Unsupported boron-carbon σ-coordination to platinum as an isolable snapshot of σ-bond activation.

σ-Complexes of transition metals-key intermediates in metal-mediated bond activation and homogeneous catalysis-have traditionally been isolable only when chelating or when one of the participating atoms is hydrogen. Here, by treating the Lewis-basic transition metal complex [Pt(PEt(3))(4)] with an electron-poor borirene, we isolate a complex with an unsupported borirene ligand bound, not through the unsaturated C=C bond, but exclusively via a B-C single bond. Using NMR spectroscopy, X-ray crystallography and density functional theory calculations, we show, herein, that coordination of the borirene ligand is based on electron donation from the B-C σ bond to the metal, aided by a strong Pt-to-B dative interaction. The complex is the first isolable non-agostic σ-complex featuring two p-block elements and has broad implications as a model for the metal-mediated activation of strong p-block-p-block σ-bonds.