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Solution
(a) In the complex [Co(CN)6]3−, Co is in +3 oxidation state. Since CN− is a strong field ligand, it will lead to pairing of electrons in 3d orbital of Co3+ ion. Thus, the electronic configuration of the ion will be t2g6 eg0. Since there are no unpaired electrons, there will no -d-d transition and the complex will absorb light UV light and not from visible region.
(b) In the complex [Ti(H2O)6]3+, Ti is in +3 oxidation state. Since H2O is a weak field ligand, the electronic configuration of the ion will be t2g1 eg0. Since [Ti(H2O)6]3+ is an octahedral complex, the CFSE of the complex will be −0.4Δ0.
In the complex [Cu(H2O)4]2+, Cu is in +2 oxidation state. Since H2O is a weak field ligand, the electronic configuration of the ion will be e4 t25. Since [Cu(H2O)4]2+ is a tetrahedral complex, the CFSE of the complex will be (−2.4Δt + 2.0Δt = −0.4Δt)
We know that Δt = (4/9)Δ0. Therefore, CFSE for octahedral complex, Δ0 will be higher than CFSE for tetrahedral complex, Δt.
Since wavelength absorbed is inversely proportional to CFSE, [Ti(H2O)6]3+ will absorb light of lesser wavelength than [Cu(H2O)4]2+. And, we know that the colour of complex is complementary to that is absorbed. Therefore, [Ti(H2O)6]3+ will be violet while [Cu(H2O)4]2+ will appear blue in colour.
(c) In the presence of NH3, the 3d electrons pair up leaving two d orbitals empty to be involved in d2sp3 hybridisation forming inner orbital complex in case of [Co(NH3)6]3+. Thus, the electronic configuration of the ion will be t2g6 eg0 and CFSE for the complex will be −2.4Δ0.
Since [Cu(H2O)4]2+ is a tetrahedral complex, the CFSE of the complex will be (−2.4Δt + 2.0Δt = −0.4Δt)
We know that Δt = (4/9)Δ0.
So, Δ0 = (9/4)Δt = (9/4)(−0.4) = −0.9Δ0
It is given that CFSE for [Co(NH3)6]3+(−2.4Δ0) is 27000 cm−1.
So, CFSE for [Cu(H2O)4]2+(−0.9Δ0) will be .
(d) The complexes can be arranged in the order of wavelength of absorption on the basis of field strength of ligands.
CN− > NH3 > H2O
Therefore, the complexes will absorb wavelength in the following order:
[Co(CN)6]3− > [Co(NH3)6]3+ > [Cu(H2O)4]2+
(a) In the complex [Co(CN)6]3−, Co is in +3 oxidation state. Since CN− is a strong field ligand, it will lead to pairing of electrons in 3d orbital of Co3+ ion. Thus, the electronic configuration of the ion will be t2g6 eg0. Since there are no unpaired electrons, there will no -d-d transition and the complex will absorb light UV light and not from visible region.
(b) In the complex [Ti(H2O)6]3+, Ti is in +3 oxidation state. Since H2O is a weak field ligand, the electronic configuration of the ion will be t2g1 eg0. Since [Ti(H2O)6]3+ is an octahedral complex, the CFSE of the complex will be −0.4Δ0.
In the complex [Cu(H2O)4]2+, Cu is in +2 oxidation state. Since H2O is a weak field ligand, the electronic configuration of the ion will be e4 t25. Since [Cu(H2O)4]2+ is a tetrahedral complex, the CFSE of the complex will be (−2.4Δt + 2.0Δt = −0.4Δt)
We know that Δt = (4/9)Δ0. Therefore, CFSE for octahedral complex, Δ0 will be higher than CFSE for tetrahedral complex, Δt.
Since wavelength absorbed is inversely proportional to CFSE, [Ti(H2O)6]3+ will absorb light of lesser wavelength than [Cu(H2O)4]2+. And, we know that the colour of complex is complementary to that is absorbed. Therefore, [Ti(H2O)6]3+ will be violet while [Cu(H2O)4]2+ will appear blue in colour.
(c) In the presence of NH3, the 3d electrons pair up leaving two d orbitals empty to be involved in d2sp3 hybridisation forming inner orbital complex in case of [Co(NH3)6]3+. Thus, the electronic configuration of the ion will be t2g6 eg0 and CFSE for the complex will be −2.4Δ0.
Since [Cu(H2O)4]2+ is a tetrahedral complex, the CFSE of the complex will be (−2.4Δt + 2.0Δt = −0.4Δt)
We know that Δt = (4/9)Δ0.
So, Δ0 = (9/4)Δt = (9/4)(−0.4) = −0.9Δ0
It is given that CFSE for [Co(NH3)6]3+(−2.4Δ0) is 27000 cm−1.
So, CFSE for [Cu(H2O)4]2+(−0.9Δ0) will be .
(d) The complexes can be arranged in the order of wavelength of absorption on the basis of field strength of ligands.
CN− > NH3 > H2O
Therefore, the complexes will absorb wavelength in the following order:
[Co(CN)6]3− > [Co(NH3)6]3+ > [Cu(H2O)4]2+