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 S-z = 3/2 corresponding Kohn-Sham determinant is expanded in the basis of Lowdin-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}. (C) 2004 Elsevier B.V. All rights reserved.