Question

Compound Max Wave length Absorbance

CuSo4 (0.05M) = 800nm

CuSO4 + EdTA = 732nm

CuSO4 + Sodium nitrate (0.05M) = 780nm

CuSO4 + Ethylenediamine (0.05) = 550nm

CuSO4 + Ethylenediamine (16.5M) = 571nm

Please help on how to figure out these questions.

1) Calculate the corresponding crystal field splitting energy values for each of the copper (II) complexes with the different ligands used above.

2) Based upon the information above, created a spectrochemical series for the copper (II) ligands.

Please help and explain!!

Answer 1

1) crystal field splitting energy values calculation

The CFSE of an octahedral complex can be calculated using the following formula

CFSE = -nt2g*0.4+neg*0.6

-nt2g = no. of electrons present in the t2g level

neg = no. of electrons present in the eg level

For the reaction between CuSO4 and NaNO2 we get [Cu(NO2)6]^4-

CuSO4(aq) + NaNO2(aq) -------> Cu(NO2)6(aq) + Na2SO4(aq)

In Cu(NO2)6, Cu is in +2 oxidation state and it contain 9 electrons. These 9 electrons can be arranged in the 5 d-orbitals as t2g^6 eg^3. Thus its CFSE is

CFSE = -6*0.4+3*0.6 = -0.6 = 6Dq

For the reaction between CuSO4 and Ethylenediamine(En) we get [Cu(En)3]^2+

CuSO4(aq) + H2N-CH2-CH2-NH2(aq) -------> Cu(En)3(aq)

In [Cu(EN)3]^2+, Cu is in +2 oxidation state and it contain 9 electrons. These 9 electrons can be arranged in the 5 d-orbitals as t2g^6 eg^3. Thus its CFSE is

CFSE = -6*0.4+3*0.6 = -0.6 = 6Dq

The CFSE value is same for all given ligand irrespective of their field strengths i.e., 0.6 or 6Dq

2) Based upon the information above, a spectrochemical series for the copper (II) ligands can be predicted

The wavelengths of absorption is given that

CuSo4 (0.05M) = 800nm

CuSO4 + EdTA = 732nm

CuSO4 + Sodium nitrate (0.05M) = 780nm

CuSO4 + Ethylenediamine (0.05) = 550nm

CuSO4 + Ethylenediamine (16.5M) = 571nm

The wavelength of radiation is directly proportional to the energy gap between t2g and eg levels in the octahedral field. This energy gap is depends upon the field strength of the ligand

As field strength increases the frequency of radiation absorbed by the complex increases or as field strength increases the wavelength of radation absorbed decreases.

Thus, En(0.05 M) > En(16.5 M) > EDTA > NO2- > SO4^2-

En (0.05 M) showing shortest wavelength and hence its field strength is strong while SO4^2- showing longer wavelength and hence its field strength is weak