[Ru-II(hedta)L](n-) complexes (hedta(3-) = N-(hydroxyethyl)ethylenediaminetriacetate; L = NO+, n = 0; L = NO, n = 1; L = NO-, n = 2) have been prepared by the displacement of H2O/D2O via NO(g) or NO2-. This is the first reported entire set of NO+, NO, and NO- as ligands for the same metal center- in a constant oxidation state and with a constant ligand environment (here Ru-II and the amino carboxylate hedta(3-)). From the NO stretching frequencies of isolated salts, the net influence of back-donation by Ru-II and its sigma-withdrawal was observed to be that the bond order for the NO+ complex is virtually the same (ca. 2.46) as that for the NO complex (ca. 2.48). The back-donation to NO- is also small, as is that to NO, but orbital mixing of NO- and Run is sufficiently important to induce a singlet NO- complex. Values for the nu(NO) in cm(-1) for the N-14- and N-15-labeled complexes, respectively, are as follows: NO+, 1846, 1827; NO, 1858, 1842; NO-, 1383, 1370. Combined results of N-15, C-13, and H-1 NMR spectra of the complexes in D2O show that [Ru-II(hedta)((NO+)-N-15)] is a single cis-equatorial isomer with its (NO+)-N-15 resonance at 249.6 ppm vs [N-15]formamide. The two-electron-reduced [Ru-II(hedta)((NO-)-N-15)](2-) complex exists as trans- and cis-equatorial isomers having (NO-)-N-15 resonances at 609.4 and 607.4 ppm. The N-15 resonances appear at 260.0 ppm for the (NO+)-N-15 ligand and at 348.8 ppm for the bound (NO2-)-N-15 ligand in the [Ru-II(hedta)((NO2-)-N-15)((NO+)-N-15)(D2O)] complex. Differential pulse voltammetric waves for the [Ru-II(hedta)L](n-) series occur at -0.37 V for the Ru-II(NO-)/Ru-II(NO) couple, at -0.10 V for the Ru-II(NO)/Ru-II(NO+) couple, and at +0.98 V for the Ru-II(NO+)/Ru-III(NO+) couple. The coordinated nitrosyl ion/nitro equilibrium (L')Ru-II(NO+) + 20H(-) - (L')Ru-II(NO2-) + H2O (KNO2-) was observed for L' = hedta(3-), as for previous examples with L' = violurate, polypyridyl ligands, and (CN-)(5) and (NH3)(5) ligand sets. KNO2- = 1.44 x 10(13) for L' = hedta(3-). log-(KNO2-) is linearly related through the ion-pairing equilibrium constant expression to -(z1z2), the charge product of the reacting ions (here the (L')Ru(NO+) complex and OH-) from -4 through +3, excluding the (NH3)(m) ammine series with m = 4 and 5. The opposite behavior of the ammines is attributed to strong solvent H-bonding that changes for reactant and product in the nitrosyl/nitro equilibrium. The pK(a) of coordinated nitrous acid in [Ru-II(hedta)(HONO)](-) is calculated to be -0.80, a 3.85 log unit enhancement over free HONO due to the Ru-II charge. An MO explanation is presented to interrelate the {Fe-III-(NO- triplet)} complexes, the {Ru-II-(NO- singlet)} type observed for [Ru-II(hedta)((NO-)-N-15)](2-), and the NO+ complexes of other strong-field metals. When both d(z2) and d(x2-y2) metal orbitals reside below the NO pi* pair, the electronic repulsions favor a bent NO- triplet ligand. If both metal orbitals reside above the NO pi* pair, the orbital mixing and back-donation induce a coordinated NO- singlet ligand, and if the NO pi* pair reside between the two sigma-based d orbitals, an NO+ ligand and reduced metal center obtain.