hi-Tech SF-51 apparatus. Each kinetic run was repeated 3 to 4 times.

Infrared (IR) spectra (4000–400 cm–1

) and far infrared (FIR) spectra (400–100 cm–1

) in nujol film

were recorded with Bruker IFS 113 V spectrometer.

The magnetization measurements were determined in the temperature range 2–300 K at 1 Tesla using

Princeton Applied Research Model 150 A Foner-type vibrating sample magnetometer [12] calibrated

with nickel metal standard. Temperatures were measured with a calibrated gallium arsenide diode.

1664 B. Banaœ et al.

All magnetic susceptibility data were corrected for the diamagnetism of constituent atoms using

Pascal’s constants [13].

Electron Paramagnetic Resonance powder spectra were measured at room temperature and at 123 K

on a JES-ME-3X spectrometer in the X-band at � 9.20 GHz.

RESULTS AND DISCUSSION

Kinetic investigations. Spectrophotometric measurements of the oxidation rate

of [Re2OCl10]4– with hydrogen peroxide in acidic solutions revealed that the reaction

is zero order towards the rhenium complex. The reaction rate was independent of the

[Re2OCl 10]4– concentration, and plots of At versus t were linear during more than

90% of the reaction course. The observed zero order rate constant was proportional to

the hydrogen peroxide concentration, (Table 1), what suggests the first order depend-

ence on [H2O2].

d OCl

dt

[ ]Re2 103�

k[H2O2] (1)

The dependence of the first order rate constant on acidity (Fig. 1a) is given by the em-

pirical expression

k = A + B[H+] (2)

values of A and B calculated from linear least squares fit of the experimental data to

the eq. (2) are 18.1 s–1

and 3.91 mol–1

dm3

s–1

, respectively with correlation coeffi-

cient R2

= 0.9832, at 25�C and I = 0.5 mol dm–3

(HClO4/NaClO4).

Table 1. Kinetic data for the oxidation reaction of [Re2OCl10]4–

with H2O2 in 0.2 mol dm–3

HClO4.I = 0.5 mol dm

–3(HClO4/NaClO4).

Temp.

°C

105 � [Re2OCl10

4�]

(mol dm–3

)

103 � [H2O2]

(mol dm–3

)

107 � kexp

(mol dm–3

s–1

)

105kexp/[H2O2]

(s–1

)

8.5 7.92 2.94 1.10 � 0.06 3.74

15 7.92 2.94 1.40 � 0.06 6.68

25

7.92 2.94 2.24 � 0.07 7.62

7.89 0.49 0.40 � 0.02 8.16

7.89 0.98 0.64 � 0.03 6.53

7.89 1.96 1.51 � 0.09 7.70

7.89 2.94 2.27 � 0.10 7.72

7.89 3.92 3.28 � 0.13 8.36

5.14 1.96 1.40 � 0.05 7.14

3.84 1.96 1.29 � 0.07 6.58

2.61 1.96 1.34 � 0.06 6.84

Av. 7.41 � 0.66

36 7.92 2.94 3.86 � 0.21 13.1

The mixed-valence rhenium(IV, V) complexes 1665

The zero order rate constant towards the investigated complex allows to assume,

that the most important for the kinetics of the reaction is the process of OH� radicals

creation. There are significant proofs for free hydroxyl group formation in the hydro-

gen peroxide solutions [14]. Those groups are a strong oxidizing species character-

ized by a high oxidative potential which react with a reaction rate controlled by the

diffusion process with a huge number of substances and also with metal ions. The rate

constants of the reactions with the OH� radicals are mostly of the value 108–1010

mol–1 dm3 s–1 [15–17].

It is suggested, that the process of OH� radicals formation may be catalysed by

hydrogen ion or some small amounts of metal ions, including monomeric Re(IV) ion.

The last ion may be formed in the equilibrium process [18]:

Re2OCl104� + H2O 2Re(OH)Cl5

2�

Since the oxidation process of the investigated complex with the OH�

radicals

OH�

+ Re2OCl104� Re2OCl10

3� + OH–

is very fast it cannot be considered in the kinetic equation (1).

