Question

Consider the spectrum of [Cr(NH3).]* in aqueous solution below and answer the questions that follow 350 nm 465 mm logie/l mol-'cm') 400 2/nm 200 (50 000 cm') (25 000 cm 600 (17 000 cm" (1) Use the appropriate Tanabe-Sugano diagram to assign the absorbance bands in the spectrum above (11) Which spin-allowed transition is not observed in the spectrum? Suggest a reason why only two absorption bands are observed (111) Determine the B Racah parameter Show all calculations (iv) Determine the Ao parameter Show all calculations

Answer 1

1. The given complex is [Cr(NH₃)₆]³⁺.

Since NH₃ is a neutral ligand the oxidation state of Cr will be +3.

The outer electronic configuration of Cr is [Ar] 3d⁴ 4s², hence for Cr³⁺ it is [Ar] 3d³. Thus it is a d³ system.

The simplified tanabe sugano diagram is shown below for d³⁼ configuration (The abscissa is Δ_o/B and ordinate is E/B).

4T d3 70 60 4T 50 40 20 4P 10 4A2 10 30 4F 40 20 L 30

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

1. ⁴A_2g   → ⁴T_2g (465 nm)
2. ⁴A_2g  →   ⁴T_1g (350 nm)
3. ⁴A_2g   →   ⁴T_1g (P)

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

Answer for 3rd and 4th part of the question is given in the following pictures.

02C/220 Ome is att 465 nm amdother transihims Om a 35 0 mm kmew thal Cve Caave mamber C Ca)to7 Thus r8 20 7d 465 x 10 1505.3763 cm Ef ल ह 2 tear 50x 10 PLNJFS 0F 857.48m daho of the dw transhims yfaing th Cwe a8571. 428 e 1.32a33 A a1505.3763 ww.o

The mato of these tewo transihi Can be Ared to the tomab- Sugano eiaga cerationhsing a ruler alema the abseissa Cx-a unh a poimt is und at whichthe measovedng y devel separatoms have the sameraha. This ie achieed a A/30 A this poim h Volue of lB digm t ansihim can be read 2150S.3763 aq B 13 741 56 cm al505.3763 nd ransthem is 1 Valuev 851429 40 857142 4.28tm Se 40 B ovavage 741.56+ 714.24 7a7.12 cm 5

thal 4 kmow Tacah paramth B further Ce also kmow that th Dole Valu at ehich ahe between tun ranshin becom eancal to 33 is at 30. 0) DO the B value in () sulshluhay Oo 3 0 727. 92 30 x 727.2 DO d1837.6 cm-