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Cobalt(II) halide complexes with ethyleneselenourea
) inorg, nucl Chem Vol. 41, pp 1111-1114 Pergamon Press Ltd. 1979. Printed in Great Britain
COBALT(II) HALIDE COMPLEXES WITH ETHYLENESELENOUREA FRANCESCO A. DEVILLANOVA and GAETANO VERANIt Istituto Chimico Policattedra, Via Ospedale 72, 09100 Cagliari, Italy (Received 5 January 1978; received[or publication 26 lanuary 1979) Abstract--The complexes obtained from ethyleneselenourea and cobalt(ll) halides have been prepared and characterized. They have formula CoL2X2 (X = CI, Br, I), tetrahedral stereochemistry and they are Se-bonded to the cobalt(ll). They are comparatively studied with the previously reported complexes of cobalt(lI) with ethylenethiourea, by IR and visible spectroscopy and magnetic susceptibilities. Using the average linand field approximation, the ethyleneselenourea is located at the very low end of the nephelauxetic series. INTRODUCTION Carlin and Holt[l] Studied the electronic spectra and the magnetism of tetrahedral Co(II) halide complexes with ethylenethiourea. These complexes had been previously prepared by Nardelli et aL [2], who identified the sulphur as donor atom by X-ray crystallographic techniques. Their far IR spectra were investigated by Adams and Cornell[3] and Carlin[l] located the ethylenethiourea at the low end of the nephelauxetic series, in agreement with the large polarizability of the sulphur. Pursuant to our interest[4, 5] in sulphur and selenium as coordinating atoms in heterocyclic pentaatomic rings :r
•
as HN.CH2.CH2"X'CY (where Y = S or Se and X = CH2, O, S, NH, NCH3 or NC2Hs), the complexes of the Co(II) halides with ethyleneselenourea have been prepared and investigated, with the aims (i) to verify the influence of the S- and Se-coordination to the Co(II) on the typical vibrations of the ethylenethiourea and ethyleneselenourea; (ii) to compare the donor ability between sulphur and selenium and locate the ethyleneselenourea in the nephelauxetic series. EXPERIMENTAL Ethylenethiourea and ethyleneselenourea were prepared according to literature[4, 17, 18]. fTo whom correspondence should be addressed.
All the complexes were obtained by refluxing in absolu~te ethanol the cobalt(ll) halide with the ligand (1:2). Measurements. The IR spectra were recorded on a PerkinElmer 325 spectrophotometer using KBr discs (4000--400cm -t) and as Nujol mulls between Csl pellets (450-200cm-'). The magnetic susceptibilities measurements were carried out by use of the Gouy method at different temperature (107-260°K) using liquid air. The visible spectra in acetone solution were recorded with a Perkin-Elmer model 402 spectrophotometer. The electronic spectra in the solid state were recorded with a Shimadzu MPS50L spectrophotometer in the range 4000-27000cm-~. The conductivity measurements were carried out with a WTW, LBR type conductivity bridge for 10-3 M solutions. RESULTS AND DISCUSSION The analytical data and some physical properties of the CoOI) complexes with ethyleneselenourea (ESU) are reported in Table I. The elemental analyses indicate that they are four-coordinate species and their colours suggest a tetrahedral stereochemistry. The conductivity measurements show them to be non-electrolytes, with a very low degree of solvolysis; this fact agrees with the formulation Co(ESUhX2, as found for the isologous ethylenethiourea complexes[l]. IR spectra. In Table 2, the principal bands due to the HN.CSe.NH group are reported for the free ligand and the complexes, together with the analogous bands of the
Table 1. Analytical data and some physical properties of the ESU complexes
Cor,,pound
Co]our
Co(ESU)2CI 2
turquoise
Co(ESU)2Br2
grass-green
Co(ESU)212
