The Phosphonium Dibenzylide Anion as a Ligand in Organobarium Chemistry.
The structural anal. of [Ph2P(4-methylbenzylide)2]2Ba, I, represents that of the 1st organobarium compd. not belonging to the cyclopentadienyl series. The structure consists of two C2-sym. phosphonium ligands (of opposite chirality) encapsulating the Ba2+ ion. The Ba2+ ion is in bonding contact with the ylidic and benzylic ring C atoms. The benzylidic C adopts a planar sp2 bonding geometry, in the solid state as well as in soln. In soln., a dynamic process exchanges the two antipodes of the chiral dibenzylide ligands. Ab initio calcns. and NPA charge analyses show that the di-ylide ligands in I should be described as species with highly polar P+-C- bonds. Calcns. and detailed anal. of the benzyl ring geometries in the crystal structure show that the neg. charge on the benzyl group is mainly localized on the ylidic C. The bonding geometries at P are distorted from tetrahedral geometry such as to increase the effect of C(lone pair) -> p* (PR2) hyperconjugation. Thus, ylene character in the P-C bonds of dibenzylide anions is not negligible. I was prepd. from Ph2(4-MeC6H4CH2)P:CHC6H4Me-4 and Ba[N(TMS)2]2.
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