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In the past decade new syntheses and numerous structural determinations have enlivened studies in the still relatively young field of nitrido-transition-metal complexes. Aside from the terminal function of the nitrido ligand MEN:, this group also occurs as linear p,-bridging ligand in symmetric and asymmetric coordination; examples are known with almost rightangled bridge function; and, finally, it also functions as p3-bridging ligand. Accordingly, the fresh impulses given to synthetic chemistry by nitrido complexes are also many-sided: such complexes are used, inter aha, for the preparation of phosphaniminato and thionitrosyl complexes as well as for the synthesis of metallaheterocycles of the type MN,S, and MN,P2 with delocalized x-systems. In technetium chemistry complexes with terminal nitrido group are employed as radiopharmaceuticals, and, owing to the strong trans influence of the M=N: group, nitrido complexes of molybdenum are suitable as catalysts in olefin metathesis. Finally, nitrido complexes are also of wide interest in theoretical studies.

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Numerous nitrido complexes of transition metals show very short metal-nitrogen bond lengths, suggesting M=N-triple bonds. At present, compounds of this type are being intensively investigated. In particular the molybdenum complexes are considered as model substances for the study of at least an intermediate step of N,-assimilation. This article contains a review of the structure and bonding, as well as syntheses and reactions of these complexes. The terminal nitrido ligand occurs, for example, in the tetragonal pyramidal complex anions [MNCI,] -(M = Mo, W, Re, Ru, OS)['-~] (see Fig. 1). The M=N bond is ex-Fig. 1. Structure of anions [MNX41-(M=Mo, Re, Ru, 0 s ; X = F, CI, Br, I) with terminal nitrido ligands.Angew. Chem. Int. Ed. Engl. 20, 413-426 (1981) 0 Verlag Chemie GmbH, 6940 Weinheim. 1981 057~0833/81/0505-0413 $ 02.50/0 teresting to compare these nitrido complexes with the tetrameric 0x0 (6) and fluoro complexes (7), in which the differences between the alternating bond lengths decrease in the succession [MoNCI, . POC13]4[81, [NbOCl, . POCl&"] and [NbFs]4[121: Mo-N-Mo Nb-N-Nb Nb-F-Nb 166 216 174 209 207 207 [pm]The solvating POCl, molecules are always arranged trans to the n bonded ligand and are coordinated to the metal center via the oxygen atom (see Fig. 3). Accordingly, the bonding in the trinuclear nitridomolybdenum complex [((Et,NCS2)3Mo~N]2Mo(SzCNEt2)3]3+r which contains the Mo-N-Mo-N=Mo group as a structural element can be understood[131. The structures of these types of compounds are discussed in Section 6.3. 414 Angew. Chem. Inr. Ed. Engl. 20. 413-426 (1981) Received: December 1, 1980 [A 365 IE] German version: Angew. Chem. 93, 451 (1981)