Nitric oxide complexes with transition metals

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). 

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. 

The nitric oxide molecule shows many similarities to carbon monoxide in its ability to form complexes with transition metals. Nitric oxide has an extra electron, occupying a n antibonding orbital, which is relatively easily lost. In the case of terminally bound NO, simple MO theory predicts that whilst M—NO+ will be linear, M—NO- may be a bent bond. The potential thus... 

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... 

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. 

Nitric oxide rapidly reacts with transition metals, which have stable oxidation states differing by one electron (see Chapters 2 and 3). Nitric oxide is unusual in that it reacts with both the ferric (Fe " ) and ferrous forms (Fe " ) of iron. TTie unpaired electron of nitric oxide is partially transferred to the metal forming a principally ionic bond. Complexes of ferric iron with nitric oxide are called nitrosyl compounds and will nitrosate (add an NO group) many compounds, while reducing the iron to the ferrous state (Wade and Castro, 1990). 

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. 

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. ... 

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. 

Alkali, alkaline earth, and a number of transition metals are not extracted by this complex into SC-C02.49 Similarly, experiments examining the behavior of various FP oxides (e.g., Zr02, Mo03, Ru02, and CeOj) under the same conditions have led to the conclusion that with the exception of Nd, decontamination factors of 400 or more (vs. uranium) can be readily obtained for all common FP elements.53 The TBP-nitric acid adduct thus exhibits significant extraction selectivity for uranium. 

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],... 

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