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A relation between the N-H stretching frequency and stability of metal complexes of β-alanine
SpectrochimicaActa,
1961, Vol.
17, pp. 895 to 903. Pergamon Press Ltd. Printed in Northern Ireland
RESEARCH NOTES
A relation between the N-H stretching frequency and stability of metal complexes of j343lanine (Received 15 March 1961)
IN A study [l] of the infra-red spectra of hydrazine complexes of the first transition series of metals in the 15-35 ,u region, a linear relationship between the nitrogen-metal stretching frequency (Y~_~) and the heats of formation of these complexes was found. The lowering of the N-H stretching frequency (v~_~), in the case of amino acid complexes of copper and nickel has been taken as an evidence of the covalent nature of the N-M bond [2]. This suggests that vN_n will be similarly related to N-M bond energy and consequently to the overall stability constant of the corresponding complex. However, no such correlation has been reported so far. Using a lithium fluoride prism, we obtained the infra-red absorption spectra of the Cu2+, Ni2+, Co2+ and Pd2+ complexes of /?-alanine of the type M(NH,CH,CH,COO), in the 3000-3400 cm-l region. The observed vN_n and the corresponding overall stability constants (klk2) of the complexes are given in Table 1. Table 1 Compounds 1. 2. 3. 4. 5.
Pd(NH,CH,CH,COO), Cu(NH,CH,CH,COO), Ni(NH,CH,CH,COO), Co(NH,CH,CH,COO), NH,CH,CH,COONa
Log k,k, 18.90 12.90 8.03 7.00 -
[3] [4] [4] [5]
v*-~ (cm-‘) 3240 3270 3333 3338 3413
3090 3231 3183 3291 3181 3285 -
The molecular formulae of these compounds were confirmed by micro-analysis and no attempt was made to isolate the different rotational isomers. The complexes of Fez+, Mn2+ are not included in this study because of the difficulty of their preparation in solid state due to their very low stability. It is observed that vN_= shows a shift (as compared to v~_~ = 3413 cm-l, in the sodium @-alaninate) to lower values in the order Pd2+ > C$f > Ni2+ > Co2+, which is in agreement with the reported “IRVINU-WILLIAMS” order [6] of stabilities [I] L. SACCONI and A. SABATINI, Nature 186,549 (1960). [2] D. N. SEN, S. MIZUSHI~A,C. CURRAN and J. V. QUAQLIANO,J. Am. Chem. Sot. 77, 211 (1955). [3] V.S. SEARMA,H.B. MATHUR~~ A.B. BISWAS. Unpublishedd&te. [4]A. ALBERT, Biochem. J. 47, 531 (1950). [5] H. IR~ING,R.J.P. WILLIAMS,D.J. FERRETT~~~A.E. WILLIAMS,J. Chem.Soc.3494(1954). [6] H. IRVING and R. J. P. WILLIAMS,J. Chem. Sot. 3192 (1953). 895
Researchnotes of bivalent metal complexes.
On plotting log k,k, vs. I++_~ of the first absorption band (Fig. 1) we get a linear relationship with the exception of the Pd2+-complex. The reason for this enhanced stability of Pd2+-complex should be examined in the light of the influence of some other structural factors and/or the formation of a comparatively stronger covalent bond. In fact, the C=O stretching frequency of the Pdz+-complex shows a peak at 1620 cm-l, thereby indicating that O-M bond has an appreciable covalent [2] character, whereas the same bond is essentially ionic in the case of the Cu2+, Ni2+ and Co2+ complexes. Considering a series of complexes and the relationship AH - TAX = -RT In K, it is seen that their stability constants are functions of both AH and AS. However, if the ligand remains the same and also there is no significant change in size and structure of the complexes (i.e. AS is practically constant) the variation in their stabilities is primarily due to the changes in the strength of the metal-to-donor bond represented by AH. The stronger is this bond, the more is the electron demand of nitrogen on the N-H linkage electrons, resulting in a corresponding decrease in the N-H stretching frequency. Thus vr._n in square planar complexes [7] of #Lalanine with the transition metal ions shows a linear relationship
I7 -
2 3
4
13 sSI 3300
I 3950
I 3200 ’
I 3350
Fig. 1. N-H stretchingfrequency(cm-‘).
with the overall stability constants because their magnitude is now a function of AH alone,
the contribution of the entropy factor remaining more or less constant. Any deviation shown by a complex may be taken as an indication of its having ELdifferent structure. Further, if by plotting log k,k, vs. AvN_u (the observed shift from vN_n of sodium palaninete) a straight line is obtained, its slope and intercept would be related to AH and AS, respectively. The result of this study supports the conclusion that it is possible to use the nitrogenhydrogen stretching frequency which occurs in the commonly used range of an infra-red spectrophotometer to determine nature and strength of the N-M bond and hence the overall stability constant in metal-amino acid complexes. Acknowkdg@rnent--We
thank Mr. C. I. JOSEfor his help in this investigation. V. S. SXXAWA H. B. ~WATHUR A. B. BISWAS
National Chemical Laboratory
Poona-8, India [7]
t9F41&iARTELL
and M. CALVIN, Chemistry of the Metal Chelate CompMmds.
