Question

4. (2 points) The absorbance maxima observed for the following octahedral cobalt complexes ([Co(NH)* max [Co(NH3)3Cl] nmax = 680 nm) can be identified with transitions involving d-electrons. = 470 nm, [Co(NH);CI]2* Amax = 560 nm, [Co(NH),Cl2]* max = 630 nm, and What can be said about the trend observed in these complexes with relation to their crystal field splitting energies? Rank from LOWEST to HIGHEST splitting energies. а. b. Given the complexes above and the trend in absorbance maxima for these complexes, can the spectrochemical series be used to explain the trend? Explain why or why not in a sentence

Answer 1

ANSWER - Question 4a:

The splitting energy for each complex (ΔE) is defined as

$\Delta E=\frac{hc}{\lambda}$

where:

• h = Planck's constant = 6.626x10^-34 J.s
• c = speed light = 2.998x10⁸ m/s
• λ = wavelength

Then,

Complex	λmax	ΔE
[Co(NH3)6]3+	470 nm	$\frac{6.626x10^{-34}\, J.s\times 2.998x10^{8}\, \frac{m}{s}}{470x10^{-9}\, m}=\mathbf{4.23x10^{-19}\, J}$
[Co(NH3)5Cl]2+	560 nm	$\frac{6.626x10^{-34}\, J.s\times 2.998x10^{8}\, \frac{m}{s}}{560x10^{-9}\, m}=\mathbf{3.55x10^{-19}\, J}$
[Co(NH3)4Cl2]+	630 nm	$\frac{6.626x10^{-34}\, J.s\times 2.998x10^{8}\, \frac{m}{s}}{630x10^{-9}\, m}=\mathbf{3.15x10^{-19}\, J}$
[Co(NH3)3Cl3]	680 nm	$\frac{6.626x10^{-34}\, J.s\times 2.998x10^{8}\, \frac{m}{s}}{680x10^{-9}\, m}=\mathbf{2.92x10^{-19}\, J}$

Splitting energy rank (lowest to highest):

$[Co(NH_{3})_{3}Cl_{3}]<[Co(NH_{3})_{4}Cl_{2}]^{&plus;}<[Co(NH_{3})_{5}Cl]^{2&plus;}<[Co(NH_{3})_{6}]^{3&plus;}$

Conclusion

• The splitting energy of the octahedral cobalt complex decreases with the substitution of NH₃ by Cl. This means that separation in energy of d-orbitals decrease with the number of Cl in the complex.

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ANSWER - Question 4b:

Yes, It could be used to explain the trend due to spectrochemical series predicts that complex with Cl (weak ligand) will have a smaller splitting energy than complex with NH₃ (strong ligand).