Question

Use Tanabe-Sugano diagrams to predict the number of transitions for the following species:
a) Mn(H2O)6 ^2+
b) Cu(H2O)6 ^2+
c) Fe(CN)6 ^3-

Answer 1

In the [Mn(H₂O)₆]²⁺ metal complex, manganese has an oxidation state of +2, thus it is a d⁵ ion. H₂O is a weak field ligand (spectrum shown below), and according to the Tanabe–Sugano diagram for d⁵ ions, the ground state is ⁶A₁. there is no sextet spin multiplicity in any excited state, hence the transitions from this ground state are expected to be spin-forbidden and the band intensities should be low

the cobalt ion has the oxidation state of +2, and it is a d⁷ ion. From the high-spin (left) side of the d⁷ Tanabe–Sugano diagram, the ground state is ⁴T₁(F), and the spin multiplicity is a quartet. The diagram shows that there are three quartet excited states: ⁴T₂, ⁴A₂, and ⁴T₁(P). From the diagram one can predict that there are three spin-allowed transitions. However, the spectra of [Co(H₂O)₆]²⁺ does not show three distinct peaks that correspond to the three predicted excited states. Instead, the spectrum has a broad peak (spectrum shown below). Based on the T-S diagram, the lowest energy transition is ⁴T₁ to ⁴T₂, which is seen in the near IR and is not observed in the visible spectrum. The main peak is the energy transition ⁴T₁(F) to ⁴T₁(P), and the slightly higher energy transition (the shoulder) is predicted to be ⁴T₁ to ⁴A₂