Diazenido ligands, transition metal

The third area is the synthesis and characterization of aryldiazenido complexes of transition metals. In 1964 King and Bisnette isolated the first metal complex with an aryldiazenido ligand. The interest of organometallic chemists was concentrated mainly on the isolation and characterization of stable aryldiazenido complexes and not on potential metastable intermediates involved in metal-catalyzed dediazonia-tions. The situation is different, however, for metal complexes with alkyl-diazenido ligands. Complexes with aryl- and alkyldiazenido ligands are the subject of Chapter 10 in the forthcoming second book (Zollinger, 1995). 

In contrast to nitrosyls, the absence of a transferable oxygen atom in N2R ligands allows the preparation of stable diazenido complexes of oxophilic, early transition metals see for example Cp2TiCl(N2Ph). Furthermore, there are as yet no diazotate (RN=NO-) forming reactions anolo-gous to the nitrite forming reactions in nitrosyl chemistry (see equations 112 and 113). 

Table 10-1. Transition metal part of the Periodic Table. Bold-faced symbols Diazenido complexes described. Symbols in light type brackets No organometallic compounds with diazenido ligands found in the literature (also not for lanthanides and actinides).

The nonspecialist reader of this section may come to the conclusion that the area of transition-metal complexes with alkyldiazenido ligands is not very large. This impression is reinforced by the fact that these complexes lose the diazo group relatively easily, as mentioned in the beginning of the section. It is wrong, however, if one considers the fact that diazoalkanes have been used, and still are, extensively in the preparation of carbenes via metastable diazenido intermediates of metal-complex catalysts in laboratory scale syntheses, in the Fischer-Tropsch reaction, and in olefin metathesis (see, e.g., Herrmann, 1978 Doyle 1986a, 1986b and Sects. 8.7 and 8.8 of this book). 

Organometaiiic chemistry started some forty years ago and developed rapidly, particularly that part involving transition metals. This can be illustrated by the facts that, in the period 1981-1992, not less than 1319 stable organometaiiic compounds containing rhodium, a relatively rare member of the platinum group, were described (Sharp, 1995), or that stable complexes containing diazenido ligands with at least 19 transition metals are known (Sutton, 1993). On the other hand, no aryldiazenido complex of copper has been described, in spite of the fact that such coordination compounds may be formed in the Sandmeyer, Pschorr, and Meerwein reactions. The latter have been known, in part, for more than a century We are aware, of course, that the search for the structure of catalysts is methodically very different from that for stable compounds. This is likely to be the reason that in the majority of review papers either structures of stable compounds or catalysts are discussed but correlations between these areas of interest are not. ... 

Might theoretical chemistry bring a significant contribution to our general knowledge of transition metal complexes with diazenido and dinitrogen ligands It is likely that this will indeed be the case, but hardly in the near future. 

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