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Palladium(II) complexes with dithiooxamide, N,N′-dimethyl- and N,N′-dicyclohexyl-dithiooxamide
J. inorg, nucl. Chem., 1973, Vol. 35, pp. 973-977.
Pergamon Press.
Printed in Great Britain
PALLADIUM(II) COMPLEXES WITH DITHIOOXAMIDE, N,N'-DIMETHYL- A N D N,N'DICYCLOHEXYL-DITHIOOXAMIDE G. PEYRONEL, A. C. FABRETTI and G. C. PELLACANI Instituto di Chimica Generale ed Inorganica, University of Modena, 41100 Modena, Italy (Received 15 February 1972) Abstract-Pailadium(II) gives diamagnetic square-planar complexes: PdL2, PdL~(CIO4)~ and PdL3 (CIO4)~ with dithiooxamide; PdLX~ (X = Cl, Br, I) and PdL~X2 (X = CI, Br, CIO4) with N, N'dimethyl- and N,N'-dicyclohexyl-dithiooxamide. I.R. ~(CN) and v(CS) bands show that these dithiooxamides act as S,N bidentate ligands, v(PdN) and v(PdS) hands are assignable in the regions 480425 and 410-320 cm-1, respectively, v(PdX) bands, in PdLX2 complexes, are characteristic of terminal metal-halogen bonds. Electronic spectra show only one band assignable to d-d transitions.
INTRODUCTION IN THE s q u a r e - p l a n a r c o m p l e x e s PdLX2 (X = CI, Br, I) a n d PdLz(CIO4)~ tetram e t h y l - a n d t e t r a e t h y l - d i t h i o o x a m i d e [ 1 ] b e h a v e as b i d e n t a t e s u l p h u r ligands. T h e p a l l a d i u m ( I I ) c o m p l e x e s with d i t h i o o x a m i d e (DH4), N , N ' - d i m e t h y l - ( M e t z D H z ) a n d N , N ' - d i c y c l o h e x y l - d i t h i o o x a m i d e ( D C H z D H 2 ) h a v e n o w b e e n prep a r e d a n d s t u d i e d by p h y s i c a l m e t h o d s in o r d e r to a s c e r t a i n h o w t h e s e l i g a n d s are c o o r d i n a t e d a n d to c o m p a r e t h e m with the similar n i c k e l ( I I ) c o m p l e x e s [2]. EXPERIMENTAL All reagents were of the best chemical grade. Acid used: HAc (glacial), HC1 (37%), HBr (48%), HI (57%), HCIO4 (60%). The following preparation methods were used: (a) the complexes PdL~X2 (L = MehDHz, X = CI, Br, C104; L = DCHzDH2, X = CIO4) and Pd(MehDH2)I2 were obtained by adding the metal salt solution in the hydrogen halides or perchloric acid to an acetic acid solution of the ligand (M:L ratio 1: 2 to 1: 1; HX: HA ratio 1.5:8.5); (b) the complexes PdLXz (L = Met2DHz, DCH~DH2; X = C1, Br) were obtained by adding the acetic acid solution of the ligand to the metal solution in the hydrogen halide (M :L = 2: 1; HX: HA = 1-5: 8.5); (c) the complexes Pd(DCH2DH2) Is, Pd(DCHzDHz)zX2 (X = C1, Br) and Pd(DH3)z were obtained by adding the metal salt solution in the hydrogen halide to the enthanolic solution of the ligand ( M : L = 1:2 to 1:1; HX:ethanol = 1:9); (d) the complex Pd(DH4)3(C104)z was obtained by adding a solution of the metal salt (3 mM) in HCIO4 (3 ml) to the solution of the ligand (4 mM) in 45 ml HAc + 5 ml HCIO4; (e) the complex Pd(DH4)2(CIO4)z was obtained by adding a solution of the metal salt (3 mM) in 10 ml HCIO4 to the solution of the ligand (4 mM) in 45 ml HAc + 5 ml HCIO4. The complexes can not be recrystailized. Magnetic susceptibilities and molar conductivities were measured and infrared and electronic spectra were recorded by conventionalmethods as previously described [3]. RESULTS AND DISCUSSION A l l the c o m p l e x e s d e s c r i b e d here are new. A n a l y s e s a n d c o l o u r s are r e p o r t e d in T a b l e 1. It is r e m a r k a b l e that in a s t r o n g acid like H C I , D H 4 gives o n l y Pd1. A.C. Fabretti, G. C. Peilacani and G. Peyronel, J. inorg, nucl. Chem., 33, 4247 (1971). 2. G. Peyronel, G. C. Pellacani and A. Pignedoli, lnorg. Chim. Acta. 5, 627 (1971). 3. G. Peyronel, G. C. Pellacani, A. Pignedoli and G. Benetti, lnorg. Chim. ,4cta 5, 263 (1971). 973
