Nitric oxide as a non-innocent ligand in (bio-)inorganic complexes: spin and electron transfer in Fe(II)-NO bond

The nature of electron density transfer upon bond formation between NO ligand and Fe(II) center is analyzed on the basis of DFT calculation for two {Fe-NO}(7) complexes with entirely diverse geometric and electronic structures: Fe(II)P(NH3)NO (with bent Fe-N-O unit) and [Fe(II)(H2O)5(NO)](2+) (with linear Fe-N-O structure). Proper identification of an electronic status of the fragments, "prepared" to make a bond, was found necessary to get meaningful resolution of charge and spin transfer processes from a spin-resolved analysis of natural orbitals for chemical valence. The Fe(II)P(NH3)NO adduct (built of NO(0) (S=1/2) and Fe(II)P(NH3) (S=0) fragments) showed a strong π*-backdonation competing with spin transfer via a σ-donation, yielding significant red-shift of the NO stretching frequency. [Fe(II)(H2O)5(NO)](2+) (built of NO(0) (S=1/2) antiferromagnetically coupled to Fe(II)(H2O)5 (S=2) fragment) gave no noticeable charge or spin transfer between fragments; a slight blue-shift of the NO stretching frequency could be related to a residual π-donation due to weak π-bonding.