The mixed-valence diiron centers of nitrosyl derivatives of deoxy hemerythrin have been studied by variable-temperature variable-field Mossbauer spectroscopy. The adduct formed by the reaction of deoxy hemerythrin (deoxyHr) with nitric oxide (deoxyHrNO) produced spectra consistent with a binuclear center of the form Fe2+(S-1 = 2)-{FeNO}(7)(S-2 = 3/2) where the two iron species are antiferromagnetically coupled. We have analyzed spectra within the framework of the spin Hamiltonian H = JS(1).S-2 + Sigma(i=1)(2) = {S-i.(D) over tilde(i).S-i + beta S-i.(g) over tilde(i).H + S-i.(a) over tilde(i).I-i + I-i.(P) over tilde(i).I-i - beta(n)g(n)H.I-i} that includes exchange, zero-field splitting, electronic Zeeman, magnetic hyperfine, electric quadrupole, and nuclear Zeeman interactions, respectively. We have obtained the following parameters that reproduce simultaneously Mossbauer spectra and EPR g values: J = 27.8 cm(-1), D-1 = +5.96 cm(-1), E-1 = +0.18 cm(-1), D-2 = +18.93 cm(-1), E-2 = +2.65 cm(-1), (a) over tilde(1)/g(n)beta(n) = -(18.5,10.4,13.8) T, (a) over tilde(2)/g(n)beta(n) = -(29.7,25.0,similar to 22) T, R((a) over tilde(1) - (P) over tilde(1)) = (18,12,67)degrees, R((a) over tilde(2) --> (P) over tilde(2)) = (55,53,29)degrees and R((D) over tilde(1) --> (D) over tilde(2)) = (0,90,28)degrees. In addition, at 100 K, we determined the isomer shifts (delta(Fe)) 1.21 and 0.68 mm/s and quadrupole splittings (Delta E-Q) +2.66 and +0.61 mm/s for the Fe2+ and {FeNO}(7) sites, respectively. We have also analyzed spectra with the S-eff = 1/2 Hamiltonian for the ground scare R-eff = beta S-eff.(g) over tilde(eff).H + Sigma(i=1)(2){S-eff.(A) over tilde(i)(eff).I-i + I-i.(P) over tilde(i).I-i - beta(n)g(n)H.I-i} and have obtained parameters in the effective spin representation. In addition, we discuss the spectra of a second adduct of deoxyHr obtained by its reaction with NO in the presence of fluoride (deoxyHrFNO) which exhibits similar, but not equal, parameters to those of deoxyHrNO. Contrary to the parent form (deoxyHr) where the two ferrous ions are Mossbauer equivalent, the nitrosyl adducts have two iron sites which produce distinctly different spectra and have quite different magnetic and electronic properties. The isomer shifts of the {FeNO}(7) sites are significantly reduced with respect to the high-spin ferrous configuration and enhanced with respect to the high-spin ferric configuration. To elucidate the physical origin of Mossbauer parameters from {FeNO}(7)(S = 3/2) sites we have applied Kohn-Sham density functional theory to the representative complex Fe(C9H21N3)(NO)(N-3)(2) [Pohl and Wieghardt, J. Chem. Sec., Dalton Trans. 1987, 187]. The unusual isomer shifts have been traced to strong valence electron delocalization within the {FeNO}(7) unit, whereby some electrons are almost equally shared by metal (i.e., d(xz), and d(yz)) and NO pi* orbitals.