An Umpolung Mechanism for Oxidative Addition

In the presence of an appropriate base, electrophilic transition metal complexes can heterolytically activate nonpolar bonds H–H/H–X/X–X (X = SiR3, or CR3 as examples). In MeCN, the extreme electrophile {PtII(EtXantphos)2}2+ ({PtII}2+) appears to spontaneously heterolyze H2, yielding {H–PtII(EtXantphos)2}+ ({H–PtII}+) and a solvated proton. As the solution acidifies, a solvated proton can return to the hydride associated {H–PtII}+ to generate the oxidative addition product {(H)2PtIV(EtXantphos)2}2+ ({H–PtIV–H}2+). The net reaction – a heterolytic rebound – corresponds to an umpolung mecha-nism for oxidative addition, with the substrate acting as nucleophile. The rate constants for association of H2 and D2 to {PtII}2+ were measured as 8.4 and 8.5 ( 10-3 psi-1 hr-1), respectively. This minimal kinetic isotope effect (kH/kD = 0.99) is consistent with an intermediate σ-complex featuring negligible backbonding. Our observations comprise a framework for under-standing small molecule activation at the electrophilic extreme.