NO forms reversible complexes with non-heme ferrous enzymes and model complexes which exhibit unusual S = 3/2 ground states. These nitrosyl derivatives can serve as stable analogs of possible oxygen intermediates in the non-heme iron enzymes. Two complexes, Fe(Me(3)TACN)(NO)(N-3)(2) and FeEDTA-NO, have been studied in detail using X-ray absorption, resonance Raman, absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies and SQUID magnetic susceptibility. These studies have been complemented by spin restricted and spin unrestricted SCF-X alpha-SW electronic structure calculations. The description generated involves high spin Fe3+ antiferromagnetically coupled to NO- S = 1. This description is strongly supported by experiment (X-ray absorption pre-edge multiplets and edge energy and shape, extended X-ray absorption fine structure bond lengths, resonance Raman force constants, ligand field and charge transfer spectral assignments), which also show that starting from the Fe3+-NO- limit the NO- is involved in a strong donor bonding interaction to the Fe3+ : This donor bonding interaction involves the 4 sigma(+) molecular orbital of the NO- which is sigma antibonding with respect to the NO bond and greatly strengthens this bond. The Fe-NO bond is bent in both complexes with an angle of 156 degrees which the resonance Raman studies and SCF-X alpha-SW calculations show derives from the stabilizing effect of bending on the in-plane bonding interaction of the NO- pi* orbital with the d orbital on the iron. : This donor bonding interaction involves the 4 sigma(+) molecular orbital of the NO- which is sigma antibonding with respect to the NO bond and greatly strengthens this bond. The Fe-NO bond is bent in both complexes with an angle of 156 degrees which the resonance Raman studies and SCF-X alpha-SW calculations show derives from the stabilizing effect of bending on the in-plane bonding interaction of the NO- pi* orbital with the d orbital on the iron.