974
G. PEYRONEL, A. C. FABRETTI and G. C. PELLACANI Table I. Analytical results, found and calculated (in brackets) Complexes
Colour
Pd(DH3)~ Pd(DH4)2(C104)~ Pd(DH4)a(C104)2 Pd(Met2DH2)C12
Red Red Red Redorange Redorange Red Darkyellow Brown Orangeyellow Redorange Redorange Red Redorange Brown Orangeyellow
Pd(Met2DH2)Br~ Pd(Met2DH~)I2 Pd(MeqDH~)~CI~ Pd(Met2DH2)~Br2 Pd(Met~DH~)z(CIO4)2 Pd(DCH2DH2) CI2 Pd(DCHsDH~)Br2 Pd(DCH2DH2)I2 Pd(DCH~DH2)2CI~ Pd(DCH2DH~)2Br~ Pd(DCH2DH2)2(CIO4)2
Pd(%)
S(%)
C(%)
H(%)
30"31 (30.68) 36.39(36.98) 19.42 (19.51) 23.34(23"50) 15.94 (15.99) 28"50 (28.85) 32.29 (32.69)
14.76 (14.75)
2.42 (2.48)
25'30 (25.67) 20.63 (20.93) 12.65(12.61)
11.84 (11.58)
2.05 (1.95)
17.07 (17.06)
2.99 (2.87)
23.04(23.05)
35.98 (36-39)
5.32 (5.24)
19.10(19.33) 16.60 (16.51) 10.00(9.95)
30.16 (30.51)
4.37 (4.32)
22.23 (22-45) 27.11 (27.07) 18.86(18.91) 22.31 (22.80) 17.64(17.68) 21"66 (21"32)
14.89(14-26) 17.44(17-19) 12.98 (12.74) 15.67(15-36) 12.35(12.17) 14.84(14-68)
(DH3)2, while in HC104 it gives two different compounds Pd/-~(CIO4)2 and Pd/-~(C104)2, depending on a greater or lower concentration of HCIO4, respectively. Met2DH~ and DCH2DH2 give two types of complexes: PdLX2 (X = CI, Br, I) and P d ~ X 2 (X = CI, Br, C104), the formation of which depends on whether the ligand is added to the metal or vice versa. All the complexes are diamagnetic. In D M F solution PdL2(CIO4)2 complexes behave as uni-divalent electrolytes (AM~ 1 5 0 f l - l m o l e - l c m 2) and Pd(DHa)2 as a non-electrolyte (AM=9f1-1 mole -1 cmZ). All the other complexes are only slightly soluble, or soluble with decomposition in D M F , so that their conductivities are unreliable. The infrared spectra. All these complexes are remarkably similar to one another with regard to the shifts of their principal bands (Table 2), and since the latter are assignable [4, 5] to modes influenced by their respective coordination, these may be considered to be very similar. From the splitting of the v(NH), v(CN) and v(CS) bands one may conclude that the ligands act as bidentate chelaing agents, being coordinated to the metal via nitrogen and sulphur atoms [2]. The far i.r. bands of the ligands are practically unaltered in the complexes which show new bands in the region of 480-425 cm -1 assignable to v(PdN) modes [2, 6] and new bands in the region of 410-315 cm -~ assignable to v(PdS) 4. T.A. Scott, Jr. and E. L. Wagner, J. chem Phys. 30, 465 (1959). 5. H. O. Desseyn and M. A. Herman, Spectrochim. Acta 23A, 2457 (1967). 6. J. R. During and D. W. Wertz, Appl. Spectrosc. 22, 627 (1968).
Pd(II) complexes of DH4, MetzDH~ and DCH~DH~
r~
~
~
975
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0 e~
0
z
~
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=
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.
