Question

Is aqua-meth-myoglobin (Fe3+) paramagnetic? In this case, the iron has all 6 coordination positions occupied by ligands: 4 nitrogen atoms from the pyrrolic rings that compose the porphyrin, 1 nitrogen atom in the axial position from the imidazolic system of the distal histidine and in the other axial position is water (or OH - in the case of hyroxy-meth-myoglobin). Being different ligands, I don't know whether to consider that the ligands are strong or weak field and implicitly I don't know how the configuration of the iron will be after coordination. My assumption is that it remains with 5 unpaired electrons, if the ligands are weak field.

Answer 1

Aqua-meth-myoglobin (Fe3+) can be paramagnetic if the ligand field splitting is less than the pairing energy, which might allow the iron to remain high spin with five unpaired electrons. The nature of ligands and the specific coordination environment will influence the spin state of iron.

Step-by-step explanation:

The student has asked whether aqua-meth-myoglobin (Fe3+) is paramagnetic. Since the iron in this complex is in the +3 oxidation state and coordinated by a variety of ligands, including nitrogen from the pyrrolic rings of porphyrin and a nitrogen from the distal histidine, as well as a water molecule, this leads to a certain electronic configuration of iron.

Typically, in biological systems, iron(III) complexes, like myoglobin, tend to form very stable octahedral complexes and often display low spin configurations when bound to strong-field ligands, resulting in fewer unpaired electrons. The iron in oxymyoglobin, when six-coordinated, becomes low spin.

However, with weak-field ligands such as water, iron(III) could potentially remain high spin with five unpaired electrons, depending on the ligand field strength and the resulting crystal field splitting. In the case of aqua-meth-myoglobin, if we consider water a weak field ligand and histidine as moderate, the iron might indeed remain high spin hence paramagnetic, assuming our ligand field splitting is less than the pairing energy (Δoct less than P).