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Reactivity and far infrared spectra of PtII and PdII complexes with nitrogen donor ligands
J. inorg, nucl.Chem., 1969,Vol. 3 t, pp. 2423 to 2426. PergamonPress. Printedin Great Britain
R E A C T I V I T Y A N D F A R I N F R A R E D S P E C T R A OF pill A N D Pd u C O M P L E X E S W I T H N I T R O G E N DONOR LIGANDS T. B O S C H I , G. D E G A N E L L O
a n d G. C A R T U R A N
Laboratorio di C h i m i c a e Tecnologia dei Radioelementi C . N .R., Padova, Italy
(First received 25 November 1968" in revised form 13 January 1969) A b s t r a c t - F a r i,r. spectra of the series of c o m p l e x e s cis-[M(py).~Xz], [M(en)X.~], and [M(bipy)X2l (M = Pt, Pd; X = CI, Br, 1, py = pyridine, en = ethylenediamine, bipy = 1,2-bipyridyl) are reported. T h e higher reactivity of [Pt(bipy)X2] over the other platinum substrates toward halide displacement by dithioxamide is interpreted as resulting from the stabilization of the transition state rather than from weakening of the M-X bond.
NUCLEOPmLIC substitutions of the square-planar complexes [Pt(en)Cl2], cis[Pt(py)2CI2] and [Pt(bipy)CI2] in CH.~OH (en = ethylenediamine, p y = pyridine, bipy = bipyridyl) with dithioxamide, of the type:
L
Pt. L/
S=C/
01
+ ~01
L
I B=C~
~
Pt, /
Nil.,
+ 2CI"~S=C~
NH
have shown that (i) reactions are of the second order and (ii) the ratios of rate constants kz (M -1 sec -~) are 1 : 1 : 100, respectively. These results have been interpreted by suggesting that the higher reaction rate displayed by [Pt(bipy)Cl,,] is due to the aromatic character of the five-atom ring of the platinum-a-diimine chelate framework (Fig. 1)[1]:
I
Pt
/c%f
~
el
Fig. 1.
Such electronic interaction, which can take place between the 7r-electron system of bipyridyl and the filled non-bonding 5dxz and 5duz platinum orbitals would 1. P. H a a k e a n d P. A. Cronin, lnorg. Chem. 2, 879 (1963). 2423
2424
T. BOSCH1, G. DEGANELLO and G. CARTURAN
bring about a significant charge delocalization from platinum to the system of the bidentate ligand. This should render the metal more susceptible to nucleophilic attack by the entering dithioxamide. However, it should be noted that the higher reactivity shown by the bipyridyl complex might be due either to destabilization of the substrate in the ground state or to stabilization of the activated complex. With the aim of gaining some information on the relative weakening of Pt-X bond in the afore-mentioned complexes, we have undertaken a comparative examination of the Pt-X stretching frequency in these derivatives. The analogous palladium complexes have also been studied.
RESULTS AND DISCUSSION
The M - X stretching frequencies of the complexes are listed in Table 1. For the complexes [Pt(en)Xo] (X = CI, Br, I), [Pt(bipy)l~] and [Pd(en)12] the assignments were made by observing the spectral shifts in the region (400-70 cm -1) caused by changing the halide. For the platinum complexes, data in Table 1 show that with the exception of a few cases two i.r. active M - X stretching vibrations (of symmetry A1 +BI) are observed, as expected for a C~v symmetry. The separation of these bands decreases with increasing atomic masses and in some cases the two frequencies happen to coincide. The M - X stretching vibrations do not always reflect the electronic situation of the metal-halogen bond, owing to possible coupling with near frequencies of the same symmetry. Nevertheless these frequencies can be taken with confidence as reliable indexes of the Pt-X bond strength, as was established for the series of complexes cis- and trans-[PtL2X2] 4. As for the chloroderivatives of Pt H it may be seen that v(M-X) decreases in the order bipy > py > en suggesting the same sequence of bond order. The greater M - X bond order in complexes having N-aromatic ligands (bipy and py) relative to N-aliphatic (en) is consistent with the hypothesis that in the former case delocalization of~--electron charge from the metal to the neutral ligand can occur to a greater extent than in the latter. Such electron drift would strengthen the dative bond C1---> M. This order should imply that, in explaining the higher reactivity of [Pt(bipy)Cl2], any electronic (hybridization) mechanism involving some Pt-C1 bond loosening in the ground state has to be dismissed. Hence, the higher reactivity of [Pt(bipy)C12] relative to the analogous pyridine and ethylenediamine complexes cannot be ascribed to any weakening of the bond involving the central metal and the leaving chloride. Therefore, the kinetical results obtained by Haake et al. [ 1] can only be interpreted on the grounds of a greater stabilization of the transition state for the bipyridyl complex relative to the other substrates. The aromaticity of the above-mentioned o~-diimine chelate ring does not manifest itself in the ground state, but rather during the attack by the entering dithioxamide. This amounts to say that such aromaticity develops upon request by the electronic conditions which the reactions set up. For the palladium complexes v(M-X) is in the order bipy ~ py > en. This trend is at variance with that found for platinum complexes. Therefore it is to be expected that these palladium complexes will display a similar reactivity ratio as that found for platinum.
