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Dioxouranium(VI) and Zinc(II) complexes of bis(o-hydroxynaphthalde)oxaloyldihydrazone: synthesis and spectral characterization
Polyhedron Vol. 12, No. 19, PP. 2351-2354, Printed in Great Britain
1993 0
0277-5387193 $6.00 + .OO 1993 Pergamon Press Ltd
DIOXOUk4NIUM(VI) AND ZINC@) COMPLEXES OF BIS(o-HYDROXYNAPHTHALDE)OXALOYLDIHYDRAZONE: SYNTHESIS AND SPECTRAL CHARACTERIZATION RAM A. LAL,* LALIT M. MUKHERJEE, (Mrs) ALAKNANDA N. SIVA, ASIM PAL and SYAMAL ADHIKARI Department of Chemistry, Tripura University, Agartala-799 004, Tripura, India and KRISKAN K. NARANG and (Mm) MEENA K. SINGH Department of Chemistry, Banaras Hindu University, Varanasi-221 005, (UP), India (Received 6 April 1993 ; accepted 2 June 1993)
Abstract-The monometallic complexes [M(napoxlhH2)(H20)& - 2nH20 [where M = UO:+ have been synand Zn2+ , and napoxlhH, = bis(o-hydroxynaphthalde)oxaloyldihydrazone] thesized and characterized on the basis of the results obtained from molar conductance data, electronic, IR and ‘H NMR spectral studies. It is proposed that the complex [U02 (napoxlhH2)(H20)2]n - 2nH20 involves eight coordinated uraniums with six ligand atoms arranged in the equatorial plane perpendicular to the linear uranyl group involving dihydrazone in the enolic form with cis configuration, whereas [Zn(napoxlhH2)(H20)& * 2nH20 is octahedral involving enolic dihydrazone in the staggered configuration. The phenolicOH groups remain uncoordinated.
Acyl-, aroyl- and phthaloyldihydrazones containing azomethine, amide and phenolic functions are polyfunctional ligands. ’ These are capable of yielding polymeric polynuclear complexes involving ligand bridging and oxo-bridging.2*3 Metal complexes of salicylaldehyde dihydrazones, which yield high molecular weight oligomeric products by polymerization, have been studied in some detail, but those containing relatively bulkier aromatic aldehydes and ketones have been much less studied. By the introduction of bulky fragments into the dihydrazone skeleton4 it is possible to prepare polynuclear homo- and heterobimetallic complexes of dihydrazones having discrete molecularity. Bis (o-hydroxynaphthaldehyde)oxaloyldihydrazone is one such ligand capable of holding two or more metal atoms in close proximity, depending upon the preparation conditions. To the best of our knowl-
*Author to whom correspondence should be addressed.
edge, there is no report on a single complex formed by this ligand. The present paper reports the results of an investigation into the reaction of uranyl acetate and zinc acetate with bis(o-hydroxynaphthaldehyde)oxaloyldihydrazone, capable of existing in the staggered (A) and cis (B) conformations, and the structural assessment of the resultant products. RESULTS
AND DISCUSSION
The complexes isolated in the present study (Table 1) are air stable and decompose above 300°C. They are soluble in DMSO and DMF only. The complexes showed weight loss corresponding to two water molecules at cu 110°C and two water molecules at cu 180°C respectively. The water molecules lost at ca 110°C suggest that they are part of the lattice structure, while those lost at 180°C suggest that they are part of the coordination sphere. Their molar conductance values in DMSO are 2.5
