Electronic Rearrangement in Steps of Reductive Elimination of Polar Electrophiles Leads to Refinement of Redox Events

The
oxidative addition/reductive elimination of polar molecules
such as methyl iodide at late metal centers has a strongly supported
SN2 mechanism for many key organometallic complexes, including
important industrial catalysts. In the reductive elimination direction,
it is proposed that a ligand initially dissociates, typically a halide,
followed by subsequent nucleophilic attack at the ligand trans to
the now vacant site. The prevailing view is the metal reduction occurs
upon transferring the electrophile in the SN2 step. Herein,
we report the use of an ensemble of computational techniques to characterize
the electronic structure of the reactants and intermediates along
this reductive elimination pathway. These calculations demonstrate,
unexpectedly, that the initiating loss of an anionic ligand from the
octahedral highly oxidized structure leads to an electronic rearrangement
that shifts electron density from the apical ligand back toward the
metal resulting in an inversion of the electron flow between the metal
and apical ligand. The anisotropic shift in electron density to the
metal disproportionately affects the apical position, which is best
described as a Pt → Me dative bond. With this Pt → Me
bonding description, our interpretation of the IUPAC oxidation state
formalism would assign the intermediate as PtII. Although
counterintuitive, the formal and functional reduction of the metal
thus occurs upon halide dissociation.