Synthesis and Applications of Simultaneous σ- and π-Donor Ligands: C-M Dative Double Bonds

In this area, our research has been strongly inspired by the theoretical work of Frenking about the nature of carbodiphosphorane 1 (Scheme 1). His studies revealed that in compound 1 and analogues the central carbon atom retains its four valence electrons that are thus all available for coordination. In fact, carbodiphosphoranes are known to react with two Lewis acids such as AuCl affording diaurated derivatives. However, their ability to donate their four electrons to the same electrophile in a simultaneous σ- and π-donation had not been described.
In this regard, we envisaged that the use carbodiphosphoranes may provide sufficient stabilization to attenuate the reactivity and allow the isolation of dihydrido borenium cations [L→BH₂]⁺ (L = carbodiphosphorane), a series of compounds that cannot be isolated when classical σ-donating ligands are employed.

Our strategy is already producing some positive results with other low valent cations. Up to now we have been able to synthesize compound 4 which already depicts a σ- and π-dative bond between the central carbon atom and the GeCl moiety. Reaction with dimethylamino pyridine affords adduct 5 where the π interaction is not existing anymore.