2351
R. A. LAL et al.
2352
Cis
Staggered A
and 1.2 RP ’ cm2 mall ‘, indicating that they are non-electrolytes in this solvent.’ The fact that the complexes are single-phase species has been confirmed by the fact that the DMSO solutions of all the complexes show a uniform spot on a TLC plate covered with silica gel and developed using a mixture of petroleum ether, ether and methanol in the ratio 2:2: 1. The dihydrazone shows bands at 330 and 370 nm in DMSO solution. A DMSO solution of the zinc complex shows bands at 320,329,375 and 450 nm, while the uranyl complex shows two bands at 340 and 405 nm, respectively. The bands in the region 320-375 nm have their origin due to electronic transitions within the ligand, while those in the region 4OW50 nm have their origin due to ligand-to-metal charge transfer transitions. In the uranyl complex the band characteristic of the uranyl ion is obscured by ligand-to-metal charge transfer transitions. 6 The IR and ‘H NMR spectral data for the dihydrazone and its complexes are summarized in Table 2. The IR spectrum of the napoxlhH, shows bands in the regions 3500-3300sbr, 3382s, 3332s 3278s, 1655vs, 1616s and 1528 cm-’ due to v(NH)+v(OH), v(C=O), v(C=N) and amide II + v(C-O)(phenolic). The ‘H NMR spectrum of the free ligand in DMSO-d6 exhibits four and two
Table
1. Analysis,
B
proton signals at 6 12.72 and 9.75 ppm downfield of TMS, assigned to &NH)+a(OH) and 6(-C==N), respectively.3 The multiplet due to the naphthyl protons appears in the region 6 7.15-8.20 ppm. The signal at 6 12.72 ppm is relatively broad, indicating the presence of hydrogen bonding in the free dihydrazone. The absence of the strong amide I band and the appearance of a strong to very strong band in the region 1602-l 580 cm- ’ and a very strong band in the region 1535-l 533 cm- ’ confirm the enolization of the ligand on complexation. The C=N band shows a negative shift by ca 14-36 cm- ‘, indicating azomethine nitrogen coordination. The [UO, (napoxlhH2)(H,0)2]n - 2nH20 complex shows a strong band at 937 cm-’ corresponding to the v3 stretching frequency of UO :’ . The complex [U0,(napoxlhH2)(H20)2]n - 2nH20 shows doublets at 6 12.65,12.79 ppm and 9.79,9.97 ppm, corresponding to two phenolic-OH and two azomethine protons ; the corresponding peaks in [Zn(napoxlhH2)(H,0)2], * 2nH,O appear as singlets at 6 12.70 ppm and 6 9.70 ppm. The average 60H peak position in [U0,(napoxlhH2)(H20)& *2nH,O remains almost at the same position or slightly upfield shifted in the [Zn(napoxlhH2) (H,O),], - 2nH20 complex compared with that in
decomposition point, molar conductance data and per cent yields of dioxouranium(V1) zinc(I1) complexes of bis(o-hydroxynaphthaldehyde)oxaloyldihydrazone
Analysis : Found (talc.) % D.P. S. No.
Complex
1
[UOl(napoxlhH,)(H,O),],
2
and colour
[Zn(napoxlhH2)(H20)&,*2nH,0
- 2nH,O
(“C)
Yield (%)
> 300
85
> 300
90
M
C
H
32.3 (31.1) 11.3 (11.6)
37.1 (37.6) 51.8 (51.3)
3.2 (3.1) 4.3 (4.3)
N
Molar conductance (W ’ cm’ mall ‘) 2.5
(:::) 1.3 (1:::)
and
Dioxouranium(V1) and Zn” complexes
2353
the free ligand, thus ruling out the possibility of involvement of the phenolic-OH group in coordination.’ The average azomethine proton peak position in uranyl complex is downfield shifted by ca 0.13 ppm, while in the zinc complex it is slightly upfield shifted by ca 0.05 ppm. Such a feature H associated with the 6 -C=N signal in the complexes indicates coordination through the nitrogen atom of the azomethine group to the metal centre. H The downfield shift of 6 -C&N in the uranyl complex suggests that there is a drainage of electron density from the nitrogen atom of the azomethine grouping to the metal centre, while in the zinc complex the naphthyl ring electron density flows through the azomethine nitrogen atom to the metal centre. H The splitting of the 6 OH and 6 -C=N signals into doublets in the uranyl complex is related to the ability of the UO:+ ion to assemble the dihydrazone in the equatorial plane, which forces a cis conformation on the dihydrazone,8 whereas the zinc ion is unable to do so and the dihydrazone coordinates to the zinc centre in the staggered configuration. On the basis of the physicochemical data presented and discussed above the uranyl complex maybe suggested to involve an eight-coordinate uranium atom’ with dihydrazone in cis conformation in the enolic form, while the zinc complex may be proposed to be octahedral involving a staggered arrangement of the dihydrazone in the enolic form around the zinc ion, respectively. EXPERIMENTAL
-
cu
UOz(OAc)22H20, Zn(OAc),2H,O, diethyl oxahydrazine o-hydroxyhydrate and late, naphthaldehyde were BDH, AR or E Merck grade reagents. Oxaloyldihydrazine was prepared by reacting diethyl oxalate (1 mol) with hydrazine Bis(o-hydroxynaphthaldehydrate (2 mol). hyde)oxaloyldihydrazone was prepared by refluxing a hot dilute ethanol solution of oxaloyldihydrazine (1 mol) with o-hydroxynaphthaldehyde (2 mol). The yellow precipitate obtained was recrystallized from ethanol and dried in an electric oven at ca 7O”C, m.p. > 300°C. Found : C, 68.0 ; H, 4.2 ; N, 13.3.Calc.forC,,HIBN,0,:C,67.6;H,4.2;N, 13.1%. Uranium and zinc in the complexes were determined by standard literature methods. Carbon, hydrogen and nitrogen were determined by microanalysis. Water molecules were determined by heating the samples in an electric oven at cu 110°C
