Nitric oxide transition metal complexes

Ford, P.C. and Lorkovic, I.M. (2002) Mechanistic aspects of the reactions of nitric oxide with transition-metal complexes, Chem. Rev., 102, 993, and references therein. 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Studies related to the mechanisms of nitric oxide reactions with transition metal complexes in this laboratory were supported by grants from the U.S. National Science Foundation, by a Collaborative UC/Los Alamos National Laboratory Research grant, by a grant from the U.S. Japan Cooperative Research Program (Photoconversion/ Photosynthesis) (NSF INT 9116346), and by a grant from the ACS Petroleum Research Fund. We thank the students and postdoctoral fellows at UC Santa Barbara who participated in this research and acknowledge collaborative studies with Dr. David Wink (National Cancer Institute, Bethesda MD, USA), Dr. Mikio Hoshino (RIKEN, Wako-shi, Japan) and Dr. Jon Schoonover (Los Alamos National Laboratory). 

One of the most important biomolecules for which fluorescent sensing [94-96] is of great importance is nitric oxide [97-101]. Nitric oxide can react with several organic dyes, switching on their fluorescence as a result of a triazole ring closure reaction [94], There are also useful and selective NO optical sensors based on transition metal complexes (Figures 16.22 and 16.23) [94-96],... 

General. Nitric oxide readily forms complexes with transition metals and is in many ways similar to carbon monoxide. It has a single electron in a tt antibonding electron, which is easily lost. When NO is not bridging, molecular orbital theory would predict a linear M-NO+ moiety and that M-NO would be bent. In principal, this would seem to allow a ready indication of the metal oxidation state from an X-ray... 

The catalytic implications of the alternative coordination modes of nitric oxide in transition metal complexes were first noted by Collman (12). He argued that the linear bent transformation, concomitant with a change in the formal oxidation state of nitrogen from (III) to (I), results in the withdrawal of electron density from the metal center and facilitates the coordination of another ligand into a vacant site. Thus, the mixed carbonyl nitrosyl complex [Co(CO)3(NO)] undergoes thermal CO substitution by an associative mechanism, whereas the iso-electronic, homoleptic carbonyl [Ni(CO)4] reacts by a dissociative pathway (13). 

If we pass gaseous nitric oxide through a solution of a coordinatively unsaturated transition metal complex, simple addition may occur. Since nitric oxide contains an unpaired electron, this method is most commonly applied to paramagnetic starting materials. Some examples of this reaction (16, 17) are... 

Alkanes can be oxidized in the presence of some transition metal complexes in aqueous and acidic media For example, in concentrated sulfuric acid the oxidative properties of the complexes are enhanced. Solutions of derivatives of palladium(II), platinum(III), manganese(III) and mercury(II) as well as some other compounds (hydrogen peroxide, ammonium persulfate, nitric acid and even concentrated sulfuric acid itself) can be used as oxidants. In the cases of metal-free oxidants the active species are apparently electrophiles such as NO2 or SO3I-C (for nitration of aromatics, see, for example, recent publication [40] and references therein). 

A nitric oxide (NO) QD-based sensor was developed via the NO-stimulated ligand substitution of a transition metal complex assodated with CdSe/ZnS QDs [142]. The red-colored tris-(N-(dithiocarboxy) carcosine) iron(III)[Fe(DTCS)3] was linked to ammonium-capped QDs by ionic interaction. As a consequence, the functionalized QDs readed with NO by an ET process, followed by ligand substitution to yield the colorless paramagnetic bis(dithiocarbamato) nitrosyl iron(I) complex as a capping layer. This process triggered the luminescence of the QDs that enabled the detection of NO (Figure 6.11). 

As noted above, Priestley discovered the first transition metal complex of nitric oxide and in 1848 Lionel Playfair (1818-1898) reported red crystalline salts of [Fe(CN)5(NO)], 1 (commonly described as nitroprusside) using the following routes based on either concentrated nitric acid or nitrite salts ... 

Martin RL, Taylor D (1976) Bending of linear nitric oxide ligands in four-coordinate transition metal complexes. Crystal and molecular structure of dinitrosyldithioacetylaceto-natocobalt(—I), Co(NO)2. Inorg Chem 15 2970-2976... 

Nitric oxide fixation by non-transition metal complexes was reported by Ilyakina et al. [98]. Lead(II) and zinc(II) catecholato complexes readily react with nitric oxide to give the corresponding nitrosyl-containing mono-o-semiquinonates the process of binding and transfer of NO was monitored by EPR spectroscopy [98]. 

Ford, P. C., and 1. M. Lorkovic. "Mechanistic Aspects of the Reactions of Nitric Oxide with Transition-Metal Complexes." Chemical Reviews, 102 2002,993-1017. 

Metal nitrosyls are the transition metal complexes of nitric oxide (NO) containing a metal-nitrogen bond. Roussin s red salt, Na2[Fe2(NO)4S2], and Roussin s black salt, Na[Fe4(NO)7S3], were the earliest known metallic nitrosyls. In line with the inclusion of metal carbonyl complexes under the category of organometaUic compounds, the metallic nitrosyl complexes are also recently included in organometaUic compounds. The nitric oxide cation is isoelectronic with carbon monoxide. Hence, there is quite a bit of resemblance in the chemistry of metaUic carbonyls and nitrosyls. However, the contrasts in these chemistries are also noteworthy. 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

Unlike nitric oxide, NO, the monomeric radical sulfur nitride, NS, is only known as a short-lived intermediate in the gas phase. Nevertheless the properties of this important diatomic molecule have been thoroughly investigated by a variety of spectroscopic and other physical techniques (Section 5.2.1). The NS molecule is stabilized by coordination to a transition metal and a large number of complexes, primarily with metals from Groups 6, 7, 8 and 9, are known. Several detailed reviews of the topic have been published. ... 

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