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976
G. PEYRONEL, A. C. FABRETTI and G. C. PELLACANI Table 3. Electronic spectra of the solid compounds
Pd(DH3)z 18870sh 22570 Pd(DH4)n(CIO4)2 1 7 2 4 0 20410sh Pd(Met2DHz)CI2 18520sh 21510 Pd(Met2DHz)Br~ 18180sh 21510 Pd(Met~DH2)I~ 1 8 0 2 0 s h 19610 Pd(MehDH2)2C12 21740 Pd(Met2DH2)2 Br2 19050sh 21050 Pd(MehDH2) ~(CIO4)~19050sh 21740 Pd(DCH2DH~)C12 21050 Pd(DCHzDH)Br2 20000sh Pd(DCH~DH2)I2 1 8 8 7 0 20410 Pd(DCH~DH~)2CI2 19050sh 21460 Pd(DCH2DH2)2Br2 18980sh 20410 Pd(DCH2DH~)~(CIO4)2 21600
22220sh, 25970sh, 30770 24390,35090 23810,28980sh, 34130 24100,32790 23530sh, 25000sh, 26670sh 22990sh, 24570sh, 33000 23260sh, 27030sh 22470,24570,33330 22220,24570,35710 21050sh, 25000sh, 30300 23260sh, 27030sh 22470sh, 27030sh 22990,30300sh
modes [1,7, 8]. Pd-halogen stretching vibrations for PdLX2 (X = CI, Br, I) are found in the expected range for terminal Pd-X bonds; this, and the fact that there are two bands assignable to asymmetric and symmetric Pd-C1 stretching vibrations for PdLCI~, lead one to the conclusion that these complexes have a C2~ symmetry [ 1, 7, 9]. The complexes PdLI~ show a v(PdI) band with a very close shoulder for Met2 DH2- and only a single band for the DCH~DH2-complex. Other cis complexes show similar anomalies. Two very close v(PtBr) bands (224 and 220 cm -1) were reported for the cis Pt(NH3)2Br~ complex [7], and only one v(MBr) band for the cis M(PhSe-CoH6-SePh)X~ (M = Pd, Pt) complexes [7]. The complexes PdLC12 show a difference between the two ~,(PdCI) bands of 14 cm -1 for the Met~DH~and of 32 cm -1 for the DCH~DH2-complex, while the two bromides have the same v(PdBr) values. Apparently the splitting of the v(MX) bands in the ciscomplexes may depend on the combination of several factors: the nature of the halogen atom, the inductive effect and the steric hindrance of the ligand and the environment in the crystal. Two new strong bands, as for the Ni(II) complexes [2], appear in the Pd(II) complexes at 1213-1100 cm -1. These new bands are assigned to z,(CRN) modes; they are enhanced by the complexation through nitrogen being stronger than that through sulphur. This agrees with a v(PdN) mode at an energy higher than that of the v(PdS) mode. Two v(CnN) bands correspond to the two differently bonded halves of the ligand molecule. Although palladium is a class B metal, with these dithiooxamides it gives, like nickel(II)[2], an S,N-coordination instead of the S,S-coordination observed for the tetrasubstituted dithiooxamides [ 1]. The i.r. bands of the perchlorate ion show a lowering of its Ta symmetry, probably, due to an interaction of this anion with the metal or with some positively-charged H R N group [2]. Similar interactions may also occur for the other anions. 7. D. M. Adams, Metal-Ligands and Related Vibrations, pp. 302, 325, 326. E. Arnold, London (1967). 8. G.W. Watt and B. J. McCormick, Spectrochim. Acta 21, 753 (1965). 9. W. Kitching, G. J. Moore and D. DoddreU, lnorg. Chem. 9, 521 (1970).
Pd(II) complexes of DH4, Met2DH~ and DCH2DH2
977
The i.r. spectrum of the compound Pd(DH4)a(CIO4)z shows the superposition of the bands of the complex Pd(DH4)z(CIO4)z and of the free ligand DH4. One molecule of the ligand is therefore present in the crystal as solvating agent and the complex must be formulated as Pd(DH4)z(C104)2"(DH4). The electronic spectra. The solid complexes in nujol mulls (Table 3) show a weak band or shoulder at 17-19 kK and a strong band at 19.5-22.5 kK. The great difference in their respective intensities suggests that the second band may be charge-transfer rather than ligand-field in character. Only the shoulder can be assigned to a d - d transition. The fact that this first d - d band is at a higher energy for palladium(II) (17-19 kK) than for nickel(II) (15-16.8 kK) [2] agrees with the spectrochemical order of these metal ions. Acknowledgements-This work was supported by financial aid from the Consiglio Nazionale delle Ricerche of Italy.
