NUCLEOPHILIC-ATTACK AT THE CENTRAL ALLYL CARBON-ATOM IN [(ETA-3-ALLYL)ML2]- EXPERIMENTAL FACTS AND NEW THEORETICAL INSIGHTS( COMPLEXES (M =PD, PT) )

The reaction of ketene silyl acetals with [(eta3-allyl)PtL2]+OAc- lead s to formation of platina-(II)cyclobutanes upon the nucleophilic attac k of the enolate at the central allyl carbon atom (C(c)). As a result, from a new theoretical analysis for complexes of the type [(eta3-ally l)ML2]+ (M = Pd, Pt), the C(c)-centered MO (C3-pi(perpendicular-to)) is not destabilized as usually assumed. Rather, it competes with anoth er MO, (d(pi)-nC3) (centered on metal and terminal C(t) carbon atoms), to be the first LUMO of the system. On this basis, the attack of a nu cleophile at the C(c) atom is almost as probable as attack at the C(t) atoms. The initial MO picture of the precursor complex can only sugge st the regioselectivity of the reaction, saying nothing about the attr active/repulsive interactions encountered by the incoming nucleophile in the pathway toward metallacyclobutane. Within the limits imposed by the EHMO method, a simple qualitative description is offered of the i ntended correlations among metal-carbon and carbon-nucleophile bonding /antibonding MO's during the formation of the metallacyclobutane produ ct. An energetic barrier, which varies as a function of the sigma-dono r strength of the nucleophile, is observed, and its origin is describe d.