2354
R. A. LAL et al.
and 18O”C, respectively, and passing the vapour through a trap containing anhydrous copper sulphate, which turned blue, and estimating the weight loss. The molar conductance of the complexes at lo- 3 M dilution in DMSO was measured using a Direct Reading Conductivity meter 303 with a diptype conductivity cell. IR spectra were recorded on a Perkin-Elmer 983 spectrophotometer in the range 4000-180 cm-’ using KBr discs. The ‘H NMR spectra were recorded on a CFT-20, 90 MHz spectrometer using DMSO-d6 solution. Electronic spectra were recorded on a Shimadzu MPS-5000 multipurpose spectrophotometer in DMSO. Preparation
of the complexes
The complexes were prepared by the following general method. M(OAc)z-2H,O (UO:+, Zn*‘) (0.05 M, 50 cm3) in ethanol containing a trace of acetic acid was dissolved by gentle heating and stirring. Oxaloyldihydrazine (0.05 M, 55 cm3) in hot distilled water was added to the above solution accompanied by stirring for a period of 10 min. The reaction mixture was refluxed for 1 h and then filtered under hot conditions, washed with hot water, ethanol and finally with ether. The precipitate obtained above was suspended in ethanol, to which o-hydroxynaphthaldehyde solution (0.1 M, 75 cm’) in ethanol was added. The reaction mixture was refluxed. The progress of the reaction was followed with the help of TLC. The reaction was complete after 6 h in the case of the uranyl complex, while it required 20 h in the case of the zinc complex. The product was filtered in hot conditions, washed with hot ethanol, ether and dried in an air oven at ca 70°C.
Acknowledgements-The authors thank the Department of Atomic Energy, Government of India, Bombay, for financial assistance, the Head, Regional Sophisticated Instrumentation Centre, Lucknow, for providing carbon, hydrogen and nitrogen analytical data and ‘H NMR spectra and the Head, Regional Sophisticated Instrumentation Centre, North-Eastern Hill University, Shillong, for IR spectra. REFERENCES 1. R. L. Dutta and Md. M. Hossain, J. Sci. Ind. Res. 1985, 44, 635 ; F. C. J. M. Van Veggel, M. Bos, S. Harkema, H. Vande Bovenkamp, W. Verboom, J. Reedijk and D. N. Reinhoudt, J. Org. Chem. 1991, 56, 225. 2. K. Lal, S. P. Gupta and S. K. Sahni, Polyhedron 1986, 5, 1499 ; R. A. Lal, R. K. Thapa and S. Das, Znorg. Chim. Acta 1989,132, 129. 3. G. Paolucci, P. A. Vigato, G. Ross&to, U. Casellato and M. Vidali, Znorg. Chim. Acta 1982, 66, L71; A. Yacouta-Nour, M. M. Mostafa and A. K. T. Maki, Trans. Met. Chem. 1990, 15, 34; Spectrochim. Acta 1988,44A, 1291. 4. V. Sam, T. Picher, P. Palanca, P. Gomez-Romero, E. Llopis, J. A. Ramirez, D. Beltran and A. Cervilla, Znorg. Chem. 1991,30,3113. 5. W. J. Geary, Coord. Chem. Rev. 1971,7,81. 6. B. I. Kim Bong, C. Miyake and S. Imoto, J. Znorg. Nucl. Chem. 1975,37,763 ; I. S. Ahuja, C. L. Yadava and S. Tripathi, Ind. J. Chem. 1988,27A, 171. R. A. Lal, M. Husain, K. B. Singh and S. S. Bhattacharjee, Polyhedron 1991, 10, 779. R. L. Lintvedt, W. E. Lynch and J. K. Zehetmair, Irtorg. Chem. 1990, 29, 3009. A. F. Wells, Structural Inorganic Chemistry, 3rd edn. Oxford University Press, Oxford (1966) ; C. Cattalini, U. Croatto, S. Degetto and E. Tandello, Inorg. Chim. Acta 1971,5, 19.