The temperature dependence of the first order rate constant k = kexp/[H2O2] (Table 1)

yields the activation parameters: EA = 33.4� 1.7 kJ mol–1, H� = 30.9 � 1.7 kJ mol–1

and S� = –219 � 5 J mol–1 K–1 at 298 K. The above values were obtained on the basis

of the independent kinetic runs at each temperature using the linear Arrhenius plot of

log(kexp/[H2O2]) vs 1/T, and Eyring plot of log(kexp/[H2O2]T) vs 1/T.

1666 B. Banaœ et al.

Figure 1. The dependence (a) of the first order rate constant on hydrogen ion concentration for the oxidation

reaction of [Re2OCl10]4–

with hydrogen peroxide in water-HClO4 solution at I = 0.5 mol dm–3

(NaClO4/HClO4) and (b) of the second order rate constant on hydrogen ion concentration for

the oxidation reaction of [Re2OCl10]4–

with Cr(VI) in water-HClO4 solution at I = 0.1 mol dm–3

(NaClO4/HClO4).

(a) (b)1

05�k

ex

p/[

H2O

2]

(s–

1)

kex

p/[

Cr(

VI)

](m

ol–

1d

m3

s–1)

[H+] (mol dm

–3)[H

+] (mol dm

–3)

The kinetic investigations of the oxidation of the [Re2OCl10]4– by Cr(VI), in a

broad range of reagents concentration and pH, revealed the first order kinetics with

respect to the rhenium complex. For the [Re2OCl10]4– concentration in the range

(1.1–5.2)�10–5 mol dm–3 and constant hydrogen ion concentration (acetate buffer),

with Cr(VI) ion concentration in the excess over the rhenium complex, the

pseudo-first order rate constant kexp was proportional to the Cr(VI) concentration and

the reaction kinetics was exactly of the second order (Table 2):

d OCl

dtk OCl Cr VI

[ ][Re ][ ( )]

Re2 103

2 104

�� (3)

where [Cr(VI)] is total Cr(VI) ions concentration, k = kexp/[Cr(VI)].

The kinetic data for the oxidation reaction of [Re2OCl10]4– with Cr(VI) in acetate

buffer are presented in Table 2. The dependence of the second order rate constant on

acidity in water-HClO4 solution, presented in Fig. 1b, is given by the empirical ex-

pression:

k = A[H+]

2+ B[H

+] + C (4)

Values of A, B and C, calculated from nonlinear least squares fit of the experimental

data to the equation (4), are: A = 1.62�105

mol–3

dm9

s–1

, B = 3.11�103

mol–2

dm6

s–1

and C = 56.4 mol–1

dm3

s–1

(correlation coefficient R2

= 0.9996) at ionic strength

0.1 mol dm–3

(HClO4 + NaClO4). The three terms in eq. (4) can be attributed to the

formation of differently protonated activated complexes.

Table 2. Kinetic data for the oxidation reaction of [Re2OCl10]4–

with Cr(VI) in 0.05 mol dm–3

acetate buffer.

Temp.

°C

105 � [Re2OCl10

4�]

(mol dm–3

)

104 � [Cr(VI)]

(mol dm–3

)

104 � [H

+]

(mol dm–3

)

102 � kexp

(s–1

)

kexp/[Cr(VI)]

mol–1

dm3(s

–1)

35 2.31 5.00 1.57 6.90 � 0.29 138

25

2.26 8.00 1.57 4.21 � 0.13 52.6

1.13 5.00 1.57 2.58 � 0.12 51.6

2.26 5.00 1.57 2.86 � 0.08 57.2

5.52 5.00 1.57 2.49 � 0.08 49.8

2.26 2.50 1.57 1.31 � 0.05 52.4

2.26 5.00 0.697 2.56 � 0.08 51.2

2.26 5.00 0.406 2.54 � 0.07 50.8

17 2.18 5.00 1.57 2.09 � 0.09 41.8

12 2.49 5.00 1.57 1.31 � 0.05 26.2

The mixed-valence rhenium(IV, V) complexes 1667

The kinetic and stoichiometric data together with the acid-base properties of the

reagents and previous findings for the reduction of Cr(VI) ions by various one elec-

tron reducing agents [19] suggest the reaction mechanism with (5), (6) and (7) as the

rate determining steps:

H2CrO4 H+

+ HCrO4� KH2A = 4.76

HCrO4� H

++ CrO4

2� KHA– = 9.61�10–7

kaHCrO4

� + 2H+

+ Re2OCl104� H3CrO4 + Re2OCl10

3� (5)

kb

HCrO4� + H

++ Re2OCl10

4� H2CrO4� + Re2OCl10

3� (6)

kc

HCrO4� + Re2OCl10

4� HCrO42� + Re2OCl10

3� (7)

Cr(V) + Re2OCl104� Cr(IV) + Re2OCl10

3�

Cr(IV) + Re2OCl104� Cr(III) + Re2OCl10

3�

Cr(IV) + Cr(VI) Cr(V)

In solution of ~10–4 mol dm–3 chromium(VI), only HCrO4� need to be considered as

the reactive species in the pH range 1–5.

According to the equation:

[HCrO4� ] =

K Cr VI

K H

H A

H A

2

2

[ ( )]

[ ]� �

where Cr(VI) = [H2CrO4] + [HCrO4� ], [HCrO4

� ] is equal to [Cr(VI)] at [H+] << KH2A.

Dihydrogen chromate H2CrO4 is significant species at pH � 0.

Taking into account the proposed mechanism and the stoichiometry of the reac-

tion, the kinetic equation (8) can be obtained:

d OCl

dt

[ ]Re2 103�

= 3(ka[H+]2 + kb[H+] + kc)[Cr(VI)]Re2OCl104� ] (8)

where: k = 3(ka[H+]2

+ kb[H+] + kc) (9)

1668 B. Banaœ et al.

Comparing eq. (9) with eq. (4) one gets: ka = A/3, kb = B/3 and kc = C/3.

At I = 0.1 mol dm–3 (NaClO4/HClO4) and temperature 25�C: ka = 5.40�104

mol–3 dm9 s–1, kb = 1.04�103 mol–2 dm6 s–1, kc = 18.8 mol–1 dm3 s–1.

The results are in agreement with the Espanson hypothesis that the reduction of

chromium(VI) to chromium(V) by one electron reagents requires the addition of two

protons to HCrO4� [20,21]. A similar conclusion regarding the protonation of chro-

mium(V) is suggested by the hydrogen ion dependence of the rate law for reduction

by NpO2� [22]. The reaction has been studied in perchloric acid solution from 0.4–2.0

mol dm–3. In this acid concentration range, the predominant chromium(VI) species

are HCrO4� and H2CrO4. The proposed kinetic mechanism requires for the reduction

of chromium(VI) to chromium(V) that the activated complex adds one proton to

H2CrO4 or two protons to HCrO4� to give H3CrO4 as the chromium(V) species.

The temperature dependence of the second order rate constant, kexp/[Cr(VI)] (Table 2),

yields the activation parameters: Ea = (47.3 � 6.1) kJ mol–1, H� = (44.8 � 6.1) kJ

mol–1 and S� = (–17 � 9) J mol–1 K–1 at 298.2 K. The above values were obtained on

the basis of the independent kinetic runs at each temperature in the 0.05 mol dm–3 ace-

tate buffer ([H+] = 1.57 mol dm–3) using linear Arrhenius plot of log(kexp/[Cr(VI)])

vs. (1/T), and Eyring plot of log(kexp/[Cr(VI)]T) vs . 1/T.

Magnetic properties. Rhenium(IV) ion with 5d3 electron configuration of the4A2g ground term has three unpaired electrons. The fourth oxidation state is particu-

larly stable in association with most ligands and it adopts octahedral symmetry [23].

