Question

A number of Fe(II) enzymes use a His, His, carboxylate triad as their metal binding site in oxygen activation. Give an explanation as to why Fe(II) which prefers six coordinate geometry might only use 3 protein ligands in the active site (you may use extradiol dioxygenases as an example if you wish).

Answer 1

Fe(II) which prefers six coordinate geometry might only use 3 protein ligands in the active site

it is due to

complex [Fe(H2O)6]2+ demonstrates dynamic Jahn-Teller distortion and shows up octahedral.

For this situation, the bending is little since the decline happens in t2g orbitals. Keep in mind Fe2+ in above complex is a high turn d6 framework with t2g4 eg2 design.

Fe2+ is 3d6 . Has 0 unpaired electron in l.s. complexes .

hence the magnetic moment would be close to 0 µB