a)
-I
yellow green
Found (Calcd)%
M.p.(°C)
AMa)
C
H
N
(dec)
16.8 (16.8)
2.9 (2.8)
13.2 (13.1)
180
1.5
175
3.0
175
7.0
13.8
2.4
iO.7
(13.9)
(2.3)
(10.9)
11.7
2.0
9.1
(ll.8)
(2.0)
(9.2)
2 -i in acetone at 15°C. era moles
1111
1112
F. A. DEVILLANOVAand G. VERANI Table 2. Principal bands (4000-200 cm -j) of the Co(II) complexes with ETU and ESU"
Compound
vNH
ETUb)
3250vs
6NH + vCN 152Ovs-i5OOvs
vCY
ACY
5OTvs
337s
~CoY
vCoX
232ms
309s-.289ma
367sh-359ms
24Os
251s-24Os
37Om-363m
234ms
216s
Co(ETU)2C12
3385m-3310vs 3260vs
153Ors
502s
368ms
Co(ETU)2Br 2
3385s-3285s
1526vs
492s
Co(ETU)2I 2
3380m-3285s
1527vs
493s
325Ovs
1515vs-15OOvs
357m
276ms
3405s-328Osh
1520sh-1507vs
347ms
278sh-269ms
306s-294s
ESUb) Co(ESU)2CI 2
3360s-3240vs
Co(ESU)2Br 2
3405s-3280sh 3370s-3260vs
152Osh-1505vs
345m
287w-276mw
237s-216s
Co(ESU)212
3~lOs-3330s
1515vs--1505vs
344m
289w-278mw
21Os-< 200
a)
All the bands present down to 400 cm -I at 252w, 245mw and 220.5w cm
b)
Assignmentsaccordlng to ref. 6.
-i
are reported. ESU has also weak absorptions
Table 3. Magnetic data and electronic spectra of Co(II) tetrahedral complexes
v2
Compound
cm b)
-1
v3
cm
-i
Dq cm
B' -i
cm
-i
5870
14700
339
693
0.~1
Co(ETU)2Br2 b)
5740
14200
332
665
0.69
Co(ETU)212 b)
5560
13510
321
629
0.65
Co(ETU)2C12
Co(ETU)~ +
-
374 c) (378) d)
63q c)" (651)d)
0 66 c"J
" d) (o.67)
Co(ESU)2CI 2
5810
14280
335
669
0.69
Co(ESU)2Br 2
5560
13510
321
629
0.65
Co(ESU)212
5480
12820
3]?
5~96
O. 61
-
361
566
0.59
Co(ESU)~ +
a)
B (free ion) 967 cm -I according to ref. (ii). Reported from ref. (i).
c)
Calculated from Co(ETU)2X 2 parameters reported by Carlin (1)0
d)
Obtained by Carlin (i) for [Co(ETU)4](CIO~) 2.
!113
Cobalt(II)halide complexeswith ethyleneselenourea ethylenethiourea (ETU) and related complexes. It is known that these last are tetrahedral and S-bonded [1-3] to the Co(II). Upon S-coordination, the band, due to 8NH + vCN, undergoes upward shifts and vCS slightly moves towards lower wavenumbers, according to an increase of the ~r-bond on CN link and a decrease on CS. The same shifts, verified for the analogous bands in the ESU complexes, clearly denote the Se-coordination to the Co(II). The identation of the vNH-bands both for ETU and for ESU complexes might be due to different NH environments. From the C:~ local symmetry of Co Y2X2, two vCoX (A,+BO and two vCoY (A~+B2) vibrations, all IR active, are expected (cobalt and halogens lie on the xz plane). Our assignments for uCoY and vCoX are reported in the last two columns of Table 2. There, only one band is attributed to vCoS. The assignments of ETU complexes, previously reported by Adams[3], do not agree with ours, except for Co(ETUhCI2. The comparison of the two series of complexes allowed us to identify the vCoX vibrations, since they must be common to both the series; among vCoYvibrations, only the band at ca. 235 cm -~, common to the ETU complexes, has been attributed to vCoS in good agreement with literature[7]; the corresponding band in ESU complexes probably falls down to 200 cm -~. The second expected vCo Y vibration could con-
tribute to the absorptions at ca. 365 and 280 cm-' in ETU and ESU complexes respectively. This hypothesis is supported by the behaviour of the bands at 337 and 276 cm -~ of the free ligands. In fact, these bands, which undergo upward shifts on Y-coordination in pseudotetrahedral complexes with l i b group metals [4], are here split. Magnetic susceptibil#ies The magnetic measurements have been made at different temperatures (107-260°K) and the molar susceptivility values have been corrected for diamagnetism and temperature independent paramagnetism (TIP). The /z and 0 values, obtained from the straight lines of l / x ~ °rr vs T, are reported in Table 