Coordination and Organometallic Chemistry of Metal−NO Complexes

The chemistry of transition-metal-NO complexes, \nor metal nitrosyls, has taken on added significance in recent years because of the important role that nitric oxide has been found to play as a signaling molecule in biological systems. Recent work has also established that the characteristic chemistry of nitrosyl \ncomplexes is often markedly different from that \nexhibited by their isoelectronic carbonyl analogues. It has been almost 10 years since these complexes were last reviewed in the book Metal Nitrosyls,1 and so it seemed to us that an update of this rapidly expanding field would be appropriate at the present time. Consequently, this article picks up where Metal Nitrosyls left off and summarizes comprehensively the literature dealing with nitrosyl complexes from \n1991 to mid-2001. Some particularly relevant work reported in late 2001 has been included where appropriate during the final stages of revising and polishing the text. As outlined in the table of contents, the pertinent information concerning the compounds themselves is first partitioned in terms of the position of the central transition metal in the periodic \ntable and then in terms of the ligands other than NO present in the complexes. In the interests of keeping the article to a manageable size, most of the routine preparative and characterization data for individual \ncompounds are simply referenced rather than being presented and discussed in detail. The review concludes with a general section that applies, in principle, to all classes of transition-metal-NO complexes, namely the characteristic reactivities of bound \nNO groups. Whenever possible, emphasis is placed \non the unique physical and chemical properties \nimparted to nitrosyl complexes by the presence of the strongly electron-withdrawing NO ligands. If not specified otherwise, the abbreviations employed throughout are those summarized in the ACS Style Guide.2 Several general articles concerning nitrosyl complexes have appeared between 1991 and 2001. These include theoretical calculations on nitrosyl fragments3- \n5 and submissions concerning electron counting \nin nitrosyl complexes.6 Several review articles \nhave also been written concerning a large array of nitrosyl-related topics, such as linkage isomerism,7infrared spectroscopy,8 biological aspects of nitrosyl complexes,9 etc.10,11 \nWe begin our coverage below with the group 5 \nmetals, since no well-defined nitrosyl complexes of group 3 or 4 are currently known. This state of affairs is probably due to the following factors: (1) These metals are very oxophilic. (2) Most nitrosylating agents are potent oxidizers and potential oxygenatom \nsources. (3) The low-valent chemistry of the \nearly metals is not as well developed as it is for groups 6-9. Consequently, requisite precursor complexes are not readily available, and most attempts to make early-metal nitrosyls lead to the formation of oxo-containing products. A few reports of nitrosyls \nfrom groups 1-4 do exist, though. The interaction of NO with Na+ and K+ in the zeolite ETS-10 at 100 K results in the formation of the weakly bound adducts M+âââNO and M+âââON.12 Also, the species Be(NO), Mg(NO), Sc(NO), Sc(è2-NO), and Ti(NO) have been detected in argon matrixes as the products resulting from the reactions of laser-ablated metal atoms with nitric oxide.13,14 The complex Cp2Zr(NO)(PMe3) has been invoked as an intermediate during the reaction between Cp2Zr(PMe3)2 and NO.15

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