1993 0
0277-5387193 $6.00 + .OO 1993 Pergamon Press Ltd
DIOXOUk4NIUM(VI) AND ZINC@) COMPLEXES OF BIS(o-HYDROXYNAPHTHALDE)OXALOYLDIHYDRAZONE: SYNTHESIS AND SPECTRAL CHARACTERIZATION RAM A. LAL,* LALIT M. MUKHERJEE, (Mrs) ALAKNANDA N. SIVA, ASIM PAL and SYAMAL ADHIKARI Department of Chemistry, Tripura University, Agartala-799 004, Tripura, India and KRISKAN K. NARANG and (Mm) MEENA K. SINGH Department of Chemistry, Banaras Hindu University, Varanasi-221 005, (UP), India (Received 6 April 1993 ; accepted 2 June 1993)
Abstract-The monometallic complexes [M(napoxlhH2)(H20)& - 2nH20 [where M = UO:+ have been synand Zn2+ , and napoxlhH, = bis(o-hydroxynaphthalde)oxaloyldihydrazone] thesized and characterized on the basis of the results obtained from molar conductance data, electronic, IR and ‘H NMR spectral studies. It is proposed that the complex [U02 (napoxlhH2)(H20)2]n - 2nH20 involves eight coordinated uraniums with six ligand atoms arranged in the equatorial plane perpendicular to the linear uranyl group involving dihydrazone in the enolic form with cis configuration, whereas [Zn(napoxlhH2)(H20)& * 2nH20 is octahedral involving enolic dihydrazone in the staggered configuration. The phenolicOH groups remain uncoordinated.
Acyl-, aroyl- and phthaloyldihydrazones containing azomethine, amide and phenolic functions are polyfunctional ligands. ’ These are capable of yielding polymeric polynuclear complexes involving ligand bridging and oxo-bridging.2*3 Metal complexes of salicylaldehyde dihydrazones, which yield high molecular weight oligomeric products by polymerization, have been studied in some detail, but those containing relatively bulkier aromatic aldehydes and ketones have been much less studied. By the introduction of bulky fragments into the dihydrazone skeleton4 it is possible to prepare polynuclear homo- and heterobimetallic complexes of dihydrazones having discrete molecularity. Bis (o-hydroxynaphthaldehyde)oxaloyldihydrazone is one such ligand capable of holding two or more metal atoms in close proximity, depending upon the preparation conditions. To the best of our knowl-
*Author to whom correspondence should be addressed.
edge, there is no report on a single complex formed by this ligand. The present paper reports the results of an investigation into the reaction of uranyl acetate and zinc acetate with bis(o-hydroxynaphthaldehyde)oxaloyldihydrazone, capable of existing in the staggered (A) and cis (B) conformations, and the structural assessment of the resultant products. RESULTS
AND DISCUSSION
The complexes isolated in the present study (Table 1) are air stable and decompose above 300°C. They are soluble in DMSO and DMF only. The complexes showed weight loss corresponding to two water molecules at cu 110°C and two water molecules at cu 180°C respectively. The water molecules lost at ca 110°C suggest that they are part of the lattice structure, while those lost at 180°C suggest that they are part of the coordination sphere. Their molar conductance values in DMSO are 2.5
