Nitrito Complexes Research Articles

Introducing Weakly Ligated Tris(trifluoromethyl)copper(III)

AbstractTris(trifluoromethyl)copper(III), Cu(CF3)3, was prepared for the first time and fully characterized in the form of weakly coordinated highly reactive solvent neutral adducts (DMF)2Cu(CF3)3 and (MeCN)Cu(CF3)3. Free Cu(CF3)3 is an elusive molecule of distorted T‐shaped geometry with Lewis acid properties, which is stabilized by solvation. Substitution of solvent molecules with monoanionic ligands in mild conditions afforded the first Cu(III)(CF3)3 carboxylates [Cu(CF3)3(OAc)]− and [Cu(CF3)3(OBz)]− and Cu(III) nitrito complex [Cu(CF3)3(ONO)]−. An improved synthesis of valuable Grushin's trifluoromethylation reagents via the intermediacy of (MeCN)Cu(CF3)3 was developed. (DMF)2Cu(CF3)3 demonstrated promising properties in photoinduced radical C‐H trifluoromethylation. Oxidation state theoretical studies showed the Cu(CF3)3 compounds to be borderline between classical Cu(III) and inverted ligand field Cu(I) assignment.

Structural and theoretical exploring of noncovalent interactions in Chlorido- and Nitrito-rhenium(I) tricarbonyl complexes bearing 2,3-Butadiene-bis(2-nitrobenzylidene)hydrazine Ligand: Intramolecular Re–κ1-endo-ONO(lone pair)…π*(C[tbnd]O) interaction

Herein, we report the synthesis, characterization and combined structural and full computational analysis of noncovalent interactions in a new hydrazine ligand and its two chlorido- and endo-nitrito-rhenium(I) tricarbonyl complexes. The analysis of crystal structures has been accompanied by comprehensive computational studies of the noncovalent interactions utilizing the quantum theory of atoms in molecules (QTAIM), natural bond orbitals (NBO), independent gradient model (IGM), and electron localization function (ELF) to shed light on the nature of the interactions. On the other hand, comprehensive energy decomposition analysis (EDA) by extended transition state coupled with natural orbitals for chemical valence (ETS-NOCV) scheme in a series of crystallographically retrieved dimers of the complexes was done to unveil the physical origin of these interactions and the factors affecting the interaction strength. In case of the Re- κ1-nitrito complex, the Interacting Quantum Atoms (IQA) scheme was used for detailed analysis of the interesting intramolecular Re–κ1-endo-ONO(lone pair)…π* (CO) interaction in the nitrito complex.

Nitric Oxide Dioxygenation by O2 Adducts of Manganese Porphyrins.

We describe here nitric oxide dioxygenation (NOD) by the dioxygen manganese porphyrin adducts Mn(Por)(η2-O2) (Por2- = the meso-tetra-phenyl or meso-tetra-p-tolylporphyrinato dianions, TPP2- and TTP2-). The Mn(Por)(η2-O2) was assembled by adding O2 to sublimed layers of MnII(Por). When NO was introduced and the temperature was slowly raised from 80 to 120 K, new IR bands with correlated intensities grew concomitant with depletion of the υ(O2) band. Isotope labeling experiments with 18O2, 15NO, and N18O combined with DFT calculations provide the basis for identifying the initial intermediates as the six-coordinate peroxynitrito complexes (ON)Mn(Por)(η1-OONO). Further warming to room temperature led to formation of the nitrato complexes Mn(Por)(η1-ONO2), thereby demonstrating the ability of these metal centers to promote NOD. However, comparable quantities of the nitrito complexes Mn(Por)(η1-ONO) are also formed. In contrast, when the analogous reactions were initiated with the weak σ-donor ligand tetrahydrofuran or dimethyl sulfide present in the layers, formation of Mn(Por)(η1-ONO2) is strongly favored (∼90%). The latter are formed via a 6-coordinate intermediate (L)Mn(Por)(η1-ONO2) (L = THF or DMS) that loses L upon warming. These reaction patterns are compared to those observed previously with analogous iron and cobalt porphyrin complexes.