Pergamon Press.
Printed in Great Britain
PALLADIUM(II) COMPLEXES WITH DITHIOOXAMIDE, N,N'-DIMETHYL- A N D N,N'DICYCLOHEXYL-DITHIOOXAMIDE G. PEYRONEL, A. C. FABRETTI and G. C. PELLACANI Instituto di Chimica Generale ed Inorganica, University of Modena, 41100 Modena, Italy (Received 15 February 1972) Abstract-Pailadium(II) gives diamagnetic square-planar complexes: PdL2, PdL~(CIO4)~ and PdL3 (CIO4)~ with dithiooxamide; PdLX~ (X = Cl, Br, I) and PdL~X2 (X = CI, Br, CIO4) with N, N'dimethyl- and N,N'-dicyclohexyl-dithiooxamide. I.R. ~(CN) and v(CS) bands show that these dithiooxamides act as S,N bidentate ligands, v(PdN) and v(PdS) hands are assignable in the regions 480425 and 410-320 cm-1, respectively, v(PdX) bands, in PdLX2 complexes, are characteristic of terminal metal-halogen bonds. Electronic spectra show only one band assignable to d-d transitions.
INTRODUCTION IN THE s q u a r e - p l a n a r c o m p l e x e s PdLX2 (X = CI, Br, I) a n d PdLz(CIO4)~ tetram e t h y l - a n d t e t r a e t h y l - d i t h i o o x a m i d e [ 1 ] b e h a v e as b i d e n t a t e s u l p h u r ligands. T h e p a l l a d i u m ( I I ) c o m p l e x e s with d i t h i o o x a m i d e (DH4), N , N ' - d i m e t h y l - ( M e t z D H z ) a n d N , N ' - d i c y c l o h e x y l - d i t h i o o x a m i d e ( D C H z D H 2 ) h a v e n o w b e e n prep a r e d a n d s t u d i e d by p h y s i c a l m e t h o d s in o r d e r to a s c e r t a i n h o w t h e s e l i g a n d s are c o o r d i n a t e d a n d to c o m p a r e t h e m with the similar n i c k e l ( I I ) c o m p l e x e s [2]. EXPERIMENTAL All reagents were of the best chemical grade. Acid used: HAc (glacial), HC1 (37%), HBr (48%), HI (57%), HCIO4 (60%). The following preparation methods were used: (a) the complexes PdL~X2 (L = MehDHz, X = CI, Br, C104; L = DCHzDH2, X = CIO4) and Pd(MehDH2)I2 were obtained by adding the metal salt solution in the hydrogen halides or perchloric acid to an acetic acid solution of the ligand (M:L ratio 1: 2 to 1: 1; HX: HA ratio 1.5:8.5); (b) the complexes PdLXz (L = Met2DHz, DCH~DH2; X = C1, Br) were obtained by adding the acetic acid solution of the ligand to the metal solution in the hydrogen halide (M :L = 2: 1; HX: HA = 1-5: 8.5); (c) the complexes Pd(DCH2DH2) Is, Pd(DCHzDHz)zX2 (X = C1, Br) and Pd(DH3)z were obtained by adding the metal salt solution in the hydrogen halide to the enthanolic solution of the ligand ( M : L = 1:2 to 1:1; HX:ethanol = 1:9); (d) the complex Pd(DH4)3(C104)z was obtained by adding a solution of the metal salt (3 mM) in HCIO4 (3 ml) to the solution of the ligand (4 mM) in 45 ml HAc + 5 ml HCIO4; (e) the complex Pd(DH4)2(CIO4)z was obtained by adding a solution of the metal salt (3 mM) in 10 ml HCIO4 to the solution of the ligand (4 mM) in 45 ml HAc + 5 ml HCIO4. The complexes can not be recrystailized. Magnetic susceptibilities and molar conductivities were measured and infrared and electronic spectra were recorded by conventionalmethods as previously described [3]. RESULTS AND DISCUSSION A l l the c o m p l e x e s d e s c r i b e d here are new. A n a l y s e s a n d c o l o u r s are r e p o r t e d in T a b l e 1. It is r e m a r k a b l e that in a s t r o n g acid like H C I , D H 4 gives o n l y Pd1. A.C. Fabretti, G. C. Peilacani and G. Peyronel, J. inorg, nucl. Chem., 33, 4247 (1971). 2. G. Peyronel, G. C. Pellacani and A. Pignedoli, lnorg. Chim. Acta. 5, 627 (1971). 3. G. Peyronel, G. C. Pellacani, A. Pignedoli and G. Benetti, lnorg. Chim. ,4cta 5, 263 (1971). 973