3. The correlation coefficients of the straight lines were in the range 0.998 1.000. The #~fr's lie in the expected range for Co(II) tetrahedral complexes [8]. Electronic spectra The spectra were recorded as solid and in acetone solution without adding an excess of ligand (see Table 3). The ~ values are coherent with a tetrahedral structure and they increase on passing from chlorine to iodine. The p2 and v3 bands are reported on the top of the Table 3 as 4Tl(F)*-4A2 and 4TI(P)*--4A2 respectively, hypothesi;,ing a Td symmetry. Really the complexes have lower symmetry than Td and if we consider only the local
Table 4. Electronic spectra and parameters for the Co(ll) tetrahedral complexes Complex
Co(ESU)2Cl 2
SoLvent
Ueff a)
e
4TI(F)~qA 2
4TI(P)+BA 2
B.M.
°K
v2(cm -)
~3b)(cm-l) 16.129(355)
Acetone
14.859(350) 13.928(276) Solid
4.70
q.7
5814
16.000 15.267 12.500
Co(ESU)2Br 2
15.480(525)
Acetone
14.528(575) 13.587(500)
Solid
4.98
-20
5555
15.270 14.490
12.345 Co(ESU)2I 2
15.576(642)
Acetone
14.535(812) 13.699(684) Solid
4.89
5.7
5480 14.180 12.120
a)
r corr e)]~. Calculated from the equation U = 2.84LX M (T -
h)
The molar extinction coefficients (£ cm -Imole-l) are reported in parentheses.
1114
F. A. DEVILLANOVAand G. VERANI
symmetry of CoSe2X2, they belong to the C2~ point group. This lowering of symmetry contributes to enrich the electronic spectra, which are themselves complicated also when the Co(II) is surrounded by four identical ligands[9, 10]. However, in order to calculate Dq, B' and /] we did not account of the lowering of symmetry and the values of u2 and us to solve the equations of Tanabe and Sugano were taken by visual estimation, according to Cotton et al. [9]. In Table 4 the chosen ~,2 and J'3 and the electronic structure parameters are reported together with the parameters of the ETU complexes previously reported[l]. All of these quantities fall in the usual ranges expected for tetrahedral Co(II) complexes. The spectrochemical properties of the ESU complexes do not differ substantially from the analogoues of ETU. In order to locate the ethyleneselenourea in the spectrochemical and nephelauxetic series, we tried without success to obtain the species Co(ESU)42+ with anions as CIO4- or BF4-; hence we used the average ligand field approximation to estimate the spectrochemical parameters of Co(ESU)42+ species, as suggested by Cotton[9]. This method has been tested on the well known Co(ETU)2Xe complexes, studied by Carlin[l] and here reported in Table 4. The agreement between the calculated values, averaged on the Co(ETU)2X2 (X = CI, Br, I), and the values obtained by Carlin for [Co(ETU)4](CIO4)2is fairly good. Hence, we have calculated the parameters of the Co(II) tetrahedrically surrounded by four molecules of ethyleneselenourea in order to insert this ligand in the spectrochemical and nephelauxetic series. The ESU position in the spectrochemical series for Co(II) in a tetrahedral field for similar ligands is: pyrrolidine-2-thione(326)[12] < N-methyl-pyrrolidine-2thione(338)[12] < ethyleneselenourea(361) < ethylenethiourea(372) [1] < N,N'-dimethylthioacetamide(387)[13] < ~-thiocaprolactam(391) [ 14] < selenourea(403) [15] < thiourea(425) [ 16]. As expected, the ethyleneselenourea/selenourea preceeds the ethylenethiourea/thiourea in the spectrochemical series. ethylenethiourea/thiourea in the spectrochemical series. The//value for Co(ESU)42+ (0.59), equal to that found for Co(SU)4z+ (SU = selenourea)[15], lower than that
found for Co(ETU)42+ (0.67), suggests a polarizability larger for selenium than for sulphur and indicates more appreciable orbital overlap for ESU complexes. Then, the position of the/] value for ESU is at the low end of the nephelauxetic series.
