FERROMAGNETIC AND ANTIFERROMAGNETIC EXCHANGE IN DECAMETHYLBIMETALLOCENES

With the aim of studying next-neighbor magnetic interactions in polyme ric metallocenes the paramagnetic decamethylbimetallocenes (M'M') have been chosen as most simple model compounds. They have been synthesize d 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 spec troscopy. 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 stabl e 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 antiferromagne tic and underwent ligand exchange to nickelocene and bisfulvalenedinic kel. Unlike the usually green nickelocenes Ni'Ni' was deep red-violet owing to a new band at 528 nm. Measurements of the magnetic susceptibi lity (chi(m)) and the magnetization established a rare example of ferr omagnetic interaction within a purely organometallic compound for Co'C o'. 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 measureme nts of the H-1 NMR signal shifts and of chi(m) yielded similar J value s for Ni'Ni'. MO calculations were carried out for M'M', and the resul ts were converted into theoretical NMR spectra of the bridging fulvale ne ligand depending on the spin-carrying MO. This allowed the full ass ignment of the NMR signals and showed that the spin is delocalized to more than one MO. The MOs were shown to have different magnetic coupli ng capabilities, and the different magnetic behavior of M'M' was attri buted to the near-degeneracy of the magnetic orbitals.