Question

For [Fe2L3](PF6)4, L= C25H20N4

Amax= 0.232 at wavelength 570.6 nm, 17525.41cm-1"

E= 11600 L/mol/cm

Comment on the value of E. Do you think the colour in this complex arises from d-d transitions? Does your answer make sense considering the structure of your ligand?

Answer 1

In the given complex, the iron is present in +2 oxidation state, which gives a d​​​​​​​​​​​​​​​6 configuration and is capable for having d-d transitions. Since d-d transitions are forbidden due to centrosymmetry present in octahedral complexes, these transitions give less color intensities and hence show lower E values.

A= ECL , where A is the absorption , E is molar extinction coefficient, C is the Concentration and L is the length of the sample holder.

Complexes which show higher E values must have high absorption ,here in the given complex the value of E is not due to just d-d transitions but also due the ligand involved. The given ligand is azobenzene which is composed of two phenyl rings interconnected by a N=N bond. It is a π acceptor ligand and will show π-π and π-π*  ​​​​​​transitions which have high absorption value since they are Laporte allowed and spin allowed as well. Hence the ligand will have a great effect on the E value