| Abstract
| - Antiferromagnetic spin exchange between two (Me5C5)2YbIII fragments in molecules of the type [(Me5C5)2Yb]2[μ-L2-], where L2- is a dianionic heterocyclic base that bridges two ytterbocene fragments, has been observed. The low values of the Néel temperature (12−18 K) imply small values for the coupling constant.
- Several adducts are isolated between heterocyclic nitrogen bases and Cp*2Yb. The adductsfall into several general classes. The membership within the class is related to the reductionpotential of the given heterocyclic base relative to that of Cp*2Yb. Analytically pure 1:1adducts of the type Cp*2Yb(L) are formed with pyrazine, quinoxaline, 1,5- and 1,8-naphthyridine, and 4,4‘-bipyridine as toluene-insoluble solids. The 1:1 adducts withphthalazine and azobenzene and the 1:2 adduct with pyridazine are soluble in toluene, fromwhich they may be isolated by crystallization. All of the adducts in this class areparamagnetic, and their effective magnetic moments are consistent with the formulationCp*2YbIII(L•-) (L•- = radical anion), in which spins on the individual units are uncoupled to5 K. Adducts between Cp*2Yb and phenazine, 2,2‘-azopyridine, 2,2‘-bipyrimidine, 2,2‘-azobenzene, and 2,3-bis(2-pyridino)quinoxaline are of 2:1 stoichiometry: (Cp*2Yb)2(μ-L). Thecrystal structure of (Cp*2Yb)2(μ-bipyrimidine) shows that the two metallocenes are bridgedby a planar bipyrimidine ligand, and the other 2:1 adducts are assumed to have a similarstructure. The effective magnetic moment of these 2:1 adducts shows that each Cp*2YbIIIfragment behaves as an isolated paramagnet and the bridging ligand is a diamagnetic dianionat high temperature. At low temperature the last three adducts undergo antiferromagneticcoupling with a Néel temperature of about 20 K. A spin polarization model is advanced toaccount for the electronic exchange coupling.
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