896
PrenticeHall, New York
1961, Vol.
17, pp. 895 to 903. Pergamon Press Ltd. Printed in Northern Ireland
RESEARCH NOTES
A relation between the N-H stretching frequency and stability of metal complexes of j343lanine (Received 15 March 1961)
IN A study [l] of the infra-red spectra of hydrazine complexes of the first transition series of metals in the 15-35 ,u region, a linear relationship between the nitrogen-metal stretching frequency (Y~_~) and the heats of formation of these complexes was found. The lowering of the N-H stretching frequency (v~_~), in the case of amino acid complexes of copper and nickel has been taken as an evidence of the covalent nature of the N-M bond [2]. This suggests that vN_n will be similarly related to N-M bond energy and consequently to the overall stability constant of the corresponding complex. However, no such correlation has been reported so far. Using a lithium fluoride prism, we obtained the infra-red absorption spectra of the Cu2+, Ni2+, Co2+ and Pd2+ complexes of /?-alanine of the type M(NH,CH,CH,COO), in the 3000-3400 cm-l region. The observed vN_n and the corresponding overall stability constants (klk2) of the complexes are given in Table 1. Table 1 Compounds 1. 2. 3. 4. 5.
Pd(NH,CH,CH,COO), Cu(NH,CH,CH,COO), Ni(NH,CH,CH,COO), Co(NH,CH,CH,COO), NH,CH,CH,COONa
Log k,k, 18.90 12.90 8.03 7.00 -
[3] [4] [4] [5]
v*-~ (cm-‘) 3240 3270 3333 3338 3413
3090 3231 3183 3291 3181 3285 -
The molecular formulae of these compounds were confirmed by micro-analysis and no attempt was made to isolate the different rotational isomers. The complexes of Fez+, Mn2+ are not included in this study because of the difficulty of their preparation in solid state due to their very low stability. It is observed that vN_= shows a shift (as compared to v~_~ = 3413 cm-l, in the sodium @-alaninate) to lower values in the order Pd2+ > C$f > Ni2+ > Co2+, which is in agreement with the reported “IRVINU-WILLIAMS” order [6] of stabilities [I] L. SACCONI and A. SABATINI, Nature 186,549 (1960). [2] D. N. SEN, S. MIZUSHI~A,C. CURRAN and J. V. QUAQLIANO,J. Am. Chem. Sot. 77, 211 (1955). [3] V.S. SEARMA,H.B. MATHUR~~ A.B. BISWAS. Unpublishedd&te. [4]A. ALBERT, Biochem. J. 47, 531 (1950). [5] H. IR~ING,R.J.P. WILLIAMS,D.J. FERRETT~~~A.E. WILLIAMS,J. Chem.Soc.3494(1954). [6] H. IRVING and R. J. P. WILLIAMS,J. Chem. Sot. 3192 (1953). 895
Researchnotes of bivalent metal complexes.
On plotting log k,k, vs. I++_~ of the first absorption band (Fig. 1) we get a linear relationship with the exception of the Pd2+-complex. The reason for this enhanced stability of Pd2+-complex should be examined in the light of the influence of some other structural factors and/or the formation of a comparatively stronger covalent bond. In fact, the C=O stretching frequency of the Pdz+-complex shows a peak at 1620 cm-l, thereby indicating that O-M bond has an appreciable covalent [2] character, whereas the same bond is essentially ionic in the case of the Cu2+, Ni2+ and Co2+ complexes. Considering a series of complexes and the relationship AH - TAX = -RT In K, it is seen that their stability constants are functions of both AH and AS. However, if the ligand remains the same and also there is no significant change in size and structure of the complexes (i.e. AS is practically constant) the variation in their stabilities is primarily due to the changes in the strength of the metal-to-donor bond represented by AH. The stronger is this bond, the more is the electron demand of nitrogen on the N-H linkage electrons, resulting in a corresponding decrease in the N-H stretching frequency. Thus vr._n in square planar complexes [7] of #Lalanine with the transition metal ions shows a linear relationship
I7 -
2 3
4
13 sSI 3300
I 3950
I 3200 ’
I 3350
Fig. 1. N-H stretchingfrequency(cm-‘).
with the overall stability constants because their magnitude is now a function of AH alone,
the contribution of the entropy factor remaining more or less constant. Any deviation shown by a complex may be taken as an indication of its having ELdifferent structure. Further, if by plotting log k,k, vs. AvN_u (the observed shift from vN_n of sodium palaninete) a straight line is obtained, its slope and intercept would be related to AH and AS, respectively. The result of this study supports the conclusion that it is possible to use the nitrogenhydrogen stretching frequency which occurs in the commonly used range of an infra-red spectrophotometer to determine nature and strength of the N-M bond and hence the overall stability constant in metal-amino acid complexes. Acknowkdg@rnent--We
thank Mr. C. I. JOSEfor his help in this investigation. V. S. SXXAWA H. B. ~WATHUR A. B. BISWAS
National Chemical Laboratory
Poona-8, India [7]
t9F41&iARTELL
and M. CALVIN, Chemistry of the Metal Chelate CompMmds.
896
PrenticeHall, New York