On the basis of the density-functional theory, the properties of the reaction product [Fe(H2O)(5)(NO)](2+) of the classical "brown-ring" reaction are studied via the B3LYP hybrid method. Here we have found that the Fe-N-O bond in the optimized structure of [Fe(H2O)(5)(NO)](2+) is linear. In addition, the vibrational frequency, atomic net charges, and spin density are analyzed and then the solvent effects are incorporated via the polarized continuum model self-consistent reaction field. Furthermore, the excitation energies are evaluated using the CIS method. Results when compared with experimental data indicate that the spin-quartet ground state of [Fe(H2O)(5)(No)](2+) is best described by the presence of Fe-II (S = 2) antiferromagnetically coupled to NO (S = 1/2 yielding [Fe-II(H2O)(5)(NO)](2+). This is clearly different from either [Fe-III(H2O)(5)(NO-)](2+) or conventional textbook [Fe-1(H2O)(5)(NO+)](2+) assignment.