The Origin of the Directing Effect in H2 Addition to Square-Planar d8 Complexes

Article abstract of DOI:10.1021/ja00223a022

Hydrogen adds to square-planar d⁸ complexes of the Vaska type to give oxidative addition products in which the H2 has added parallel to the X-Ir-CO axis.In principle, however, addition perpendicular to this axis should also be possible.Three models have been discussed to account for the apparent preference for parallel adducts.We show that in contrast to the chloro complex (X = Cl, parallel adduct formed), the methyl, phenyl, or hydride analogues give perpendicular adducts.In one case (X = Ph), the perpendicular adduct is formed at -80 deg C, but rearranges to the parallel adduct on warming via reductive elimination of H2 and readdition.These findings seem to invalidate the previous theoretical models that have been proposed to explain the direction of H2 addition.A new one is suggested that covers all the data gathered up to now.