Complexes with nitrogen donor ligands
2425
<
¢'4
.2
I
-6
I
=g
¢-z
oJ ¢",1
i~
m
~D
_---,
tl Y
6
6
6
i
J~ LE
Z~ II <
L;
~r..~
%
~'~
2426
T. BOSCHI, G. D E G A N E L L O and G. C A R T U R A N
EXPERIMENTAL All the complexes with the exception of[Pt(en)Br2] and [Pt(en)12] were prepared and characterized by the methods of the literature. The references are the following: [Pt(py)2Cl~][7], [Pt(py)2Br2][8], [Pt(py)212] [8], [Pt(en)C12] [9], [Pt(bipy)C12][10], [Pt(bipy)Brz] [10], [Pt(bipy)Iz] [10], [Pd(py)~CI2] [1 I], [Pd(en)C12] [ 12], [Pd(en)I2] [ 13]. [Pt(en)Br2] and [Pt(en)12] were prepared from an acetonic suspension of [Pt(en)Clz] by metathetical reaction with a large excess of LiBr and Lil respectively. Infrared spectra. Spectra were recorded in Nujol mulls between polytene plates on a Beckman IR 11 spectrophotometer.
Acknowledgements-We wish to thank Professor U. Belluco for stimulating discussion. This work was supported by a N.A.T.O. Grant No. 341. 7. 8. 9. 10. 1 I. 12. 13.
S. M. Jergensen, J. prakt. Chem. 33, 489 (1886). Gmelin, Platin 1,286 and references therein. F, Basolo, J. C. Bailar and B. R. Tarr, J.Am. chem. Soc. 72, 2433 (1950). G . T . Morgan and F. H. BurstalI,J. chem. Soc. 965 (1934). A. A. Grinberg and W. M. Schulmann, C. R.Acad. URSS 275 (1933). H. D. K. Drew, F. W. Pinkard, G. H. Preston and W. Wardlaw, J. chem. Soc. 1903 (1932). A. Gutbier and U. Woernla, Chem. Ber. 39, 2717 (1906).
R E A C T I V I T Y A N D F A R I N F R A R E D S P E C T R A OF pill A N D Pd u C O M P L E X E S W I T H N I T R O G E N DONOR LIGANDS T. B O S C H I , G. D E G A N E L L O
a n d G. C A R T U R A N
Laboratorio di C h i m i c a e Tecnologia dei Radioelementi C . N .R., Padova, Italy
(First received 25 November 1968" in revised form 13 January 1969) A b s t r a c t - F a r i,r. spectra of the series of c o m p l e x e s cis-[M(py).~Xz], [M(en)X.~], and [M(bipy)X2l (M = Pt, Pd; X = CI, Br, 1, py = pyridine, en = ethylenediamine, bipy = 1,2-bipyridyl) are reported. T h e higher reactivity of [Pt(bipy)X2] over the other platinum substrates toward halide displacement by dithioxamide is interpreted as resulting from the stabilization of the transition state rather than from weakening of the M-X bond.
NUCLEOPmLIC substitutions of the square-planar complexes [Pt(en)Cl2], cis[Pt(py)2CI2] and [Pt(bipy)CI2] in CH.~OH (en = ethylenediamine, p y = pyridine, bipy = bipyridyl) with dithioxamide, of the type:
L
Pt. L/
S=C/
01
+ ~01
L
I B=C~
~
Pt, /
Nil.,
+ 2CI"~S=C~
NH
have shown that (i) reactions are of the second order and (ii) the ratios of rate constants kz (M -1 sec -~) are 1 : 1 : 100, respectively. These results have been interpreted by suggesting that the higher reaction rate displayed by [Pt(bipy)Cl,,] is due to the aromatic character of the five-atom ring of the platinum-a-diimine chelate framework (Fig. 1)[1]:
I
Pt
/c%f
~
el
Fig. 1.
Such electronic interaction, which can take place between the 7r-electron system of bipyridyl and the filled non-bonding 5dxz and 5duz platinum orbitals would 1. P. H a a k e a n d P. A. Cronin, lnorg. Chem. 2, 879 (1963). 2423
2424
T. BOSCH1, G. DEGANELLO and G. CARTURAN
bring about a significant charge delocalization from platinum to the system of the bidentate ligand. This should render the metal more susceptible to nucleophilic attack by the entering dithioxamide. However, it should be noted that the higher reactivity shown by the bipyridyl complex might be due either to destabilization of the substrate in the ground state or to stabilization of the activated complex. With the aim of gaining some information on the relative weakening of Pt-X bond in the afore-mentioned complexes, we have undertaken a comparative examination of the Pt-X stretching frequency in these derivatives. The analogous palladium complexes have also been studied.
