Spectrochemical Series of Some d-block Transition Metal Complexes. The spectrochemical series of the ligands for Cu(II), Co(II) and Cu(II) ions were constructed by visual inspection and spectral analysis of the transition metal complexes. Results show tha

Figure 1. Geometries of the d orbitals

The excitation of an electron from a  to an   orbital accounts for the color observed for the metal complexes. Moreover, the energy gap between  and   orbitals is designated as or the Crystal Field Splitting Energy (CFSE). CFSE can be measured spectrophotometrically through the wavelength of absorption and depends upon the size and charge of the metal and the ionic nature of the ligand. Accordingly, the wavelength of light that the compound absorbs determines how large  is by equation 1.

      [1]

where h is the Planck’s constant, c is the speed of light and  is the wavelength of absorbed light. Moreover, the fact that the nature of the ligand affects the CFSE makes it possible to construct a spectrochemical series that arranges the ligands according to their ability to cause d orbital splitting in a given oxidation state. The spectrochemical series can be summarized as:

Based on the trend, the multiply-bonded ligands are stronger than nitrogen donors. Moreover, it can also be deduced that oxygen-donor ligands are weaker than nitrogen-donor ligands but stronger than sulfur donors and halides. The stronger the interaction of the ligand with the d orbitals of the metals, the stronger the field strength of that ligand. 

The spectrochemical series was determined experimentally by using the complexes formed by Cu(II), Co(II) and Ni(II) ions. The construction of the series was done qualitatively through visual inspection method and quantitatively by means of a spectrophotometric analysis.

   

EXPERIMENTAL SECTION

Reagents

Potassium oxalate, pyridine, sodium glycinate, sodium edetate, ethylenediamine and ammonia were all prepared and provided by the stockroom of the Department of Chemistry of the Ateneo de Manila University.

Apparatus:

The spectra for all complexes was obtained by utilizing Shimadzu UV-2401PC at the Chemistry Department of the Ateneo de Manila University.

Procedure

Two milliliters of the metal ion, which can be Cu(II), Co(II) and Ni(II), was put into seven test tubes labeled A-G. 2 mL of distilled water, sodium glycinate, potassium oxalate, ammonia, pyridine, sodium edetate and ethylenediamine were then added to each test tube A-G respectively.

The color of the resulting complexes from Cu(II), Ni(II) and Co(II) was arranged in “rainbow order” for the qualitative assessment.

The λmax of all complexes were then obtained from their spectra with the help of Shimadzu UV-2401PC and recorded as . For the complexes with more than one d-d transition, the peak with the longest wavelength was taken as . The f factor for each ligand was calculated from  values and by assigning an f factor of 1.00 to the complex with water molecules as ligands. The ligands were then arranged in order of increasing f factor.

RESULTS and DISCUSSION

 The qualitative test or the visual inspection method involves ranking the ligands by the color of the cobalt compound they produce. This is accomplished using the “rainbow order.” Solutions manifest the color opposite the wavelength absorbed. This method of ranking ligands is valid given the direct correlation of the wavelength of light absorbed to the energy of the electron transition in the molecule (D0).

As shown in table 1, the spectrochemical series produced from Ni(II) compounds is:

H2O < oxalate < pyridine < NH3 < EDTA < glycinate < ethylenediammine

For Cu(II) complexes, the series produced (table 2) is:

H2O < EDTA < oxalate < glycinate < pyridine < NH3 < ethylenediamine

For Co(II) complexes, the series produced (table 3) is:

H2O < EDTA < glycinate < oxalate < pyridine < NH3 < ethylenediamine

For all the three metals, ethylenediamine seems to be the strongest ligand. Ethylenediamine is a bidentate chelating agent so it has two donor sites. Three bidentate ethylenediamine wraps around the metal and encases it. In general, chelating ligands have additional stability over non-chelating or monodentate ligands.

It can be noticed that the ligand ordering for Cu(II) is reasonably consistent with the ordering for the complexes of Co(II) with just a deviation in the interchange of glycinate and oxalate which are both oxygen-donor ligands. Consequently, the deviation is somewhat expected to occur. The ligands of Ni(II) complexes show more deviations as compared to both Co(II) and Cu(II). However, other trends can be noticed like oxalate < pyridine < NH3 . This can be attributed to the basicity of the ligands. Ammonia is the most basic among the three so its capacity to donate electrons is greater as compared to oxalate and pyridine.

Moreover, the spectrochemical series produced by quantitative method or the spectrophotometric analysis for Ni(II), Cu(II) and Co(II) is nearly similar as compared to the results that yielded from the visual inspection method. For all the three metals, the position of glycinate is inconsistent. This can be explained by the fact that glycinate can react with the metal ion in different forms depending on the pH of the solution. One form of glycinate reacts with the metal ion with both oxygen and nitrogen as electron donors while the other form interacts with the metal ion through oxygen as its only electron donor. Interestingly, oxalate and water tend to interchange in the spectrochemical series in both qualitative and quantitative method. This can be explained by the fact that the crystal field splitting parameters for H2O and ox2– are virtually identical. Furthermore, the same trend oxalate < pyridine < NH3 was observed with the quantitative method. This agrees with the fact that nitrogen-donor ligands are stronger field ligands than oxygen-donor ligands since nitrogen is less electronegative than oxygen so it is more capable of donating electrons to the central metal ion.

The f factor of the ligand and g factor of the metal determine the crystal field splitting energy. Consequently, keeping the f values constant makes it possible to arrange the g factor of the metals signifying the degree at which each metal can effectively contribute to Δ0. As shown in tables 1-3, the g values can be arranged in the order: Cu < Ni < Co. For a given oxidation state, as the size of the central metal atom decreases, the more contribution it can make to Δ0.

CONCLUSION

Based on the results, the nitrogen-donor ligands were observed to be stronger than oxygen-donor ligands because nitrogen is less electronegative than oxygen so it is more capable of donating electrons to the central metal ion. Moreover, the g factor of metals are arranged as: Cu < Ni < Co. For a given oxidation state, as the size of the central metal atom decreases, the more contribution it can make to Δ0.

Tables and Figures

Table 1. Crystal Field Splitting Energy and f factor for Ni(II) complexes

        

Table 1. Crystal Field Splitting Energy and f factor for Ni(II) complexes

Table 1. Crystal Field Splitting Energy and f factor for Ni(II) complexes

Figure 2. UV-VIS Spectra of Ni (II) complexes

Figure 3. UV-VIS Spectra of Ni (II) complexes

Figure 4. UV-VIS Spectra of Ni (II) complexes

        

                        

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