Question

Consider the spectrum of [V(H2O).]3+ in aqueous solution below and answer the questions that follow 17,000 cm 24,500 cm Absorbance Warenumber cm (a) What is the free-ion term for the electronic arrangement of the metal-ion? Use the appropriate Tanabe-Sugano diagram to assign the absorbance bands in the spectrum above Which transition is not observed in the spectrum? Suggest a reason why only two absorption bands are observed Determine the B Racah parameter Show all calculations. (e) Determine the Ao parameter Show all calculations.

Answer 1

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2. The given complex is [V(H₂O)₆]³⁺.

Since H₂O is a neutral ligand the oxidation state of V will be +3.

The outer electronic configuration of V is [Ar] 3d³ 4s², hence for V³⁺ it is [Ar] 3d². Thus it is a d² system.

The simplified tanabe sugano diagram is shown below for d² configuration (g notations are used in calculations since we are considering a octahedral molecule which is centrosymmetric. The TB diagram shown below is a general one and thus the terms doesn't have g or u representations).

E 'A, T 60 T2 3T1 'S 50 3T2 40 'A, 30 G 20 'E P T2 'D 10+ 3T, 3 3F 2 1 A/B

From the diagram we can see that the four possible electronic transitions are

1. ³T_1g   → ³T_2g (17000 cm^-1)
2. ³T_1g   →   ³T_1g (P) (24500 cm^-1)
3. ³T_1g  →   ³A_1g

The energy order of the transitions are 3>2>1.

c. The spin allowed transition that is not observed in the spectrum is ³T_1g  → ³A_1g . This is because the energy separation between these states is very high and the absorption occurs at very high frequencies or wave number (in other word at very small wavelengths in UV region). It has a low intensity and since it is in the high energy portion of the spectrum where it is masked by many totally allowed transitions, it is not observed.

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