Facile ligand substitution is observed when the ruthenium azido complex (PPh 3 )[Ru]-N 3 (1; [Ru] ) Tp(PPh 3 )Ru) is treated with CH 3 CN, yielding the nitrile-substituted ruthenium azido complex (CH 3 CN)[Ru]-N 3 (2). Alkylation, [3 + 2] cycloaddition, and catalytic reactions of complex 2 have been investigated. In the case of [3 + 2] cycloaddition reactions, the metal-bound heterocyclic complexes (CH 3 CN)[Ru]-N 3 C 2 (CO 2 Me) 2 (3a), (CH 3 CN)[Ru]-N 3 C 2 HCO 2 Me (5), and (CH 3 CN)[Ru]-N 3 C 2 HCN (6) are obtained from dimethyl acetylenedicarboxylate, methyl propiolate, and fumaronitrile, respectively. The tetrazolato complex 8) is prepared from 2 and TCNE. Alkylation of 3a with organic bromides affords N-alkylated five-membered-ring organic triazoles. The reaction of CS 2 with 2 produces the thermally unstable thiothiatriazolate (CH 3 CN)[Ru]-N 3 CS(S) (10), which decomposes to the isothiocyanate (CH 3 CN)[Ru]-NCS (11). On the other hand, several cationic imine complexes {(CH 3 CN)[Ru]-(NHdCHR)} + (12a, R ) H; 12b, R ) CH 3 ; 12c, R ) HCdCH 2 ) are formed by the reaction of RCH 2 X with the negatively charged nitrogen atom of the azido ligand on Ru II . The reaction proceeds via formation of an alkylimido intermediate followed by N 2 evolution and proton transfer from the alkyl group to the imido nitrogen atom. Preliminary results on the catalytic activity of 2 are also presented. Interestingly, complex 2 catalyzes the dimerization of some terminal alkynes HCtCR in organic and aqueous medium. Whereas with R ) Ph, SiMe 3 , t-Bu isomeric mixtures of head-to-head and head-to-tail coupling products are obtained, in the case of R ) COOMe, E-selective head-to-head dimerization takes place exclusively. The structures of 1, 8, and 12c have been determined by X-ray diffraction analysis.

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