The osmium(VI) nitrido complex TpOs(N)Cl-2 (1) has been prepared from K[Os(N)O-3] and KTp in aqueous ethanolic HCl. It reacts rapidly with PhMgCl and related reagents with transfer of a phenyl group to the nitrido ligand. This forms Os(IV) metalla-analido complexes, which are readily protonated to give the analido complex TpOs(NHPh)Cl-2 (4). The nitrido-phenyl derivatives TpOs(N)PhCl and TpOs(N)Ph-2 react more slowly with PhMgCl and are not competent intermediates for the reaction of 1 with PhMgCl. Reactions of 1 with alkyl- and arylboranes similarly result in transfer of one organic group to nitrogen, leading to isolable borylamido complexes such as TpOs[N(Ph)(BPh2)]Cl-2 (11). This is an unprecedented insertion of a nitrido ligand into a boron-carbon bond. Hydrolysis of 11 gives 4. Mechanistic studies suggest that both the Grignard and borane reactions proceed by initial weak coordination of Mg or B to the nitrido ligand, followed by migration of the carbanion to nitrogen. The hydrocarbyl group does not go to osmium and then move to nitrogen-there is no change in the atoms bound to the osmium during the reactions. It is suggested that there may be a general preference for nucleophiles to add directly to the metal-ligand multiple bond rather than binding to the metal first and migrating. Ab initio calculations show that the unusual reactivity of 1 results from its accessible LUMO and LUMO + 1, which are the Os=N pi* orbitals. The bonding in 1 and its reactivity with organoboranes are reminiscent of CO.