Owing to the calculations of magnetic moment values based on the data obtained

from the experiment, it was possible to find two different magnetic cores Re(IV) and

Re(V) in the molecule of each investigated [Re2OCl10]3– complex. The values of

magnetic moments which are characteristic for one paramagnetic center with the spin

value S = 3/2 indicate the mixed valence oxidation state for two rhenium cores. As re-

ported earlier [24], magnetic moments of pure octahedral Re(V) complexes show

that most of the complexes are diamagnetic or show very low positive susceptibility

after diamagnetic corrections [25,26]. For this reason, the evaluated values of the

susceptibility data were calculated for the fine structures associated with the

zero-field splitting for the 4A2g term of paramagnetic Re(IV) center. The basic com-

plex with the form of K4[Re2OCl10] presents a typical diamagnetic properties in the

whole temperature range [27]. The temperature dependences of the magnetic suscep-

tibility for Cs3[Re2OCl10], (Ph4As)3[Re2OCl 10] and (ChinH)3[Re2OCl10] complexes

were measured in the range of 2–300 K, using Foner-type vibrating sample magne-

tometer. The values of effective moments, obtained from the calculations of magnetic

data in the temperature range studied, are equal to �eff. = 3.17 B.M. for complex 1,

3.09 B.M. for 2 and 3.12 B.M. for complex 3. The investigated compounds indicate

the discrepancy between the magnetic moments and the spin-only value (�spin-only =

[4S(S+1)]1/2, S = 3/2) equal to 3.87 B.M. The axial ligand field removes the orbital

degeneration of the 4A2g term and splits the ground term into two Kramers doublets

�3/2 and �1/2 separately. The energy separation is denoted as 2D, where D is the axial

zero-field splitting parameter [28]. The parameters C and � were determined in the

The mixed-valence rhenium(IV, V) complexes 1669

temperature range 100–300 K and are equal to 1.45 cm3 mol–1 K, –40.8 K for 1; 1.32

cm3 mol–1 K, –32.1 K for 2 and 1.45 cm3 mol–1 K, –40.3 K for 3, respectively.

The magnetic properties of those compounds are shown in Figs. 2, 3 and 4 as the

�MT on T dependence.

The �M values for the oxidized dimer Cs3[Re2OCl10] increase on lowering of the

temperature, pass through a maximum at about 10 K, and then decrease. This type of

the �M = f(T) dependence indicates the presence of antiferromagnetic coupling be-

1670 B. Banaœ et al.

0 50 100 150 200 250 300

0,000

0,005

0,010

0,015

0,020

0,025

T [K]

�M

[cm

3 mol

-1]

0,0

0,5

1,0

1,5

�M T

[cm3m

ol-1K

]

Figure 2. Thermal dependence of �M (�) and �MT (�) for Cs3[Re2OCl10] complex. The linear regression

presents the theoretical value of �MT.

0 50 100 150 200 250 300

0,0

0,1

0,2

0,3

T [K]

�M

[cm

3 mol

-1]

0,0

0,2

0,4

0,6

0,8

1,0

1,2

�MT

[cm3m

ol -1K

]

Figure 3. Thermal dependence of �M (�) and�MT (�) for [(C6H5)3As]3[Re2OCl10] complex. The linear

regression presents the theoretical value of �MT.

tween rhenium(IV) centers in the crystal lattice. The magnetic susceptibilities, in the

whole temperature range 2–300 K, have been successfully fitted using the

Hamiltonian [29]:

H = –2J�Sz, iSz, i+1 + g||�BHzSz, i + g��B(HxSx, i + HySy, i) + D(Sz2 – 1.25) (10)

The parallel and perpendicular susceptibilities (�|| , ��) for S = 3/2 were defined by

equations (11, 12):

�|| = [Ng ||2�B

2 /4kT][1 + 9exp(–2D/kT)][1 + exp(–2D/kT)]–1

exp(J/kT) (11)

�� = {[Ng B2

�2 � /kT][1 + exp(–2D/kT)]

–1+ 3Ng�

2 �B2 tanh(D/kT)/4D}{(kT/J)tanh(J/2kT) +

[sech2(J/2kT]/2} (12)

where D is the zero-field splitting, N – the Avogadro’s number, g – the spectroscopic

splitting factor, k – the Boltzmann constant and T is the absolute temperature.