Acknowledgement--We thank the National Research Council (C.N.R.) of Italy for financial support.
REFERENCES
1. R. L. Carlin and S. L. Holt, Jr., lnorg. Chem. 2, 849 (1963). 2. M. Nardelli, I. Chierici and A. Braibanti, Gazz. Chim. ltal. 88, 37 (1958). 3. D. M. Adams and J. B. CornelI, J.C.S.(A),884 (1967). 4. F. A. Devillanova and G. Verani, Trans. Met. Chem. 2, 9 (1977). 5. F. Cristiani,F. A. Devillanova and G. Verani, Trans. Met. Chem. 2, 50 (1977); F. A. Devillanova and G. Verani, J. Coord. Chem. 7, 177 (1978). 6. F. A. Devillanova and G. Verani, J.C.S. Perkin II, 1529
(1977). 7. B. P. Kennedy and A. B. P. Lever, Can. J. Chem. 50, 3488 (1972). 8. H. L. $chi~fer and G. Gliemann, Basic Principles of Ligand Field Theory. Wiley-lnterscience,Bristol, England (1969). 9. F. A. Cotton, D. M. L. Goodgameand M. Goodgame,J. Am. Chem. Soc., g3, 4690 (1961) and Refs. therein. 10. A. B. P. Lever, Inorganic Electronic Spectroscopy (Edited by M. F. Lappert). Elsevier, Amsterdam(1958). 11. W. E. Slinkard and D. W. Meek, lnorg. Chem. $, 1911(1969). 12. S. K. Madan and M. Sulich, Inorg. Chem. 5, 1662(1966). 13. S. K. Madan and D. Mueller, J. lnorg. Nucl. Chem. 28, 177 fl966). 14. S. K. Madan and C. Goldstein,J. Inorg. Nucl. Chem. 28, 1251 (1966). 15. G. B. Aitken, J. L. Duncan and G. P. McQuillan, J.C.S. Dalton, 2103(1972), and Refs. therein. 16. F. A. Cotton, O. D. Faut and J. T. Mague, Inorg. Chem. 3, 17 (1964). 17. D. L. Klaymanand R. J. Shine, J. Org. Chem. 34, 3549(1969). 18. E. C. Homing, Org. Synth., Coll.3, 394 (1955).
