Potentiometric and spectrophotometric Investigations of Nickle Complexes

Abstract

1. Nature of mononuclear complexes in nickel ethanolamine systems The work presented in this thesis indicates that nickel forms six complexes, NiA? to NiA?²?. In this respect nickel–monoethanolamine complexes resemble Ni–NH? complexes. In the following table the step constants of nickel mono , di and triethanolamine complexes have been given. For comparison purposes, the step constants of nickel–ammonia complexes have also been included. Table 64. Step constants for Ni–ammonia, Ni–MEA, Ni–DEA complexes Type of complex Ni(NH?) Ni(MEA) Ni(DEA) Ni(TEA) NiL 2.73 3.18 2.79 2.915 NiL? 2.18 2.86 1.63 1.821 NiL? 1.67 1.72 1.18 NiL? 1.13 1.47 NiL? 0.69 0.47 NiL? 0.03 0.33 Comparison of Ni–ammonia constant with those of Ni–monoethanolamine complexes one notices that the constants for Ni(MEA)?, Ni(MEA)?²?, Ni(MEA)?³? and Ni(MEA)??? are greater than the corresponding values for nickel–ammonia complexes while in the case of Ni(MEA)??? the constant for Ni(MEA)??? is smaller than that of Ni(NH?)???. These values are indicative of the fact that in complexes Ni(MEA) to Ni(MEA)? there is a slight chelate effect. The lower value of the constant for Ni(MEA)? as compared to Ni(NH?)? indicates that monoethanolamine is monodentate and being a primary amine, the formation constant is lower than that of the corresponding ammonia complex. It may be pointed out that it would have been more appropriate to compare constants of nickel–ethylamine complexes instead of Ni–ammonia complexes with those of nickel–monoethanolamine complexes. Unfortunately the constants of nickel–ethylamine complexes are not available. However, one could expect nickel–methylamine constants to be lower than those of Ni–ammonia complexes. Hence the conclusion arrived at is quite valid. The formation constants of nickel–diethanolamine and triethanolamine are also included in the same table. Since Ni(DEA)?²? and Ni(TEA)?²? are the highest complexes produced, it is reasonable to conclude that slight chelate effect will be there in these complexes. However, it is not possible to draw conclusions on the basis of step constants, since the step constants of Ni–diethylamine and triethylamine are not available in literature. It is also likely, particularly in triethanolamine, the steric effects might be responsible for the non formation of higher complexes. It is expected that with an increase in the number of ethanol groups in the amine the successive formation constants of all the complexes (1:1, 1:2, 1:3 etc.) should decrease. In the present study this is true in the case of Ni–mono and diethanolamine complexes. In the case of nickel–triethanolamine system the constants are slightly higher than in the case of Ni–DEA system. This is probably due to stronger chelate effect in the case of nickel–triethanolamine system. 2. Nature of polynuclear complexes The formation of polynuclear complexes in hydrolytic studies is very well known. Particularly the hydrolytic studies by self medium method, where very high concentrations of metal ions have eliminated the formation of mononuclear complexes. Although a few examples are known regarding the formation of polynuclear complexes in metal–ligand complex system, generalized systematic study appears to have been made, in this regard. The results presented in this thesis indicate that a system can give rise to: (i) pure mononuclear complexes, (ii) pure polynuclear complexes and (iii) mixture of mono and polynuclear complexes. In nickel–monoethanolamine system pure mononuclear complexes Ni(MEA)? to Ni(MEA)?²? are formed under Bjerrum’s conditions. When the concentration of monoethanolammonium ion is less than that of the metal ion the nature of the n–pA curves is entirely different. The n–pA curves taken at different concentrations of nickel ion do not coincide and precipitation occurs at low values of n, when the metal concentration is between 20 mM and 50 mM in presence of 10.0 mM of MEA ion. Precipitation occurs at n = 0.25 and the analysis carried out by assuming the presence of one predominating complex gives the formula as Ni?(MEA) with stability constant — 0.03. When the metal ion concentration is between 100 mM and 500 mM, with 50.0 mM of MEA ion precipitation occurs at n = 0.2. The complex produced is Ni?(MEA)? with stability constant ??? = 10²·?³ ± 0.07. It is clear therefore, that by carefully controlling the experimental conditions it is possible to obtain either mono or polynuclear complexes. In the case of Ni–DEA system, evidence for the formation of polynuclear complexes is obtained even when the concentration diethanolammonium ion is about 0.5 M and n beyond 2. It has not been possible to make a quantitative analysis of the curves when n > 2 due to experimental limitations. Analysis of the n–pA curves obtained in solutions containing diethanolammonium ion concentration equal to 10 mM indicates the formation of both mono and polynuclear complexes. (This thesis page 154, Fig. 38). When the concentration of metal ion is between 20.0 mM and 50.0 mM, the predominant complex is mononuclear. Analysis of the results by extrapolation method indicates that the polynuclear complex produced is Ni?(DEA) and the stability constant of this complex is 10¹?·?. Between 80 mM and 300 mM of the metal ion concentration with 50 mM of DEA ion only polynuclear complexes are produced. Analysis of the n–pA curves by the method of assuming one predominating complex gives the formula of the complex as Ni?