Mesomeric Tuning at Planar Bi centres: Unexpected Dimerization and Benzyl C-H Activation in [CN 2 ]Bi Complexes.

Planar bismuth(III) compounds featuring a N 3 -pincer ligand environment are an unusual class of main group compounds that exhibit Lewis acidity via a 6p orbital. The ability to tune their acidity via Hammett and Coulomb effects has been demonstrated previously. Here we test the ability to tune them via mesomeric effects by replacing the central nitrogen of the [N 3 ] pincer ligand with a less π-donating carbon atom to achieve a [CN 2 ] ligand environment. This strategy boosts the Lewis acidity at the metal to such an extent that only strongly-bound dimers can be accessed in the condensed phase, or, if dimerization is sterically prohibited, an unusual reductive elimination occurs to give Bi(I) compounds. These results highlight the fine line separating donor/acceptor and redox chemistry at planar bismuth centres.