COBALT(II) HALIDE COMPLEXES WITH ETHYLENESELENOUREA FRANCESCO A. DEVILLANOVA and GAETANO VERANIt Istituto Chimico Policattedra, Via Ospedale 72, 09100 Cagliari, Italy (Received 5 January 1978; received[or publication 26 lanuary 1979) Abstract--The complexes obtained from ethyleneselenourea and cobalt(ll) halides have been prepared and characterized. They have formula CoL2X2 (X = CI, Br, I), tetrahedral stereochemistry and they are Se-bonded to the cobalt(ll). They are comparatively studied with the previously reported complexes of cobalt(lI) with ethylenethiourea, by IR and visible spectroscopy and magnetic susceptibilities. Using the average linand field approximation, the ethyleneselenourea is located at the very low end of the nephelauxetic series. INTRODUCTION Carlin and Holt[l] Studied the electronic spectra and the magnetism of tetrahedral Co(II) halide complexes with ethylenethiourea. These complexes had been previously prepared by Nardelli et aL [2], who identified the sulphur as donor atom by X-ray crystallographic techniques. Their far IR spectra were investigated by Adams and Cornell[3] and Carlin[l] located the ethylenethiourea at the low end of the nephelauxetic series, in agreement with the large polarizability of the sulphur. Pursuant to our interest[4, 5] in sulphur and selenium as coordinating atoms in heterocyclic pentaatomic rings :r
•
as HN.CH2.CH2"X'CY (where Y = S or Se and X = CH2, O, S, NH, NCH3 or NC2Hs), the complexes of the Co(II) halides with ethyleneselenourea have been prepared and investigated, with the aims (i) to verify the influence of the S- and Se-coordination to the Co(II) on the typical vibrations of the ethylenethiourea and ethyleneselenourea; (ii) to compare the donor ability between sulphur and selenium and locate the ethyleneselenourea in the nephelauxetic series. EXPERIMENTAL Ethylenethiourea and ethyleneselenourea were prepared according to literature[4, 17, 18]. fTo whom correspondence should be addressed.
All the complexes were obtained by refluxing in absolu~te ethanol the cobalt(ll) halide with the ligand (1:2). Measurements. The IR spectra were recorded on a PerkinElmer 325 spectrophotometer using KBr discs (4000--400cm -t) and as Nujol mulls between Csl pellets (450-200cm-'). The magnetic susceptibilities measurements were carried out by use of the Gouy method at different temperature (107-260°K) using liquid air. The visible spectra in acetone solution were recorded with a Perkin-Elmer model 402 spectrophotometer. The electronic spectra in the solid state were recorded with a Shimadzu MPS50L spectrophotometer in the range 4000-27000cm-~. The conductivity measurements were carried out with a WTW, LBR type conductivity bridge for 10-3 M solutions. RESULTS AND DISCUSSION The analytical data and some physical properties of the CoOI) complexes with ethyleneselenourea (ESU) are reported in Table I. The elemental analyses indicate that they are four-coordinate species and their colours suggest a tetrahedral stereochemistry. The conductivity measurements show them to be non-electrolytes, with a very low degree of solvolysis; this fact agrees with the formulation Co(ESUhX2, as found for the isologous ethylenethiourea complexes[l]. IR spectra. In Table 2, the principal bands due to the HN.CSe.NH group are reported for the free ligand and the complexes, together with the analogous bands of the
Table 1. Analytical data and some physical properties of the ESU complexes
Cor,,pound
Co]our
Co(ESU)2CI 2
turquoise
Co(ESU)2Br2
grass-green
Co(ESU)212
a)
-I
yellow green
Found (Calcd)%
M.p.(°C)
AMa)
C
H
N
(dec)
16.8 (16.8)
2.9 (2.8)
13.2 (13.1)
180
1.5
175
3.0
175
7.0
13.8
2.4
iO.7
(13.9)
(2.3)
(10.9)
11.7
2.0
9.1
(ll.8)
(2.0)
(9.2)
2 -i in acetone at 15°C. era moles
1111
1112
F. A. DEVILLANOVAand G. VERANI Table 2. Principal bands (4000-200 cm -j) of the Co(II) complexes with ETU and ESU"
Compound
vNH
ETUb)
3250vs
6NH + vCN 152Ovs-i5OOvs
vCY
ACY
5OTvs
337s
~CoY
vCoX
232ms
309s-.289ma
367sh-359ms
24Os
251s-24Os
37Om-363m
234ms
216s
Co(ETU)2C12
3385m-3310vs 3260vs
153Ors
502s
368ms
Co(ETU)2Br 2
3385s-3285s
1526vs
492s
Co(ETU)2I 2
3380m-3285s
1527vs
493s
325Ovs
1515vs-15OOvs
357m
276ms
3405s-328Osh
1520sh-1507vs
347ms
278sh-269ms
306s-294s
ESUb) Co(ESU)2CI 2
3360s-3240vs
Co(ESU)2Br 2
3405s-3280sh 3370s-3260vs
152Osh-1505vs
345m
287w-276mw
237s-216s
Co(ESU)212
3~lOs-3330s
1515vs--1505vs
344m
289w-278mw
21Os-< 200
a)
All the bands present down to 400 cm -I at 252w, 245mw and 220.5w cm
b)
Assignmentsaccordlng to ref. 6.
