Coordination of 1,4-diazabutadiene ligands to decamethylytterbocene: Additional examples of spin coupling in ytterbocene complexes

The paramagnetic 1:1 coordination complexes of (C5Me5)(2)Yb with a series of diazabutadiene ligands, RNC(R')C(R')NR, where R = CMe3, CHMe2, adamantyl, p-tolyl, p-anisyl, mesityl when R' = H and R = p-anisyl when R' = Me, have been prepared. The complexes are paramagnetic, but their magnetic moments are less than expected for the two uncoupled spin carriers, (C5Me5)(2)Yb-III(4f(13)) and the diazabutadiene radical anions (S = (1)/(2)), which implies exchange coupling between the spins. The variable-temperature H-1 NMR spectra show that rotation about the R-N bond is hindered and these barriers are estimated. The barriers are largely determined by steric effects, but electronic effects are not unimportant.