Direct Resonance Raman Characterization of a Peroxynitrito Copper Complex Generated from O2 and NO and Mechanistic Insights into Metal‐Mediated Peroxynitrite Decomposition

AbstractWe report the formation of a new copper peroxynitrite (PN) complex [CuII(TMG3tren)(κ1‐OONO)]+ (PN1) from the reaction of [CuII(TMG3tren)(O2.−)]+ (1) with NO.(g) at −125 °C. The first resonance Raman spectroscopic characterization of such a metal‐bound PN moiety supports a cis κ1‐(−OONO) geometry. PN1 transforms thermally into an isomeric form (PN2) with κ2‐O,O′‐(−OONO) coordination, which undergoes O−O bond homolysis to generate a putative cupryl (LCuII−O.) intermediate and NO2.. These transient species do not recombine to give a nitrato (NO3−) product but instead proceed to effect oxidative chemistry and formation of a CuII–nitrito (NO2−) complex (2).

Direct Resonance Raman Characterization of a Peroxynitrito Copper Complex Generated from O2 and NO and Mechanistic Insights into Metal-Mediated Peroxynitrite Decomposition.

We report the formation of a new copper peroxynitrite (PN) complex [CuII (TMG3 tren)(κ1 -OONO)]+ (PN1) from the reaction of [CuII (TMG3 tren)(O2.- )]+ (1) with NO.(g) at -125 °C. The first resonance Raman spectroscopic characterization of such a metal-bound PN moiety supports a cis κ1 -(- OONO) geometry. PN1 transforms thermally into an isomeric form (PN2) with κ2 -O,O'-(- OONO) coordination, which undergoes O-O bond homolysis to generate a putative cupryl (LCuII -O. ) intermediate and NO2. . These transient species do not recombine to give a nitrato (NO3- ) product but instead proceed to effect oxidative chemistry and formation of a CuII -nitrito (NO2- ) complex (2).

Nitric oxide-release study of a bio-inspired copper(i)-nitrito complex under chemical and biological conditions.

The selective and efficient nitrite reduction process is ubiquitous in biological systems. To understand copper-mediated nitrite reduction, we developed a bio-inspired model system to investigate the mechanism of copper-containing nitrite reductase. A well-characterized copper(i)-nitrate complex with amino functionalized 2-(diphenylphosphino)aniline ligands, [(Ph2PC6H4(o-NH2))2Cu(ONO)], demonstrated the aniline protonation will cause NO release in an acidic environment. To further understand NO releasing ability, we also performed pH-dependency experiments and confocal imaging to release NO under physiological buffer conditions. According to titration and spectroscopic studies on the protonation reaction of complex [(Ph2PC6H4(o-NH2))2Cu(ONO)], we proposed a mechanistic pathway for proton transfer and NO release. Furthermore, DFT calculations predicted that the release of NO takes place via aniline in both organic and aqueous media. These results highlight the importance of the proton-rich microenvironment around the copper(i)-nitrite core to induce nitrate reduction in a chemical and biological environment.

A mononuclear nonheme {FeNO}6 complex: synthesis and structural and spectroscopic characterization.