-i
are reported. ESU has also weak absorptions
Table 3. Magnetic data and electronic spectra of Co(II) tetrahedral complexes
v2
Compound
cm b)
-1
v3
cm
-i
Dq cm
B' -i
cm
-i
5870
14700
339
693
0.~1
Co(ETU)2Br2 b)
5740
14200
332
665
0.69
Co(ETU)212 b)
5560
13510
321
629
0.65
Co(ETU)2C12
Co(ETU)~ +
-
374 c) (378) d)
63q c)" (651)d)
0 66 c"J
" d) (o.67)
Co(ESU)2CI 2
5810
14280
335
669
0.69
Co(ESU)2Br 2
5560
13510
321
629
0.65
Co(ESU)212
5480
12820
3]?
5~96
O. 61
-
361
566
0.59
Co(ESU)~ +
a)
B (free ion) 967 cm -I according to ref. (ii). Reported from ref. (i).
c)
Calculated from Co(ETU)2X 2 parameters reported by Carlin (1)0
d)
Obtained by Carlin (i) for [Co(ETU)4](CIO~) 2.
!113
Cobalt(II)halide complexeswith ethyleneselenourea ethylenethiourea (ETU) and related complexes. It is known that these last are tetrahedral and S-bonded [1-3] to the Co(II). Upon S-coordination, the band, due to 8NH + vCN, undergoes upward shifts and vCS slightly moves towards lower wavenumbers, according to an increase of the ~r-bond on CN link and a decrease on CS. The same shifts, verified for the analogous bands in the ESU complexes, clearly denote the Se-coordination to the Co(II). The identation of the vNH-bands both for ETU and for ESU complexes might be due to different NH environments. From the C:~ local symmetry of Co Y2X2, two vCoX (A,+BO and two vCoY (A~+B2) vibrations, all IR active, are expected (cobalt and halogens lie on the xz plane). Our assignments for uCoY and vCoX are reported in the last two columns of Table 2. There, only one band is attributed to vCoS. The assignments of ETU complexes, previously reported by Adams[3], do not agree with ours, except for Co(ETUhCI2. The comparison of the two series of complexes allowed us to identify the vCoX vibrations, since they must be common to both the series; among vCoYvibrations, only the band at ca. 235 cm -~, common to the ETU complexes, has been attributed to vCoS in good agreement with literature[7]; the corresponding band in ESU complexes probably falls down to 200 cm -~. The second expected vCo Y vibration could con-
tribute to the absorptions at ca. 365 and 280 cm-' in ETU and ESU complexes respectively. This hypothesis is supported by the behaviour of the bands at 337 and 276 cm -~ of the free ligands. In fact, these bands, which undergo upward shifts on Y-coordination in pseudotetrahedral complexes with l i b group metals [4], are here split. Magnetic susceptibil#ies The magnetic measurements have been made at different temperatures (107-260°K) and the molar susceptivility values have been corrected for diamagnetism and temperature independent paramagnetism (TIP). The /z and 0 values, obtained from the straight lines of l / x ~ °rr vs T, are reported in Table 3. The correlation coefficients of the straight lines were in the range 0.998 1.000. The #~fr's lie in the expected range for Co(II) tetrahedral complexes [8]. Electronic spectra The spectra were recorded as solid and in acetone solution without adding an excess of ligand (see Table 3). The ~ values are coherent with a tetrahedral structure and they increase on passing from chlorine to iodine. The p2 and v3 bands are reported on the top of the Table 3 as 4Tl(F)*-4A2 and 4TI(P)*--4A2 respectively, hypothesi;,ing a Td symmetry. Really the complexes have lower symmetry than Td and if we consider only the local
Table 4. Electronic spectra and parameters for the Co(ll) tetrahedral complexes Complex
Co(ESU)2Cl 2
SoLvent
Ueff a)
e
4TI(F)~qA 2
4TI(P)+BA 2
B.M.
