Pentacoordinate lead complexes neutral

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

Research into the mechanism of the Heck reaction continues and the understanding of the reaction is increasing. Recent research has revealed that in some intramolecular cases another mechanism is observed. Cationic intermediate 68 can be accessed by associative displacement via the pentacoordinate intermediate 70, leading to high enantioselectivity from a reaction that might be thought to proceed via a neutral pathway. Other studies have also identified key roles for pentacoordinate intermediates as well as anionic complexes.f ... 

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