974
G. PEYRONEL, A. C. FABRETTI and G. C. PELLACANI Table I. Analytical results, found and calculated (in brackets) Complexes
Colour
Pd(DH3)~ Pd(DH4)2(C104)~ Pd(DH4)a(C104)2 Pd(Met2DH2)C12
Red Red Red Redorange Redorange Red Darkyellow Brown Orangeyellow Redorange Redorange Red Redorange Brown Orangeyellow
Pd(Met2DH2)Br~ Pd(Met2DH~)I2 Pd(MeqDH~)~CI~ Pd(Met2DH2)~Br2 Pd(Met~DH~)z(CIO4)2 Pd(DCH2DH2) CI2 Pd(DCHsDH~)Br2 Pd(DCH2DH2)I2 Pd(DCH~DH2)2CI~ Pd(DCH2DH~)2Br~ Pd(DCH2DH2)2(CIO4)2
Pd(%)
S(%)
C(%)
H(%)
30"31 (30.68) 36.39(36.98) 19.42 (19.51) 23.34(23"50) 15.94 (15.99) 28"50 (28.85) 32.29 (32.69)
14.76 (14.75)
2.42 (2.48)
25'30 (25.67) 20.63 (20.93) 12.65(12.61)
11.84 (11.58)
2.05 (1.95)
17.07 (17.06)
2.99 (2.87)
23.04(23.05)
35.98 (36-39)
5.32 (5.24)
19.10(19.33) 16.60 (16.51) 10.00(9.95)
30.16 (30.51)
4.37 (4.32)
22.23 (22-45) 27.11 (27.07) 18.86(18.91) 22.31 (22.80) 17.64(17.68) 21"66 (21"32)
14.89(14-26) 17.44(17-19) 12.98 (12.74) 15.67(15-36) 12.35(12.17) 14.84(14-68)
(DH3)2, while in HC104 it gives two different compounds Pd/-~(CIO4)2 and Pd/-~(C104)2, depending on a greater or lower concentration of HCIO4, respectively. Met2DH~ and DCH2DH2 give two types of complexes: PdLX2 (X = CI, Br, I) and P d ~ X 2 (X = CI, Br, C104), the formation of which depends on whether the ligand is added to the metal or vice versa. All the complexes are diamagnetic. In D M F solution PdL2(CIO4)2 complexes behave as uni-divalent electrolytes (AM~ 1 5 0 f l - l m o l e - l c m 2) and Pd(DHa)2 as a non-electrolyte (AM=9f1-1 mole -1 cmZ). All the other complexes are only slightly soluble, or soluble with decomposition in D M F , so that their conductivities are unreliable. The infrared spectra. All these complexes are remarkably similar to one another with regard to the shifts of their principal bands (Table 2), and since the latter are assignable [4, 5] to modes influenced by their respective coordination, these may be considered to be very similar. From the splitting of the v(NH), v(CN) and v(CS) bands one may conclude that the ligands act as bidentate chelaing agents, being coordinated to the metal via nitrogen and sulphur atoms [2]. The far i.r. bands of the ligands are practically unaltered in the complexes which show new bands in the region of 480-425 cm -1 assignable to v(PdN) modes [2, 6] and new bands in the region of 410-315 cm -~ assignable to v(PdS) 4. T.A. Scott, Jr. and E. L. Wagner, J. chem Phys. 30, 465 (1959). 5. H. O. Desseyn and M. A. Herman, Spectrochim. Acta 23A, 2457 (1967). 6. J. R. During and D. W. Wertz, Appl. Spectrosc. 22, 627 (1968).
Pd(II) complexes of DH4, MetzDH~ and DCH~DH~
r~
~
~
975
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0
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~ _ ~
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.
