Molybdenum pentachloride in benzene reacts with nitrogen oxide to form a red nitrosyl from which a series of mononitrosyl complexes of molybdenum can be synthesised, including [MoCl4(NO)]–, [MoCl5(NO)]2–, [MoCl3(NO)L2], [MoCl4(NO)L]–, [MoCl3(NO)L3], [MoCl3(NO)L2]–, [MoCl2(NO)L3], and [MoCl(NO)L4](L = neutral ligand). The co-ordinated NO in these complexes shows no sign of reactivity with nucleophiles or with electrophiles although the original red nitrosyl is apparently converted by some tertiary phosphines into amido- and phosphineiminido-complexes. Some tungsten analogues are also described.