While the synthesis and characterization of {FeNO}7,8,9 complexes have been well documented in heme and nonheme iron models, {FeNO}6 complexes have been less clearly understood. Herein, we report the synthesis and structural and spectroscopic characterization of mononuclear nonheme {FeNO}6 and iron(iii)-nitrito complexes bearing a tetraamido macrocyclic ligand (TAML), such as [(TAML)FeIII(NO)]- and [(TAML)FeIII(NO2)]2-, respectively. First, direct addition of NO(g) to [FeIII(TAML)]- results in the formation of [(TAML)FeIII(NO)]-, which is sensitive to moisture and air. The spectroscopic data of [(TAML)FeIII(NO)]-, such as 1H nuclear magnetic resonance and X-ray absorption spectroscopies, combined with computational study suggest the neutral nature of nitric oxide with a diamagnetic Fe center (S = 0). We also provide alternative pathways for the generation of [(TAML)FeIII(NO)]-, such as the iron-nitrite reduction triggered by protonation in the presence of ferrocene, which acts as an electron donor, and the photochemical iron-nitrite reduction. To the best of our knowledge, the present study reports the first photochemical nitrite reduction in nonheme iron models.

Copper(II) Activation of Nitrite: Nitrosation of Nucleophiles and Generation of NO by Thiols.

Nitrite (NO2-) and nitroso compounds (E-NO, E = RS, RO, and R2N) in mammalian plasma and cells serve important roles in nitric oxide (NO) dependent as well as NO independent signaling. Employing an electron deficient β-diketiminato copper(II) nitrito complex [Cl2NNF6]Cu(κ2-O2N)·THF, thiols mediate reduction of nitrite to NO. In contrast to NO generation upon reaction of thiols at iron nitrite species, at copper this conversion proceeds through nucleophilic attack of thiol RSH on the bound nitrite in [CuII](κ2-O2N) that leads to S-nitrosation to give the S-nitrosothiol RSNO and copper(II) hydroxide [CuII]-OH. This nitrosation pathway is general and results in the nitrosation of the amine Ph2NH and alcohol tBuOH to give Ph2NNO and tBuONO, respectively. NO formation from thiols occurs from the reaction of RSNO and a copper(II) thiolate [CuII]-SR intermediate formed upon reaction of an additional equiv thiol with [CuII]-OH.

Synthesis, spectral and structural characterization and computational studies of rhenium(I)-tricarbonyl nitrito complexes of 2,2′-bipyridine and 2,9-dimethylphenanthroline ligands: π-Accepting character of the diimine ligands

The synthesis, characterization, structural and computational studies of mononuclear Re(I) tricarbonyl complexes of 2,2′-bipyridine (2,2′-bpy) and 2,9-dimethylphenantroline (2,9-Me2Phen), [Re(CO)3(NN)(X)], where NN=2,2′-bpy, X=Br (1) and X=ONO (3); NN=2,9-Me2Phen, X=Br (2) and X=ONO (4), are reported. The complexes characterized by crystallographic and spectroscopic methods and elemental analyses. In each complex the Re(I) centre shows the distorted octahedral geometry. Single crystal X-ray diffraction data revealed the endo-nitrito (κ1–ONO) coordination in complexes 3 and 4. It has been shown that the replacement of the bromo ligand in complexes 1 and 2, either by AgOTf/NaNO2 in a mixture of CH3CN/H2O or AgNO2 in CH2Cl2 solution under dark and inert atmosphere conditions, were resulted the corresponding endo-nitrito (κ1–ONO) complexes. Density functional theory (DFT) was used for geometry optimization of the singlet and triplet states in gas phase and the electronic structure calculations. DFT calculations showed that the HOMO-LUMO energy gap is increased by replacing of the axial bromo with the electron withdrawing nitrito ligand. The analysis of the molecular orbital (MO) compositions in terms of occupied and unoccupied fragment orbitals in each complex was performed by AOMix program. Charge decomposition analysis (CDA) based on the electron-donation and back-donation revealed that the π-accepting character of 2,2′-bpy is greater than 2,9-Me2phen in these similar complexes. The singlet excited states are examined by TD-DFT showed that the occupied orbitals involved in the transitions have a significant mixture of Re and X, and the lowest unoccupied orbital is a π∗ orbitals of the diimine ligands. Details of the excited state character are revealed by TD-DFT, calculated changes of electron density distribution using electron density difference map (EDDM) and confirmed the MLCT/LLCT and IL character of the transitions. NCIPLOT analysis based on promolecular densities showed that the intermolecular CH⋯O, O⋯O and π⋯π interactions in complexes 3 and 4 are attractive based on their electron density and reduced electron density gradient.

