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The chemistry of transition-metal-NO complexes, or 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 complexes is often markedly different from that exhibited 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 1991 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 table 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 compounds 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 NO groups. Whenever possible, emphasis is placed on the unique physical and chemical properties imparted 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.

The chemistry of transition-metal-NO complexes, or 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 complexes is often markedly different from that exhibited 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 1991 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 table 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 compounds 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 NO groups. Whenever possible, emphasis is placed on the unique physical and chemical properties imparted 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.