The Mn(II) and Mn(III) complexes of the pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy3H; H is the dissociable carboxamicle H), namely, [Mn(PaPy3)(H2O)]ClO4 (1) and [Mn(PaPy3)-(Cl)]ClO4 (2), with bound carboxamido nitrogen have been isolated and characterized. The high-spin Mn(II) center in 1 is very sensitive to dioxygen, and this complex is rapidly converted into 2 upon reaction with Cl- in air. The bound carboxamido nitrogen in 1 is responsible for this sensitivity toward oxidation since the analogous Schiff base complex [Mn(SBPy3)Cl]ClO4 (4) is very resistant to oxidation. Reaction of NO with 1 affords the diamagnetic { Mn-NO}(6) nitrosyl [Mn(PaPy3)(NO)]ClO4 (5). Complexes with no bound carboxamido nitrogen such as 4 and [Mn(PaPy3H)(Cl)(2)] (3) do not react with NO. No reaction with NO is observed with the Mn(III) complexes 2 and [Mn(PaPy3)(MeCN)](2+) either. Collectively these reactions indicate that NO reacts only with the Mn(II) center ligated to at least one carboxamido nitrogen. Both the carbonyl and N-O stretching frequencies (v(CO) and v(NO)) of the present and related complexes strongly suggest a {low-spin Mn(II)-NO.} formulation for 5. The alternative description flow-spin Mn(I)-NO+} is not supported by the spectroscopic and redox behavior of 5. Complex 5 is the first example of a {Mn-NO}(6) nitrosyl that exhibits photolability of NO upon illumination with low-intensity tungsten lamps in solvents such as MeCN and H2O. The rapid NO loss from 5 leads to the formation of the corresponding solvato species [Mn(PaPy3)(MeCN)](2+) under aerobic conditions. Oxidation of 5 with (NH4)(2)[Ce(NO3)(6)] in MeCN affords the highly reactive paramagnetic (S = 1/2) {MnNO}(5) nitrosyl [Mn(PaPy3)(NO)](NO3)(2) (6) in high yield. Spectroscopic and magnetic studies confirm a {low-spin Mn(II)-NO+} formulation for 6. The N-O stretching frequencies (V-NO) of 5, 6, and analogous nitrosyls reported by other groups collectively suggest that V-NO is a better indicator of the oxidation state of NO (NO+, NO., or NO-) in non-heme iron and other transition-metal complexes with bound NO.