15N NMR and Electrochemical Studies of [Ru(II)(hedta)](-) Complexes of NO, NO(+), NO(2)(-), and NO(-).

Ru(II)(hedta)L()(-) 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 H(2)O/D(2)O via NO(g) or NO(2)(-). 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 Ru(II) is sufficiently important to induce a singlet NO(-) complex. Values for the nu(NO) in cm(-)(1) for the (14)N- and (15)N-labeled complexes, respectively, are as follows: NO(+), 1846, 1827; NO, 1858, 1842; NO(-), 1383, 1370. Combined results of (15)N, (13)C, and (1)H NMR spectra of the complexes in D(2)O show that [Ru(II)(hedta)((15)NO(+))] is a single cis-equatorial isomer with its (15)NO(+) resonance at 249.6 ppm vs [(15)N]formamide. The two-electron-reduced Ru(II)(hedta)((15)NO(-))(-) complex exists as trans- and cis-equatorial isomers having (15)NO(-) resonances at 609.4 and 607.4 ppm. The (15)N resonances appear at 260.0 ppm for the (15)NO(+) ligand and at 348.8 ppm for the bound (15)NO(2)(-) ligand in the [Ru(II)(hedta)((15)NO(2)(-))((15)NO(+))(D(2)O)] complex. Differential pulse voltammetric waves for the Ru(II)(hedta)L()(-) 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(+)) + 2OH(-) right harpoon over left harpoon (L')Ru(II)(NO(2)(-)) + H(2)O (K(NO)()2()-) was observed for L' = hedta(3)(-), as for previous examples with L' = violurate, polypyridyl ligands, and (CN(-))(5) and (NH(3))(5) ligand sets. K(NO)()2()- = 1.44 x 10(13) for L' = hedta(3)(-). log(K(NO)()2()-) is linearly related through the ion-pairing equilibrium constant expression to -z(1)z(2), the charge product of the reacting ions (here the (L')Ru(NO(+)) complex and OH(-)) from -4 through +3, excluding the (NH(3))(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)((15)NO(-))(-), and the NO(+) complexes of other strong-field metals. When both d(z)()()2 and d(x)()()2(-)(y)()()2 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.