Metal-only Lewis pairs by reversible insertion of ruthenium and osmium fragments into metal–boron double bonds

New metal-only Lewis pairs (MOLPs: Ru?Cr and Os?Cr) are prepared by the insertion of a zerovalent ruthenium or osmium complex into chromium–boron double bonds of borylene complexes. The reaction creates new borylene complexes (the first ever for osmium), and it is crystallization-controlled; re-dissolving the complexes results in regeneration of the starting materials. A mechanism is proposed based on DFT calculations, along with a computational study of the unusual MOLPs.