Lawrence Berkeley National Laboratory

The synthesis and X-ray crystal structures of Cp-2*Yb(3,3'-Me(2)bipy) and [Cp-2 Yb(3,3'-Me(2)bipy)][Cp-2 YbCl1.6I0.4]center dot CH2Cl2 are described. In both complexes, the NCCN torsion angles are approximately 40 degrees. The temperature-independent value of n(f) of 0.17 shows that the valence of ytterbium in the neutral adduct is multiconfigurational, in reasonable agreement with a CASSCF calculation that yields a n(f) value of 0.27; that is, the two configurations in the wave function are f(13)(pi(1))(1) and f(14)(pi(1))(0) in a ratio of 0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV below the triplet state (n(f) accounts for f-hole occupancy; that is, n(f) = 1 when the configuration is f(13) and n(f) = 0 when the configuration is f(14)). A correlation is outlined between the value of nf and the individual ytterbocene and bipyridine fragments such that, as the reduction potentials of the ytterbocene cation and the free x,x'-R-2-bipy ligandsapproach each other, the value of nf and therefore the f(13):f(14) ratio reaches a maximum; conversely, the ratio is minimized as the disparity increases.