Me), affords ( i ) simple hydrazine adducts, [Mo(y-C,-H,) (N H RNR'R") (NO)X,] (1 ), (ii) hydrazido( 1 -)-complexes, [ Mo(q-C5H5) (NRN R'R") I (NO)] (2), and (iii) hydrazido(2-) complexes, [{Mo(q-C,H,)(NO)X},NNR'R"] (4). Protonation of (2) or (4) with HBF, gives the hydrazine adducts [MO(T~-C,H,)I(NHRNR'R'')(NO)] [BF,] (3). Thespecies (l), on the basis of spectral data, may contain either a unidentate or a chelating hydrazine, the latter forming a three-membered ring with the metal. Species (2) may also contain a three-membered MoNN chelate ring which on protonation affords (3), whose structure is known to involve a bidentate hydrazine bound to only one metal atom. The binuclear species (4) contain the hydrazido(2-)-ligand asymmetrically bridging the two Mo atoms, one N atom being bound to one Mo atom by a 6-and a (p,+d,)-donor bond, and the other Mo atom being attached to both N atoms via CT bonds in a three-membered ring. The chemical and structural relationships between (1 ), (2), (3), and (4) are discussed.