Interpret the spectroscopic results of the following octahedral nickel(II) compl

Question

Interpret the spectroscopic results of the following octahedral nickel(II) complexes in terms of the crystal field theory. The interpretation must include details of spectroscopic terms for the transitions involved, their corresponding wavelengths and energies (use cm-1 as units of energy) as well as their molar extinction coefficients. Compare the energies of the same transitions for the different complexes and discuss the relationship between these and the ligand field strengths of the ligands. Finally, comment on the relative intensity of the absorption in the context of spectroscopic selection rules (spin selection rule and Laporte selection). Note: The highest energy transitions for the [Ni(bipy)3]2+ complexes are obsured by charge transfer bands and cannot be determined.

Figure 1 (above): UV-vis Absorption Spectra of NiCl2.6H2O

Figure 2 (above): UV-vis Absorption Spectra of [Ni(NH3)6]Cl2

Figure 3 (above): UV-vis Absorption Spectra of [Ni(en)3]Cl2.2H2O

Figure 4 (above): UV-vis Absorption Spectra of [Ni(pn)3]Cl2.2H2O

Figure 5 (above): UV-vis Absorption Spectra of [Ni(bipy)3]2+

Explanation / Answer

Octahedral Nickel(II) complexes . Ni(0) = [Ar] 4s2 3d8

The approach that is usually followed in developing an energy level diagram for a complex is to begin with the dn configuration of the free ion and then, add the effects of interelectronic repulsions and the effects of surrounding ligands.

Ni(II) is a d8 system = [Ar] 4s0 3d8.

Electron-Electron Interactions give rise to ground state free ion term and also a number of excited state terms.

The Spectroscopic Ground state term for d8  is 3F. A number of excited state terms is also there as well ( 3P, 1G, 1D and 1S).

The ground state term 3F will split into three terms in an octahedral field : 3A2g , 3T2g , 3T1g.

3P splits into 3T1g .1D splits into 2T2g and 2Eg.   1S splits into 1A1g etc.

1.) NiCl2.6H2O :-

d2, d3, d7 and d8 complexes show three absorptions, with the 3rd peak often obscured by a very intense charge transfer band.

In the spectra two transitions are obtained.due to

Wavelength, 1 = 394nm = 25380.71 cm-1(v1) ,2 = 721nm = 13869.63 cm-1(v2) (v = wavenumber)

Energy, [E = hc / = hcv], E1 = 25380.71hc cm-1 E2 = 13869.63hc cm-1 ( h = Plank's constant ,c = velociy of light, )

Molar Extinction Coefficients = A / cl (A=absorbance, c = molar concentration, l= pathlength ) we are provided with only A from spectra not c and l .

2. [Ni(NH3)6]Cl2 :-

Wavelength,1=300nm=33333.33cm-1(v1) ,2 = 579nm =17271.16cm-1(v2),2=960nm= 10416.67cm-1

Energy, E1 = 33333.33hc cm-1 E2 = 17271.16hc cm-1 E3 = 10416.67hc cm-1

Transitions takes place are

3A2g ---> 3T2g

3A2g ---> 3T1g

3A2g ---> 3T1g (P)

3.) [Ni(en)3]Cl2.2H2O

Wavelength,1= 345nm=28985.51cm-1(v1), 2 = 544nm =18382.35cm-1(v2), 3=885nm= 11299.44cm-1

Energy, E1 = 28985.51hc cm-1 E2 = 18382.35hc cm-1 E3 = 11299.44hc cm-1

4.) [Ni(pn)3]Cl2.2H2O

Wavelength,1= 358nm=27932.96cm-1(v1), 2 = 574nm =17421.60cm-1(v2), 3=927nm= 10787.49cm-1

Energy, E1 = 27932.96hc cm-1 E2 = 17421.60hc cm-1 E3 = 10787.49hc cm-1

5.) [Ni(bipy)3]2+

Wavelength,1= 518nm=19305.02cm-1(v1), 2 = 788nm =12690.36cm-1(v2)

Energy, E1 = 19305.02hc cm-1 E2 =12690.36hc cm-1

Molar absorptivities for Ni(II) octahedral complexes are mainly in the range (1-10)L mol-1cm-1.

The transitions with high value of wavenumber are of low intensities and are in high energy portion of spectrum and thus masked by many toatlly allowed transitios.

Selection rules :

The first selection rule, known as the Laporte rule, states that the only allowed transitions are those with a change of parity , i.e. gerade to ungerade (g---->u) and ungerade to gerade (u--->g) are allowed but not g--->g and u--->u . All d orbitals have gerade symmetry therefore all d-d transitions in octahedral complexes are forbidden

Second Selection Rule ,Spin Selection Rule states that any transition for which ( S 0) is forbidden i.e for a tranisition to be allowed must involve no change in spin state. Thus three Transitions are Spin Allowed.

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