2351
R. A. LAL et al.
2352
Cis
Staggered A
and 1.2 RP ’ cm2 mall ‘, indicating that they are non-electrolytes in this solvent.’ The fact that the complexes are single-phase species has been confirmed by the fact that the DMSO solutions of all the complexes show a uniform spot on a TLC plate covered with silica gel and developed using a mixture of petroleum ether, ether and methanol in the ratio 2:2: 1. The dihydrazone shows bands at 330 and 370 nm in DMSO solution. A DMSO solution of the zinc complex shows bands at 320,329,375 and 450 nm, while the uranyl complex shows two bands at 340 and 405 nm, respectively. The bands in the region 320-375 nm have their origin due to electronic transitions within the ligand, while those in the region 4OW50 nm have their origin due to ligand-to-metal charge transfer transitions. In the uranyl complex the band characteristic of the uranyl ion is obscured by ligand-to-metal charge transfer transitions. 6 The IR and ‘H NMR spectral data for the dihydrazone and its complexes are summarized in Table 2. The IR spectrum of the napoxlhH, shows bands in the regions 3500-3300sbr, 3382s, 3332s 3278s, 1655vs, 1616s and 1528 cm-’ due to v(NH)+v(OH), v(C=O), v(C=N) and amide II + v(C-O)(phenolic). The ‘H NMR spectrum of the free ligand in DMSO-d6 exhibits four and two
Table
1. Analysis,
B
proton signals at 6 12.72 and 9.75 ppm downfield of TMS, assigned to &NH)+a(OH) and 6(-C==N), respectively.3 The multiplet due to the naphthyl protons appears in the region 6 7.15-8.20 ppm. The signal at 6 12.72 ppm is relatively broad, indicating the presence of hydrogen bonding in the free dihydrazone. The absence of the strong amide I band and the appearance of a strong to very strong band in the region 1602-l 580 cm- ’ and a very strong band in the region 1535-l 533 cm- ’ confirm the enolization of the ligand on complexation. The C=N band shows a negative shift by ca 14-36 cm- ‘, indicating azomethine nitrogen coordination. The [UO, (napoxlhH2)(H,0)2]n - 2nH20 complex shows a strong band at 937 cm-’ corresponding to the v3 stretching frequency of UO :’ . The complex [U0,(napoxlhH2)(H20)2]n - 2nH20 shows doublets at 6 12.65,12.79 ppm and 9.79,9.97 ppm, corresponding to two phenolic-OH and two azomethine protons ; the corresponding peaks in [Zn(napoxlhH2)(H,0)2], * 2nH,O appear as singlets at 6 12.70 ppm and 6 9.70 ppm. The average 60H peak position in [U0,(napoxlhH2)(H20)& *2nH,O remains almost at the same position or slightly upfield shifted in the [Zn(napoxlhH2) (H,O),], - 2nH20 complex compared with that in
decomposition point, molar conductance data and per cent yields of dioxouranium(V1) zinc(I1) complexes of bis(o-hydroxynaphthaldehyde)oxaloyldihydrazone
Analysis : Found (talc.) % D.P. S. No.
Complex
1
[UOl(napoxlhH,)(H,O),],
2
and colour
[Zn(napoxlhH2)(H20)&,*2nH,0
- 2nH,O
(“C)
Yield (%)
> 300
85
> 300
90
M
C
H
32.3 (31.1) 11.3 (11.6)
37.1 (37.6) 51.8 (51.3)
3.2 (3.1) 4.3 (4.3)
N
Molar conductance (W ’ cm’ mall ‘) 2.5
(:::) 1.3 (1:::)
and
Dioxouranium(V1) and Zn” complexes
2353
the free ligand, thus ruling out the possibility of involvement of the phenolic-OH group in coordination.’ The average azomethine proton peak position in uranyl complex is downfield shifted by ca 0.13 ppm, while in the zinc complex it is slightly upfield shifted by ca 0.05 ppm. Such a feature H associated with the 6 -C=N signal in the complexes indicates coordination through the nitrogen atom of the azomethine group to the metal centre. H The downfield shift of 6 -C&N in the uranyl complex suggests that there is a drainage of electron density from the nitrogen atom of the azomethine grouping to the metal centre, while in the zinc complex the naphthyl ring electron density flows through the azomethine nitrogen atom to the metal centre. H The splitting of the 6 OH and 6 -C=N signals into doublets in the uranyl complex is related to the ability of the UO:+ ion to assemble the dihydrazone in the equatorial plane, which forces a cis conformation on the dihydrazone,8 whereas the zinc ion is unable to do so and the dihydrazone coordinates to the zinc centre in the staggered configuration. On the basis of the physicochemical data presented and discussed above the uranyl complex maybe suggested to involve an eight-coordinate uranium atom’ with dihydrazone in cis conformation in the enolic form, while the zinc complex may be proposed to be octahedral involving a staggered arrangement of the dihydrazone in the enolic form around the zinc ion, respectively. EXPERIMENTAL
-
cu
UOz(OAc)22H20, Zn(OAc),2H,O, diethyl oxahydrazine o-hydroxyhydrate and late, naphthaldehyde were BDH, AR or E Merck grade reagents. Oxaloyldihydrazine was prepared by reacting diethyl oxalate (1 mol) with hydrazine Bis(o-hydroxynaphthaldehydrate (2 mol). hyde)oxaloyldihydrazone was prepared by refluxing a hot dilute ethanol solution of oxaloyldihydrazine (1 mol) with o-hydroxynaphthaldehyde (2 mol). The yellow precipitate obtained was recrystallized from ethanol and dried in an electric oven at ca 7O”C, m.p. > 300°C. Found : C, 68.0 ; H, 4.2 ; N, 13.3.Calc.forC,,HIBN,0,:C,67.6;H,4.2;N, 13.1%. Uranium and zinc in the complexes were determined by standard literature methods. Carbon, hydrogen and nitrogen were determined by microanalysis. Water molecules were determined by heating the samples in an electric oven at cu 110°C
