With the aim of studying next-neighbor magnetic interactions in polymeric metallocenes the paramagnetic decamethylbimetallocenes (M'M') have been chosen as most simple model compounds. They have been synthesized for vanadium, cobalt, and nickel (to yield V'V', Co'Co', and Ni'Ni', respectively) by starting from dilithium and dithallium salts of the fulvalene dianion. The latter have been characterized by C-13 NMR spectroscopy. Decamethylbiferrocene has been synthesized as a diamagnetic standard compound, and decamethylbicobaltocenium hexafluorophosphate, as a precursor to Co'Co'. While the methylated M'M' species were stable when protected from air, the synthesis of the parent binickelocene (Ni'Ni') was accompanied by the formation of the ternickelocene NiNiNi. According to H-1 NMR spectroscopy NiNi and NiNiNi were antiferromagnetic and underwent ligand exchange to nickelocene and bisfulvalenedinickel. Unlike the usually green nickelocenes Ni'Ni' was deep red-violet owing to a new band at 528 nm. Measurements of the magnetic susceptibility (chi(m)) and the magnetization established a rare example of ferromagnetic interaction within a purely organometallic compound for Co'Co'. By contrast, V'V' and Ni'Ni' were antiferromagnetic (J = -1.6 and -180 cm(-1), respectively, with IQ = -JS(A).S-B) The H-1 and C-13 NMR spectra confirmed the expected structures of Co'Co' and Ni'Ni', while the synthesis of V'V'-d(8) and H-2 NMR spectroscopy were necessary to fully establish the vanadium compound. Temperature-dependent measurements of the H-1 NMR signal shifts and of chi(m) yielded similar J values for Ni'Ni'. MO calculations were carried out for M'M', and the results were converted into theoretical NMR spectra of the bridging fulvalene ligand depending on the spin-carrying MO. This allowed the full assignment of the NMR signals and showed that the spin is delocalized to more than one MO. The MOs were shown to have different magnetic coupling capabilities, and the different magnetic behavior of M'M' was attributed to the near-degeneracy of the magnetic orbitals.