°K
v2(cm -)
~3b)(cm-l) 16.129(355)
Acetone
14.859(350) 13.928(276) Solid
4.70
q.7
5814
16.000 15.267 12.500
Co(ESU)2Br 2
15.480(525)
Acetone
14.528(575) 13.587(500)
Solid
4.98
-20
5555
15.270 14.490
12.345 Co(ESU)2I 2
15.576(642)
Acetone
14.535(812) 13.699(684) Solid
4.89
5.7
5480 14.180 12.120
a)
r corr e)]~. Calculated from the equation U = 2.84LX M (T -
h)
The molar extinction coefficients (£ cm -Imole-l) are reported in parentheses.
1114
F. A. DEVILLANOVAand G. VERANI
symmetry of CoSe2X2, they belong to the C2~ point group. This lowering of symmetry contributes to enrich the electronic spectra, which are themselves complicated also when the Co(II) is surrounded by four identical ligands[9, 10]. However, in order to calculate Dq, B' and /] we did not account of the lowering of symmetry and the values of u2 and us to solve the equations of Tanabe and Sugano were taken by visual estimation, according to Cotton et al. [9]. In Table 4 the chosen ~,2 and J'3 and the electronic structure parameters are reported together with the parameters of the ETU complexes previously reported[l]. All of these quantities fall in the usual ranges expected for tetrahedral Co(II) complexes. The spectrochemical properties of the ESU complexes do not differ substantially from the analogoues of ETU. In order to locate the ethyleneselenourea in the spectrochemical and nephelauxetic series, we tried without success to obtain the species Co(ESU)42+ with anions as CIO4- or BF4-; hence we used the average ligand field approximation to estimate the spectrochemical parameters of Co(ESU)42+ species, as suggested by Cotton[9]. This method has been tested on the well known Co(ETU)2Xe complexes, studied by Carlin[l] and here reported in Table 4. The agreement between the calculated values, averaged on the Co(ETU)2X2 (X = CI, Br, I), and the values obtained by Carlin for [Co(ETU)4](CIO4)2is fairly good. Hence, we have calculated the parameters of the Co(II) tetrahedrically surrounded by four molecules of ethyleneselenourea in order to insert this ligand in the spectrochemical and nephelauxetic series. The ESU position in the spectrochemical series for Co(II) in a tetrahedral field for similar ligands is: pyrrolidine-2-thione(326)[12] < N-methyl-pyrrolidine-2thione(338)[12] < ethyleneselenourea(361) < ethylenethiourea(372) [1] < N,N'-dimethylthioacetamide(387)[13] < ~-thiocaprolactam(391) [ 14] < selenourea(403) [15] < thiourea(425) [ 16]. As expected, the ethyleneselenourea/selenourea preceeds the ethylenethiourea/thiourea in the spectrochemical series. ethylenethiourea/thiourea in the spectrochemical series. The//value for Co(ESU)42+ (0.59), equal to that found for Co(SU)4z+ (SU = selenourea)[15], lower than that
found for Co(ETU)42+ (0.67), suggests a polarizability larger for selenium than for sulphur and indicates more appreciable orbital overlap for ESU complexes. Then, the position of the/] value for ESU is at the low end of the nephelauxetic series.
Acknowledgement--We thank the National Research Council (C.N.R.) of Italy for financial support.
REFERENCES
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