2354
R. A. LAL et al.
and 18O”C, respectively, and passing the vapour through a trap containing anhydrous copper sulphate, which turned blue, and estimating the weight loss. The molar conductance of the complexes at lo- 3 M dilution in DMSO was measured using a Direct Reading Conductivity meter 303 with a diptype conductivity cell. IR spectra were recorded on a Perkin-Elmer 983 spectrophotometer in the range 4000-180 cm-’ using KBr discs. The ‘H NMR spectra were recorded on a CFT-20, 90 MHz spectrometer using DMSO-d6 solution. Electronic spectra were recorded on a Shimadzu MPS-5000 multipurpose spectrophotometer in DMSO. Preparation
of the complexes
The complexes were prepared by the following general method. M(OAc)z-2H,O (UO:+, Zn*‘) (0.05 M, 50 cm3) in ethanol containing a trace of acetic acid was dissolved by gentle heating and stirring. Oxaloyldihydrazine (0.05 M, 55 cm3) in hot distilled water was added to the above solution accompanied by stirring for a period of 10 min. The reaction mixture was refluxed for 1 h and then filtered under hot conditions, washed with hot water, ethanol and finally with ether. The precipitate obtained above was suspended in ethanol, to which o-hydroxynaphthaldehyde solution (0.1 M, 75 cm’) in ethanol was added. The reaction mixture was refluxed. The progress of the reaction was followed with the help of TLC. The reaction was complete after 6 h in the case of the uranyl complex, while it required 20 h in the case of the zinc complex. The product was filtered in hot conditions, washed with hot ethanol, ether and dried in an air oven at ca 70°C.
Acknowledgements-The authors thank the Department of Atomic Energy, Government of India, Bombay, for financial assistance, the Head, Regional Sophisticated Instrumentation Centre, Lucknow, for providing carbon, hydrogen and nitrogen analytical data and ‘H NMR spectra and the Head, Regional Sophisticated Instrumentation Centre, North-Eastern Hill University, Shillong, for IR spectra. REFERENCES 1. R. L. Dutta and Md. M. Hossain, J. Sci. Ind. Res. 1985, 44, 635 ; F. C. J. M. Van Veggel, M. Bos, S. Harkema, H. Vande Bovenkamp, W. Verboom, J. Reedijk and D. N. Reinhoudt, J. Org. Chem. 1991, 56, 225. 2. K. Lal, S. P. Gupta and S. K. Sahni, Polyhedron 1986, 5, 1499 ; R. A. Lal, R. K. Thapa and S. Das, Znorg. Chim. Acta 1989,132, 129. 3. G. Paolucci, P. A. Vigato, G. Ross&to, U. Casellato and M. Vidali, Znorg. Chim. Acta 1982, 66, L71; A. Yacouta-Nour, M. M. Mostafa and A. K. T. Maki, Trans. Met. Chem. 1990, 15, 34; Spectrochim. Acta 1988,44A, 1291. 4. V. Sam, T. Picher, P. Palanca, P. Gomez-Romero, E. Llopis, J. A. Ramirez, D. Beltran and A. Cervilla, Znorg. Chem. 1991,30,3113. 5. W. J. Geary, Coord. Chem. Rev. 1971,7,81. 6. B. I. Kim Bong, C. Miyake and S. Imoto, J. Znorg. Nucl. Chem. 1975,37,763 ; I. S. Ahuja, C. L. Yadava and S. Tripathi, Ind. J. Chem. 1988,27A, 171. R. A. Lal, M. Husain, K. B. Singh and S. S. Bhattacharjee, Polyhedron 1991, 10, 779. R. L. Lintvedt, W. E. Lynch and J. K. Zehetmair, Irtorg. Chem. 1990, 29, 3009. A. F. Wells, Structural Inorganic Chemistry, 3rd edn. Oxford University Press, Oxford (1966) ; C. Cattalini, U. Croatto, S. Degetto and E. Tandello, Inorg. Chim. Acta 1971,5, 19.