(DEA)?, the stability constant being 10²²·?. Nickel–triethanolamine system presents quite a few interesting features. In a solution containing 5.0 mM of TEA ion and various metal ion concentration (10 mM to 40.0 mM) n–pA curves coincide till n = 0.8 showing that only mononuclear complexes are produced. When n > 0.8 the curves branch out and this region obviously corresponds to polynuclear region. Analysis of mono and polynuclear parts (up to n = 1.6 only) shows that the predominant complex in the region is mononuclear. The steep rise beyond n = 1.4 is due to the formation of polynuclear hydroxy complexes. The formation constants of the first and second mononuclear complexes are 2.92 and 1.82 respectively. When the metal ion concentration is between 100.0 mM and 300.0 mM (Fig. 41) the formula of the complex is Ni?(TEA)?²?. When the metal ion concentration is 50.0 and 75.0 mM, the slope of the plot log (C_M × n) vs. pA is 1.22 and 1.5 respectively. These values for the slope give the information about the types of the complexes present. Obviously at these concentrations of metal ion, both mono and dinuclear complexes are present in different proportions. From the above discussion it is clear that the relative proportion of ethanolammonium ion and metal ion is the governing factor in the formation of either mononuclear or polynuclear complexes. The general trend appears to be that high concentrations of ethanolammonium ion with respect to metal ion favours the formation of mononuclear complexes. On the other hand, lower concentrations of ethanolammonium ion favours the formation of polynuclear complexes. Very high concentrations of metal ion with respect to ethanolammonium ion favours mainly the formation of polynuclear complexes. It is also noticed that higher the concentration of the metal ion more favourable is the formation of higher polynuclear complexes. Another point of interest is that in solutions forming polynuclear complexes, the value of n is the lowest in the case of monoethanolamine and highest in the case of triethanolamine. This can be attributed to the fact that an increase in the ethanol groups in amine increases the solubility of the complexes formed. The work on nickel hydrolysis is also carried out in this thesis. This has been done primarily to find out the extent of interference of hydrolytic equilibria with complex formation (with ethanolamine) equilibria at lower concentrations of the nickel ion (below 0.1 M) which is not included in Sillén’s work. The work presented in this thesis shows that at low concentrations of the metal ion, the time of attainment of equilibrium is of the order of 70 hours in the case of hydrolytic equilibria. The n–pH curves obtained, soon after the addition of NaOH are not quite regular but the region of pH range where hydrolytic reaction occurs is far above (pH 8.2) from the pH range of polynuclear complex formation (with ethanolamines). It can, therefore, be concluded that except in the case of triethanolamine the hydrolytic equilibria do not interfere with the polynuclear complex formation with ethanolamines. Interpretation of data on hydrolytic equilibria has given evidence for the formation Ni?(OH)?²? with a stability constant of 10??·?. The work done by Sillén and co workers at high concentrations of the metal ion gives the evidence for the formation Ni?(OH)? which is formed in an interval of 15–20 minutes after addition of NaOH. 3. Absorption spectra of Ni–ethanolamine (mono , di and tri ) complexes In nickel–monoethanolamine system, it has been possible to obtain pure absorption spectra of five complexes i.e., Ni(MEA)? to Ni(MEA)??? and Ni(MEA)???. In all cases the spectrum consists of three bands, which are due to the following transitions: ³A?g ? ³T?g(F) ³A?g ? ³T?g(F) ³A?g ? ³T?g(P) The middle band consists of two peaks in the case of Ni(MEA)? complex. The separation between two peaks becomes less and less prominent with the increase in the number of ethanolamine groups and only a single band is observed in the case of Ni(MEA)? and onwards. Gaussian analysis has been made to confirm that the middle band is made up of two bands (Figs. 23, 25). On the basis of the theory of average ligand field, it has been possible to predict the position of the second and the third bands in all cases. It is noteworthy that the theoretical and experimental values are in good agreement. In the case of nickel–diethanolamine system the formation of Ni(DEA), Ni(DEA)? and Ni(DEA)? has been observed while in the case of triethanolamine system, only Ni(TEA) and Ni(TEA)?²? have been noticed. As in the case of nickel–monoethanolamine system, three absorption bands in each of the spectrum are noticed. Taking the value of 10Dq from experimental data, ligand field calculations have been made to predict the position of the remaining two bands. There is a good agreement between calculated and experimental values, for the positions of these bands. It is well known that the formation constant of Ni(H?O)?²? complex is small. With the addition of complexing agents H?O is replaced by the ligand. With the progressive replacement of water molecules by the ligand groups, the absorption maximum will be shifted towards shorter wavelength. The relevant data concerning nickel–ethanolamine complexes are summarised in the Table 26. 4. M.O. Calculations Molecular orbital calculations have been confined to nickel–monoethanolamine system since it has been possible to obtain the absorption spectra of Ni(MEA)?²?. There has been good agreement between the values of 10Dq calculated on the basis of M.O. theory and the experimentally observed (This thesis p. 177). As mentioned already these calculations have been made using only 2p wave function in the case of amino nitrogen. This method was preferred in view of the finding of Gray and Ballhausen that the double basis set taking 2s, 2p functions in the case of octahedral complexes gave higher calculated values of ?. The close agreement (calculated - 11,400 cm?¹ and experimental - 10,400 cm?¹) between calculated and experimental values obtained in the present work justifies the validity of using only 2p wave function for the ligand atom as the basis set, instead of using either the double basis set or hybridised wave functions.