Unrestricted density functional theory of the bonding between NO radical and ferrous ion

Bonding between NO and the Fe(II) center to form so-called {FeNO}7 unit has been studied using simplistic Fe(NO)(OH)2 system within the unrestricted density functional theory using hybrid and pure exchange-correlation functionals. To analyze in details the obtained unrestricted solution for ground state with Sz = 3/2 corresponding Kohn–Sham determinant is expanded in the basis of Löwdin–Amos–Hall paired orbitals. This expansion allowed us to assign the electron structure of {FeNO}7 unit as a mixture of charge-transfer configurations {Fe+⋯NO+} and {Fe3+⋯NO−}, and the covalent configuration {Fe2+ = NO0}.