15N NMR and Electrochemical Studies of [RuII(hedta)]-Complexes of NO, NO+, NO2-, and NO-

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Abstract
[RuII(hedta)L]n- complexes (hedta3- = 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 RuII and the amino carboxylate hedta3-). From the NO stretching frequencies of isolated salts, the net influence of back-donation by RuII and its σ-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 RuII is sufficiently important to induce a singlet NO- complex. Values for the νNO in cm-1 for the 14N- and 15N-labeled complexes, respectively, are as follows: NO+, 1846, 1827; NO, 1858, 1842; NO-, 1383, 1370. Combined results of 15N, 13C, and 1H NMR spectra of the complexes in D2O show that [RuII(hedta)(15NO+)] is a single cis-equatorial isomer with its 15NO+ resonance at 249.6 ppm vs [15N]formamide. The two-electron-reduced [RuII(hedta)(15NO-)]2- complex exists as trans- and cis-equatorial isomers having 15NO- resonances at 609.4 and 607.4 ppm. The 15N resonances appear at 260.0 ppm for the 15NO+ ligand and at 348.8 ppm for the bound 15NO2- ligand in the [RuII(hedta)(15NO2-)(15NO+)(D2O)] complex. Differential pulse voltammetric waves for the [RuII(hedta)L]n- series occur at −0.37 V for the RuII(NO-)/RuII(NO) couple, at −0.10 V for the RuII(NO)/RuII(NO+) couple, and at +0.98 V for the RuII(NO+)/RuIII(NO+) couple. The coordinated nitrosyl ion/nitro equilibrium (L‘)RuII(NO+) + 2OH- ⇌ (L‘)RuII(NO2-) + H2O (KNO2−) was observed for L‘ = hedta3-, as for previous examples with L‘ = violurate, polypyridyl ligands, and (CN-)5 and (NH3)5 ligand sets. KNO2− = 1.44 × 1013 for L‘ = hedta3-. 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 pKa of coordinated nitrous acid in [RuII(hedta)(HONO)]- is calculated to be −0.80, a 3.85 log unit enhancement over free HONO due to the RuII charge. An MO explanation is presented to interrelate the {FeIII−(NO- triplet)} complexes, the {RuII−(NO- singlet)} type observed for [RuII(hedta)(15NO-)]2-, and the NO+ complexes of other strong-field metals. When both dz2 and dx2-y2 metal orbitals reside below the NO π* pair, the electronic repulsions favor a bent NO- triplet ligand. If both metal orbitals reside above the NO π* pair, the orbital mixing and back-donation induce a coordinated NO- singlet ligand, and if the NO π* pair reside between the two σ-based d orbitals, an NO+ ligand and reduced metal center obtain.

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