The synthesis acid magnetic characterization of a series of alpha-metallocenyl nitronyl nitroxide monoradicals (M = Fe, Ru, Os) are reported. Intra-and intermolecular magnetic interactions in this series of compounds have been studied in detail by ESR spectroscopy and magnetic measurements. ESR spectra of monoradicals in dilute solutions under isotropic conditions show a small spin density on the metal centers. In the solid state, the radicals derived from ferrocene and ruthenocene show a paramagnetic behavior with very weak antiferromagnetic intermolecular interactions, as ascertained from magnetic susceptibility measurements. This behavior is rationalized, for the ferrocene derivative, by means of its crystal structure.