Reductive nitrosylation of nickel(ii) complex by nitric oxide followed by nitrous oxide release.

Ni(ii) complex of ligand ( = bis(2-ethyl-4-methylimidazol-5-yl)methane) in methanol solution reacts with an equivalent amount of NO resulting in a corresponding Ni(i) complex. Adding further NO equivalent affords a Ni(i)-nitrosyl intermediate with the {NiNO}(10) configuration. This nitrosyl intermediate upon subsequent reaction with additional NO results in the release of N2O and formation of a Ni(ii)-nitrito complex. Crystallographic characterization of the nitrito complex revealed a symmetric η(2)-O,O-nitrito bonding to the metal ion. This study demonstrates the reductive nitrosylation of a Ni(ii) center followed by N2O release in the presence of excess NO.

Nitrite to nitric oxide interconversion by heme FeII complex assisted by [CuI(tmpa)

The present computational study complements the recent experimental efforts by Karlin and coworkers to describe the interconversion of nitrite to nitric oxide by means of an iron porphyrin complex together with a Cu chemical system, i.e., the iron(II) complex (F8TPP)FeII [F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2−)] and a preformed copper(II)–nitrito complex [(tmpa)CuII(NO2)][B(C6F5)4] [tmpa = tris(2-pyridylmethyl)amine], being the latter an oxidized species of [(tmpa)CuI(MeCN)]+. By DFT calculations, we unravel how the reduction of nitrite to nitric oxide takes place through a μ-oxo heme-FeIII–O–CuII complex, following a mimetic path as in the cytochrome c oxidase. Mayer bond order (MBO) and energy decomposition analyses are used to analyze the bonding strength of such nitro derivatives to either copper or iron.

NO disproportionation at a mononuclear site-isolated Fe(2+) center in Fe(2+)-MOF-5.

The weak-field ligand environments at the metal nodes of metal-organic frameworks (MOFs) mimic the electronic environment of metalloenzyme active sites, but little is known about the reactivity of MOF nodes toward small molecules of biological relevance. Here, we report that the ferrous ions in Fe(2+)-exchanged MOF-5 disproportionate nitric oxide to produce nitrous oxide and a ferric nitrito complex. Although mechanistic studies of N-N bond forming transformations often invoke a hyponitrite species, as in nitric oxide reductase and NOx reduction catalysis, little is known about this intermediate in its monoanionic state. Together with the first report of N-N coupling between NO molecules in a MOF, we present evidence for a species that is consistent with a ferric hyponitrite radical, whose isolation is enabled by the spatial constraints of the MOF matrix.

Nitrito complexes of nickel(II), copper(II) and cobalt(II) with tridentate pyrazole based planer ligand: Structure, spectroscopy, thermal properties and imitative nuclease activity

Functional documentation of metal complexes are essential for various aspects including gene function/regulation and drug design for lethal diseases like cancer (Sigel and Martin (1982), Peters et al. (1997), Martin and Mascharak (2000), Hazra et al. (2007), Zhang et al. (2002), Patra et al. (2003)). Therefore, we have synthesized three new nitrite bound coordination complexes with the ligand 2,6-bis(3,4,5-trimethylpyrazole)pyridine (btmpp), [Ni(btmpp)(ONO)2] (1), [Cu(btmpp)(ONO)2] (2) and [Co(btmpp)(ONO)2] (3) and complexes were characterized by elemental analysis, IR spectroscopy and X-ray single-crystal diffraction analysis. Thermal stabilities of these complexes were studied by thermogravimetric (TG) and differential thermal analyses (DTA). The single crystal X-ray analyses showed that the geometry around the Ni atom was distorted octahedral with NiN3O3 coordination sphere. One of the nitrites in complex 1 is terminally bonded to the metal centre through the oxygen atom, whereas the other one is chelated as a bidentate through both oxygen atoms. In complex 2, the coordination around the Cu atom is a distorted square-pyramid (τ=0.083) involving three N atoms from btmpp and two oxygen atoms from nitrites. Both nitrite groups are terminally bonded to the metal. DNA interaction properties of the compounds were also analyzed by agarose gel electrophoresis. The results indicated that all of the complexes converted supercoiled plasmid DNA (pET21a) into its nicked-circular form without addition of any external agents at physiological conditions. Complex 2 was the most effective on the cleavage of DNA, and its activity was observed as a conversion of double stranded DNA into nicked and linear form.

