The Phosphonium Dibenzylide Anion as a Ligand in Organobarium Chemistry

The structural analysis of [Ph 2 P(4-methylbenzylide) 2 ] 2 Ba represents that of the first organobarium compound not belonging to the cyclopentadienyl series. The structure consists of two C 2 -symmetric phosphonium ligands (of opposite chirality) encapsulating the Ba 2+ ion. The Ba 2+ ion is in bonding contact with the ylidic and benzylic ring carbon atoms. The benzylidic carbon adopts a planar sp 2 bonding geometry, in the solid state as well as in solution. In solution, a dynamic process exchanges the two antipodes of the chiral dibenzylide ligands. Ab initio calculations and NPA charge analyses show that the di-ylide ligands in [Ph 2 P(4-methylbenzylide) 2 ] 2 Ba should be described as species with highly polar P + −C - bonds. Calculations and detailed analysis of the benzyl ring geometries in the crystal structure show that the negative charge on the benzyl group is mainly localized on the ylidic carbon. The bonding geometries at P are distorted from tetrahedral geometry such as to increase the effect of C(lone pair) → π* (PR 2 ) hyperconjugation. Thus, ylene character in the P−C bonds of dibenzylide anions is not negligible.