I
I
~c5~ q-- q.+
976
G. PEYRONEL, A. C. FABRETTI and G. C. PELLACANI Table 3. Electronic spectra of the solid compounds
Pd(DH3)z 18870sh 22570 Pd(DH4)n(CIO4)2 1 7 2 4 0 20410sh Pd(Met2DHz)CI2 18520sh 21510 Pd(Met2DHz)Br~ 18180sh 21510 Pd(Met~DH2)I~ 1 8 0 2 0 s h 19610 Pd(MehDH2)2C12 21740 Pd(Met2DH2)2 Br2 19050sh 21050 Pd(MehDH2) ~(CIO4)~19050sh 21740 Pd(DCH2DH~)C12 21050 Pd(DCHzDH)Br2 20000sh Pd(DCH~DH2)I2 1 8 8 7 0 20410 Pd(DCH~DH~)2CI2 19050sh 21460 Pd(DCH2DH2)2Br2 18980sh 20410 Pd(DCH2DH~)~(CIO4)2 21600
22220sh, 25970sh, 30770 24390,35090 23810,28980sh, 34130 24100,32790 23530sh, 25000sh, 26670sh 22990sh, 24570sh, 33000 23260sh, 27030sh 22470,24570,33330 22220,24570,35710 21050sh, 25000sh, 30300 23260sh, 27030sh 22470sh, 27030sh 22990,30300sh
modes [1,7, 8]. Pd-halogen stretching vibrations for PdLX2 (X = CI, Br, I) are found in the expected range for terminal Pd-X bonds; this, and the fact that there are two bands assignable to asymmetric and symmetric Pd-C1 stretching vibrations for PdLCI~, lead one to the conclusion that these complexes have a C2~ symmetry [ 1, 7, 9]. The complexes PdLI~ show a v(PdI) band with a very close shoulder for Met2 DH2- and only a single band for the DCH~DH2-complex. Other cis complexes show similar anomalies. Two very close v(PtBr) bands (224 and 220 cm -1) were reported for the cis Pt(NH3)2Br~ complex [7], and only one v(MBr) band for the cis M(PhSe-CoH6-SePh)X~ (M = Pd, Pt) complexes [7]. The complexes PdLC12 show a difference between the two ~,(PdCI) bands of 14 cm -1 for the Met~DH~and of 32 cm -1 for the DCH~DH2-complex, while the two bromides have the same v(PdBr) values. Apparently the splitting of the v(MX) bands in the ciscomplexes may depend on the combination of several factors: the nature of the halogen atom, the inductive effect and the steric hindrance of the ligand and the environment in the crystal. Two new strong bands, as for the Ni(II) complexes [2], appear in the Pd(II) complexes at 1213-1100 cm -1. These new bands are assigned to z,(CRN) modes; they are enhanced by the complexation through nitrogen being stronger than that through sulphur. This agrees with a v(PdN) mode at an energy higher than that of the v(PdS) mode. Two v(CnN) bands correspond to the two differently bonded halves of the ligand molecule. Although palladium is a class B metal, with these dithiooxamides it gives, like nickel(II)[2], an S,N-coordination instead of the S,S-coordination observed for the tetrasubstituted dithiooxamides [ 1]. The i.r. bands of the perchlorate ion show a lowering of its Ta symmetry, probably, due to an interaction of this anion with the metal or with some positively-charged H R N group [2]. Similar interactions may also occur for the other anions. 7. D. M. Adams, Metal-Ligands and Related Vibrations, pp. 302, 325, 326. E. Arnold, London (1967). 8. G.W. Watt and B. J. McCormick, Spectrochim. Acta 21, 753 (1965). 9. W. Kitching, G. J. Moore and D. DoddreU, lnorg. Chem. 9, 521 (1970).
Pd(II) complexes of DH4, Met2DH~ and DCH2DH2
977
The i.r. spectrum of the compound Pd(DH4)a(CIO4)z shows the superposition of the bands of the complex Pd(DH4)z(CIO4)z and of the free ligand DH4. One molecule of the ligand is therefore present in the crystal as solvating agent and the complex must be formulated as Pd(DH4)z(C104)2"(DH4). The electronic spectra. The solid complexes in nujol mulls (Table 3) show a weak band or shoulder at 17-19 kK and a strong band at 19.5-22.5 kK. The great difference in their respective intensities suggests that the second band may be charge-transfer rather than ligand-field in character. Only the shoulder can be assigned to a d - d transition. The fact that this first d - d band is at a higher energy for palladium(II) (17-19 kK) than for nickel(II) (15-16.8 kK) [2] agrees with the spectrochemical order of these metal ions. Acknowledgements-This work was supported by financial aid from the Consiglio Nazionale delle Ricerche of Italy.