Six-coordinate nitrito and nitrato complexes of manganese porphyrin.

Reaction of small increments of NO2 gas with sublimed amorphous layers of Mn(II)(TPP) (TPP = meso-tetra-phenylporphyrinato dianion) in a vacuum cryostat leads to formation of the 5-coordinate monodentate nitrato complex Mn(III)(TPP)(η(1)-ONO2) (II). This transformation proceeds through the two distinct steps with initial formation of the five coordinate O-nitrito complex Mn(III)(TPP)(η(1)-ONO) (I) as demonstrated by the electronic absorption spectra and by FTIR spectra using differently labeled nitrogen dioxide. A plausible mechanism for the second stage of reaction is offered based on the spectral changes observed upon subsequent interaction of (15)NO2 and NO2 with the layered Mn(TPP). Low-temperature interaction of I and II with the vapors of various ligands L (L = O-, S-, and N-donors) leads to formation of the 6-coordinate O-nitrito Mn(III)(TPP)(L)(η(1)-ONO) and monodentate nitrato Mn(III)(TPP)(L)(η(1)-ONO2) complexes, respectively. Formation of the 6-coordinate O-nitrito complex is accompanied by the shifts of the ν(N═O) band to lower frequency and of the ν(N-O) band to higher frequency. The frequency difference between these bands Δν = ν(N═O) - ν(N-O) is a function of L and is smaller for the stronger bases. Reaction of excess NH3 with I leads to formation of Mn(TPP)(NH3)(η(1)-ONO) and of the cation [Mn(TPP)(NH3)2](+) plus ionic nitrite. The nitrito complexes are relatively unstable, but several of the nitrato species can be observed in the solid state at room temperature. For example, the tetrahydrofuran complex Mn(TPP)(THF)(η(1)-ONO2) is stable in the presence of THF vapors (∼5 mm), but it loses this ligand upon high vacuum pumping at RT. When L = dimethylsulfide (DMS), the nitrato complex is stable only to ∼-30 °C. Reactions of II with the N-donor ligands NH3, pyridine, or 1-methylimidazole are more complex. With these ligands, the nitrato complexes Mn(III)(TPP)(L)(η(1)-ONO2) and the cationic complexes [Mn(TPP)(L)2](+) coexist in the layer at room temperature, the latter formed as a result of NO3(-) displacement when L is in excess.

Reactions of the unsaturated ditungsten complexes [W2Cp2(μ-PPh2)2(CO)x] (x = 1, 2) with nitric oxide: stereoselective carbonyl displacement and oxygen-transfer reactions of a nitrite ligand.