The average magnetic susceptibility is equal to �av = 1/3��� + 2/3��. Minimization

agreement factor R = �(�MTexp – �MTcalc)2/�(�MTexp)2 for complex 1 equals to

1.54�10–4 and leads to 2D = 25.8 cm–1, gav = 1.76 and J = –10.8 cm–1. The negative

value of exchange parameter for the compound 1 confirms antiferromagnetic interac-

tion in this complex. On the basis of the literature data concerning chloride and bro-

mide compounds [30–32] it can be assumed that the magnetic exchange interactions

The mixed-valence rhenium(IV, V) complexes 1671

0 50 100 150 200 250 300

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

T [K]

� M[c

m3 m

ol-1

]

0,0

0,2

0,4

0,6

0,8

1,0

1,2

�MT

[cm3m

ol -1K]

Figure 4. Thermal dependence of �M (�) and �MT (�) for (ChinH)3[Re2OCl10] complex. The linear

regression presents the theoretical value of �MT.

between ReIV cations can be realized in the crystal lattice via the bonding arrange-

ments such as Re–Cl---Re. The replacement of caesium cations by phenylarsonium

and ChinH+ cations results in the changes in packing of [Re2OCl10]3– ions in the crys-

tal lattice. This is accompanied by two effects: the diamagnetic dilution leading to the

lowering of a molar magnetic susceptibility, and the enhancement of distances be-

tween the paramagnetic Re4+ centers. In fact, the magnetic curves of compounds 2

and 3 show, that the �MT values smoothly decrease in a wide range of temperatures

and rapidly decrease at the lowest temperatures. The slight decrease of�MT can be at-

tributed to zero-field splitting effect of the ReIV ions. The lack of the maximum on the

relation�Re = f(T) suggests the absence of magnetic exchange and then the magnetic

properties are described to sufficient accuracy by the following Hamiltonian (13)

[33,34]

H + D S S Sz2 1

31� ��

�����

( ) + g||�HzSz + g��(HxSx + HySy) (13)

The parallel and perpendicular zero field susceptibilities for S =3/2 are (14,15) [35]:

�|| =N g

kT

D

kT

D

kT

�2 2

4

1 92

1

Re|| �� ��

��

� !

� ���

� !

exp

exp2

(14)

�� =N g

kT

kT

D

D

kT

D

k

�2 2

4

4 62

12

1

Re� �� � ��

��

� !

�

���

��

� �

exp

exp2

T

���

� !

(15)

where g(||, �) =2 1( )

(||, )

�

�

�

Least-squares fitting of the data leads to 2D = 36.7 cm–1

, gav = 1.72, R = 1.89 �10–4

for

the complex 2 and D = 38.8 cm–1, gav = 1.74 , R = 2.84 � 10–4 for 3.

Both molecular anions have a linear Cl–Re–O–Re–Cl group and it was observed

that the only difference is in the Re–O distance in the bridging system 1.865(2) for

[Re2OCl 10]4– and 1.832(3)� for [Re2OCl10]3– [10,36]. Metal–oxygen bond length in

the [Re2OCl10]4– ion exceeds by 0.03 � that in the [Re2OCl10]3– ion [10].

1672 B. Banaœ et al.

The IR spectra of 1, 2, 3 in Nujol film present a strong band at 316 cm–1 for 1,

330 cm–1 for 2 and 372 cm–1 for 3 corresponding to the �(Re–Cl) vibrations and a

sharp band at 172 cm–1 for 1, 175 cm–1 for 2 and 179 cm–1 for 3, "(Cl–Re–Cl) vibra-

tions.

The frequencies �asym and �sym concern M–O–M bridging core [37] observed at

about 860 and 230 cm–1, respectively. For a linear system, the symmetric stretching

vibrations, �sym, are forbidden in IR spectra, however they may be activated by crys-

tal field effects [38–40].

The frequencies �asym for examined compounds are observed at 858 for 1, 860 for

2 and 865 for 3; and �sym at 228 for 1, 235 for 2 and 239 cm–1 for 3.

The electronic spectra of both anions [Re2OCl10]4– and [Re2OCl10]3– , which have

been reported previously [41], showed the characteristic bands between 10000 and

50000 cm–1 (Fig. 5).

