INTRODUCTION Complexes containing metal-boron bonds are relatively new to organometallic chemistry. Just over 20 years ago, the first crystallographically characterized complexes containing a transition metal-boron bond were reported. Boron is typically encountered in the +3 oxidation state, and boron species commonly serve as Lewis acids via their vacant p-orbital. Indeed, metal borane complexes are known where a neutral borane ligand acts as an electron acceptor (Figure 1). Metal boryl complexes are also common. However, there are far fewer examples of metal borylene complexes, where the boron center is 2-coordinate with 2 electrons for sigma donation to the metal. Recent work on borylene chemistry has focused on overcoming the challenges of isolating kinetically stable mono-coordinate boron ligands, incorporating them into transition metal complexes, and further probing the reaction pathways available to these relatively novel species.