The dicarbonyl complex trans-[W2Cp2(μ-PPh2)2(CO)2] (Cp = η(5)-C5H5) reacted rapidly with NO (5% in N2) at 273 K to give selectively cis-[W2Cp2(μ-PPh2)2(NO)2]. In contrast, the analogous reactions of monocarbonyl [W2Cp2(μ-PPh2)2(μ-CO)] yielded either trans-[W2Cp2(μ-PPh2)2(NO)2] or the nitrito complex [W2Cp2(μ-PPh2)2(ONO)(CO)(NO)] (W-W = 2.9797(4) Å), depending on experimental conditions, with the latter presumably arising from reaction with trace amounts of oxygen in the medium. The stereoselectivity of the above reactions can be rationalized by assuming the participation of 33-electron [W2Cp2(μ-PPh2)2(CO)(NO)] intermediates which rapidly add a second molecule of NO via η(2)-C5H5 intermediates to eventually yield the corresponding dinitrosyls with inversion of the stereochemistry at the dimetal center, as supported by density functional theory (DFT) calculations. The nitrito complex was thermally unstable and evolved through oxygen transfer either to the carbonyl ligand, to yield the above dinitrosyls with release of CO2, or to the phosphide ligand, to give the phosphinito derivative cis-[W2Cp2(μ-OPPh2)(μ-PPh2)(NO)2], depending on experimental conditions. According to DFT calculations, the first process would involve transient dissociation/recombination of the nitrite ligand followed by coupling to carbonyl to give an intermediate with a chelate W{C,N-C(O)ON(O)} ring. Indeed, the nitrite ligand could be easily removed upon reaction of the nitrito complex with Na(BAr'4), but immediate decomposition also took place to render the electron-precise dicarbonyl [W2Cp2(μ-PPh2)2(CO)2(NO)]BAr'4 (W-W = 2.9663(3) Å) as the unique product (Ar' = 3,5-C6H3(CF3)2). Attempts to decarbonylate the latter complex photochemically yielded instead the oxo derivatives cis- and trans-[W2Cp2(μ-PPh2)2(O)(NO)]BAr'4 as the only isolable products (W-W = 2.980(2) and 3.0077(3) Å, respectively).

Nitric oxide generation from heme/copper assembly mediated nitrite reductase activity

Nitric oxide (NO) as a cellular signaling molecule and vasodilator regulates a range of physiological and pathological processes. Nitrite (NO2 (-)) is recycled in vivo to generate nitric oxide, particularly in physiologic hypoxia and ischemia. The cytochromec oxidase binuclear hemea 3/CuB active site is one entity known to be responsible for conversion of cellular nitrite to nitric oxide. We recently reported that a partially reduced heme/copper assembly reduces nitrite ion, producing nitric oxide; the heme serves as the reductant and the cupric ion provides a Lewis acid interaction with nitrite, facilitating nitrite (N-O) bond cleavage (Hematian et al., J. Am. Chem. Soc. 134:18912-18915, 2012). To further investigate this nitrite reductase chemistry, copper(II)-nitrito complexes with tridentate and tetradentate ligands were used in this study, where either O,O'-bidentate or O-unidentate modes of nitrite binding to the cupric center are present. To study the role of the reducing ability of the ferrous heme center, two different tetraarylporphyrinate-iron(II) complexes, one with electron-donating para-methoxy peripheral substituents and the other with electron-withdrawing 2,6-difluorophenyl substituents, were used. The results show that differing modes of nitrite coordination to the copper(II) ion lead to differing kinetic behavior. Here, also, the ferrous heme is in all cases the source of the reducing equivalent required to convert nitrite to nitric oxide, but the reduction ability of the heme center does not play a key role in the observed overall reaction rate. On the basis of our observations, reaction mechanisms are proposed and discussed in terms of heme/copper heterobinuclear structures.

Reactions of a Chromium(III)-Superoxo Complex and Nitric Oxide That Lead to the Formation of Chromium(IV)-Oxo and Chromium(III)-Nitrito Complexes

The reaction of an end-on Cr(III)-superoxo complex bearing a 14-membered tetraazamacrocyclic TMC ligand, [Cr(III)(14-TMC)(O2)(Cl)](+), with nitric oxide (NO) resulted in the generation of a stable Cr(IV)-oxo species, [Cr(IV)(14-TMC)(O)(Cl)](+), via the formation of a Cr(III)-peroxynitrite intermediate and homolytic O-O bond cleavage of the peroxynitrite ligand. Evidence for the latter comes from electron paramagnetic resonance spectroscopy, computational chemistry and the observation of phenol nitration chemistry. The Cr(IV)-oxo complex does not react with nitrogen dioxide (NO2), but reacts with NO to afford a Cr(III)-nitrito complex, [Cr(III)(14-TMC)(NO2)(Cl)](+). The Cr(IV)-oxo and Cr(III)-nitrito complexes were also characterized spectroscopically and/or structurally.

Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion

The heme(a3)/Cu(B) active site of cytochrome c oxidase is responsible for cellular nitrite reduction to nitric oxide; the same center can return NO to the nitrite pool via oxidative chemistry. Here, we show that a partially reduced heme/Cu assembly reduces NO(2)(-) ion, producing nitric oxide. The heme serves as the reductant, but the Cu(II) ion is also required. In turn, a μ-oxo heme-Fe(III)-O-Cu(II) complex facilitates NO oxidation to nitrite; the final products are the reduced heme and Cu(II)-nitrito complexes.

Nanomaterial antennae in photo-delivery of bioactive molecules

Our laboratory has developed photochemical strategies for delivery of NO and CO to physiological targets that allow control of the location, timing, and dosage of such delivery. We have explored the photoreactions of the iron/sulfur/nitrosyl clusters Fe 2 (μ-SR) 2 (NO) 4 and the Cr (III) nitrito complex trans- Cr (cyclam) ( ONO ) 2 + that release NO under visible irradiation but not at longer wavelengths. Thus, we have designed new supramolecular assemblies with water-soluble semiconductor quantum dots and lanthanide near-IR upconversion materials to serve as antennae for photosensitization of reactions from analogous precursors and increase rates of NO release using longer wavelength visible light. Certain antennae have high two photon cross-sections, a feature that allows use of NIR light (where tissue transmission is maximized) to sensitize photoreactions of these systems.

Photocrystallographic identification of metastable nitrito linkage isomers in a series of nickel(ii) complexes

Single crystal photocrystallographic experiments and solid state Raman spectroscopy have been used to determine the low temperature, metastable structures of the nickel(ii) nitrito complexes [Ni(aep)(2)(η(1)-ONO)(2)] 1#O (aep = 1-(2-aminoethyl)piperidine), [Ni(aem)(2)(η(1)-ONO)(2)] 2#O (aem = 1-(2-aminoethyl)morpholine), and [Ni(aepy)(2)(η(1)-ONO)(2)] 3#O (aepy = 1-(2-aminoethyl)pyrrolidine and where the #O denotes the oxygen-bound nitrito metastable molecule). These linkage isomers of the equivalent nitro complexes [Ni(aep)(2)(η(1)-NO(2))(2)] 1, [Ni(aem)(2)(η(1)-NO(2))(2)] 2 and [Ni(aepy)(2)(η(1)-NO(2))(2)] 3 are formed by LED irradiation at temperatures below 120 K. The behavior of the three complexes upon irradiation is generally similar, but some subtle differences have been observed. From the crystallographic studies all three complexes 1-3 exhibit the endo-nitrito linkage isomer upon irradiation, however, for 3# (a crystal structure that contains components of both 3 and 3#O) an exo-nitrito isomer is also observed. Under conditions of 90-100 K, with blue light, the conversion percentages to the nitrito isomers, 1#O, 2#O and 3#O were 16%, 22% and 30%, respectively. At temperatures below 110 K all three nitrito isomers were stable for over four hours but while 2#O and 3#O could be detected at temperatures down to 30 K, at temperatures below 60 K the metastable structure 1#O appeared to be quenched and only the nitro isomer 1 was identified in the crystal. The solid state Raman spectra for 1#, 2# and 3# confirmed the photocrystallographic results with the nitrito isomers being identified from the O-N-O deformation vibrations.