The absorption spectrum of [Cr(NH₃)₆]³⁺ in water shows two bands around 475 and 365 nm. The ground term and the spin‐allowed transitions, respectively, are

Concept:

• Orgel Diagrams are actually correlation diagram that represents relative energy levels of electronic terms for octahedral and tetrahedral complexes.
• Orgel Diagrams are used to represent a transition in metal complexes.
• It is only applicable for high spin complexes
• transitions represented by the orgel Diagram occur only from the ground state.

Explanation:

[Cr(NH3)6]³⁺ is a weak field octahedral complex. 

First we will find the ground state of Cr³⁺(d³) 

Total Orbital Momentum, L = 1+2+0 =3 ,i.e., F

Total Spin Momentum, S = \(\frac{3}{2}\)

Spin Multiplicity = 2S+1 = \(2\times\frac{3}{2}+1\)= 4

ground state term is = ⁴F

The orgel diagram for a given complex (in the octahedral field) can be drawn as:

According to the Orgel diagram, a transition occurs only from the ground state. Therefore, any transition other than the transition from A_2g will not be considered.

So the two bands appear due to the following transitions:

• 4A2g → 4T2g (F)
• 4A2g → 4T1g (F)

​Conclusion:

[Cr(NH3)6]³⁺ has ⁴F ground state term and two peaks appear due to spin-allowed transition from  4A2g → 4T2g (F) and 4A2g → 4T1g (F)