The oxidation state of Fe depends on what you attach to it. Based on your structure it should be +4, which is a little too high. I would suggest that you make the central sulfur atoms thiolates (RS) instead in which case Fe becomes +3.

dear Dr. Aryapour

I ask your question from my husband (dr. Darzi) . he works with   Gaussian.

regards

Baher

For the structure of the {Fe2S2} cluster given by you the oxidation state works out to +4 on each Fe. This cannot be possible because it is well known that Fe+4 is too highly oxidising and will oxidise the (sulfide) S2- of Fe2S2 and also the thiolate (RS)- of the thiol. For the formula [Fe2S2(SR)4]2- the oxidation state works out to +3 for each Fe.

To the best of my knowledge there is no known compound containing a {Fe(IV)-S} moiety.

Hallo, Hassan!

  It is very problematic to assign the oxidation state to transition metal central atoms in the complexes with "non-innocent" ligands due to electron density transfer between ligand and the central atom. I tried to do it using d electron populations but this treatment significantly depends on the calculation method.

  Concerning the spin multiplicity - you must try various values and use the energetically preferred one. Due to the spin contamination in unrestricted formalism (e.g. UMP2, UB3LYP)   the SCF energy must be corrected (PMP2) or recalculated in the restricted formalism to obtain correct pure-spin energies (e.g. ROB3LYP). The unrestricted formalism is used for geometry optimization due to a poor convergency of RO-DFT methods.

  Ad population analysis - NBO is good for charges but does not show ionic and highly polarized bonds (non-Lewis character) which causes problems in metal complexes. Moreover, aromatic systems are presented using alternating single-double bonds.

Mulliken population analysis gives incorrect results for unbalanced basis sets with polarisation and diffusion functions.

   Best wishes

                        mb

p.s/: There are some complexes with formal oxidation states Fe(IV) and Fe(V) - I have calculated them in last years. Another problem is, that the real (physical) ox. state of Fe in these complexes is lower (based on d-electron populations).