A hampered oxidative addition of pre-coordinated pincer ligands can favour alternative pathways of activation.

Pre-coordination to a transition metal by the terminal donor groups of a tri-dentate ligand is a common strategy to stabilise elusive groups, to achieve unprecedented bond activation and to develop novel modes of metal-ligand-cooperation for catalysis. In the current manuscript, we demonstrate that the oxidative addition of a central E-H-bond after pre-coordination to the metal centre is disfavoured for metals with d electron configuration. For exemplary pincer ligands and metals with d electron configuration, quantum chemical calculations suggest a second barrier, which is associated with the rearrangement of the saw-horse structure, obtained after oxidative addition, to the expected square planar geometry for the resulting d electron configuration. In the case of PBP-type ligands with a central LBH-group (L = RP) the reaction with Pt precursors proceeds an alternative pathway of activation, which involves the backside attack of a nucleophile to the boron atom, which facilitates the nucleophilic attack of the Pt centre and formation of a boryl complex (LBH). As the corresponding reaction with a Pt precursor leads to B-H- instead of B-L-activation and formation of complex 2 with a LBH donor, our results show that ligand-stabilized borylenes (LBH) can in principle be converted to boryls (LBH) boronium salts (LBH).