RESULTS AND DISCUSSION
The M - X stretching frequencies of the complexes are listed in Table 1. For the complexes [Pt(en)Xo] (X = CI, Br, I), [Pt(bipy)l~] and [Pd(en)12] the assignments were made by observing the spectral shifts in the region (400-70 cm -1) caused by changing the halide. For the platinum complexes, data in Table 1 show that with the exception of a few cases two i.r. active M - X stretching vibrations (of symmetry A1 +BI) are observed, as expected for a C~v symmetry. The separation of these bands decreases with increasing atomic masses and in some cases the two frequencies happen to coincide. The M - X stretching vibrations do not always reflect the electronic situation of the metal-halogen bond, owing to possible coupling with near frequencies of the same symmetry. Nevertheless these frequencies can be taken with confidence as reliable indexes of the Pt-X bond strength, as was established for the series of complexes cis- and trans-[PtL2X2] 4. As for the chloroderivatives of Pt H it may be seen that v(M-X) decreases in the order bipy > py > en suggesting the same sequence of bond order. The greater M - X bond order in complexes having N-aromatic ligands (bipy and py) relative to N-aliphatic (en) is consistent with the hypothesis that in the former case delocalization of~--electron charge from the metal to the neutral ligand can occur to a greater extent than in the latter. Such electron drift would strengthen the dative bond C1---> M. This order should imply that, in explaining the higher reactivity of [Pt(bipy)Cl2], any electronic (hybridization) mechanism involving some Pt-C1 bond loosening in the ground state has to be dismissed. Hence, the higher reactivity of [Pt(bipy)C12] relative to the analogous pyridine and ethylenediamine complexes cannot be ascribed to any weakening of the bond involving the central metal and the leaving chloride. Therefore, the kinetical results obtained by Haake et al. [ 1] can only be interpreted on the grounds of a greater stabilization of the transition state for the bipyridyl complex relative to the other substrates. The aromaticity of the above-mentioned o~-diimine chelate ring does not manifest itself in the ground state, but rather during the attack by the entering dithioxamide. This amounts to say that such aromaticity develops upon request by the electronic conditions which the reactions set up. For the palladium complexes v(M-X) is in the order bipy ~ py > en. This trend is at variance with that found for platinum complexes. Therefore it is to be expected that these palladium complexes will display a similar reactivity ratio as that found for platinum.
Complexes with nitrogen donor ligands
2425
<
¢'4
.2
I
-6
I
=g
¢-z
oJ ¢",1
i~
m
~D
_---,
tl Y
6
6
6
i
J~ LE
Z~ II <
L;
~r..~
%
~'~
2426
T. BOSCHI, G. D E G A N E L L O and G. C A R T U R A N
EXPERIMENTAL All the complexes with the exception of[Pt(en)Br2] and [Pt(en)12] were prepared and characterized by the methods of the literature. The references are the following: [Pt(py)2Cl~][7], [Pt(py)2Br2][8], [Pt(py)212] [8], [Pt(en)C12] [9], [Pt(bipy)C12][10], [Pt(bipy)Brz] [10], [Pt(bipy)Iz] [10], [Pd(py)~CI2] [1 I], [Pd(en)C12] [ 12], [Pd(en)I2] [ 13]. [Pt(en)Br2] and [Pt(en)12] were prepared from an acetonic suspension of [Pt(en)Clz] by metathetical reaction with a large excess of LiBr and Lil respectively. Infrared spectra. Spectra were recorded in Nujol mulls between polytene plates on a Beckman IR 11 spectrophotometer.
Acknowledgements-We wish to thank Professor U. Belluco for stimulating discussion. This work was supported by a N.A.T.O. Grant No. 341. 7. 8. 9. 10. 1 I. 12. 13.
S. M. Jergensen, J. prakt. Chem. 33, 489 (1886). Gmelin, Platin 1,286 and references therein. F, Basolo, J. C. Bailar and B. R. Tarr, J.Am. chem. Soc. 72, 2433 (1950). G . T . Morgan and F. H. BurstalI,J. chem. Soc. 965 (1934). A. A. Grinberg and W. M. Schulmann, C. R.Acad. URSS 275 (1933). H. D. K. Drew, F. W. Pinkard, G. H. Preston and W. Wardlaw, J. chem. Soc. 1903 (1932). A. Gutbier and U. Woernla, Chem. Ber. 39, 2717 (1906).