The band at 12500 cm–1 corresponds to a d-d transition and the bands of higher

wavelength halogen–metal and oxygen–metal are due to charge transfer transitions.

The most peculiar characteristics of the spectrum of Cs3[Re2OCl10] is a very inten-

sive band at 20150 cm–1(� 15000 per mol) of the MMCT type. The [Re2OCl10]4– ion

does not exhibit this band. The comparable intensity in the visible region of the spectrum

is often encountered in mixed-valence compounds [42]. The examples of such kind com-

plexes are K4[W2OCl10], � 20000 at 19100 cm–1 with tungstate(III, V) centers [43] and

The mixed-valence rhenium(IV, V) complexes 1673

200 300 400 500 6000,0

0,5

1,0

1,5

2,0

Abs

orpt

ion

Wavelength[nm]

Figure 5. The absorption spectra of 8.0�10–5

mol dm–3

[Re2OCl10]4–

complex in 0.1 mol dm–3

HClO4

solution (–) and after one-electron oxidation with hydrogen peroxide (���� ) and Cr(VI) ions

(-�-�-� -).

Ab

so

rptio

n

ruthenium red cation [44] [(NH3)5RuIII–O–RuIV(NH3)4–O–RuIII(NH3)5]6+ with � 21000 at 18800 cm–1. These facts suggest that some delocalization of valence elec-

trons may take place.

There were known the electron paramagnetic resonance spectra obtained for

Re4+ centers but only for the samples diluted in single crystals of (NH4)2PtCl6 [45,46]

and SnO2 [47]. Two isotopes 185Re and 187Re both have nuclear spins I = 5/2 and since

their magnetic moments are very nearly equal, they cannot be separately resolved in

these measurements [48]. EPR measurements were made for the dimeric salts of

[Re2OCl 10]3– with different cations using undiluted powder samples. For caesium

salt it was impossible to obtain the EPR spectrum, what probably can be related to the

high concentration of the Re4+ magnetic centers in the crystal lattice. The spectra of

good quality were obtained for salts having bulky ligands (Ph4As)3[Re2OCl10] and

(ChinH)3[Re2OCl10], in which the dimeric anions are practically magnetically iso-

lated in the crystal lattice. Observed in the EPR spectra single lines concern the elec-

tron transition 1/2 # –1/2 and these spectra are related to the electron transfer process

(Re4+ $ Re5+). With those effects there is connected a band at about 20000 cm–1 in

the electronic spectra. The unpaired electron oscillating in the Re–O–Re core be-

tween ReIV and ReV centers should react with both rhenium cores of the spin I = 5/2. In

the case of an interaction with the two rhenium centers, the asymmetric single broad

line should be split to eleven components {[2(I1+ I2) + 1] = 11 where I1 = I2 = 5/2}.

The effect of resonance line splitting was observed only for tetraphenylarsonium

salt of [Re2OCl10]3– at temperature of 123 K (Fig. 6).

On the very wide single line, with the peak to peak linewidth 1100 Oe, it is possi-

ble to observe superhyperfine structure composed of many lines. The main resonance

line shows a significant anisotropy and one may find the three groups of lines with the

1674 B. Banaœ et al.

1000 3000 5000 H [G]

Figure 6. EPR spectrum of a powdered sample of (Ph4As)3[Re2OCl10] compound.

T = 123 K

spectroscopic splitting parameters g1 = 4.8, g2 = 2.6 and g3 = 1.7. It is possible to ob-

serve the presence of six superhyperfine structure lines (2I1 + 1 = 6, where I1 = 5/2) of

good quality in the low field region, with the average coupling constant A = 200 Oe,

what suggests the possibility of electron interaction with one rhenium center mainly.

The reason of it may be the inequality of rhenium centers one of each (Re4+) is having

a strong magnetic properties when the other (Re5+) is diamagnetic.

Acknowledgments

This work was financially supported by the Ministry of Education and Science, Grant No. 1 T09A12430.

We express our gratitude to Prof. Adam Jezierski for valuable discussion of EPR data.

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