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 10 electron configuration. For exemplary pincer ligands and metals with d 10 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 8 electron configuration. In the case of PBP-type ligands with a central L 2 BH 2 -group (L = R 3 P) the reaction with Pt 0 precursors proceeds via 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 0 centre and formation of a boryl complex (LBH 2 ). As the corresponding reaction with a Pt II precursor leads to B-H- instead of B-L-activation and formation of complex 2 with a L 2 BH donor, our results show that ligand-stabilized borylenes (L 2 BH) can in principle be converted to boryls (LBH 2 ) via boronium salts (L 2 BH 2 + ).