Partially and Fully Reversible Solvation-Controlled Borylene Swapping and Metal-Only Lewis Pair Formation

New borylene-containing metal-only Lewis pairs based on group 6 metals and ruthenium are prepared by a partially reversible borylene transfer from group 6 borylene complexes [M­{BN­(SiMe3)2}­(CO)5] (M = Mo, W). The complexes contain strong Ru → Mo/W dative interactions. As in a previous report on related compounds, redissolving these complexes resulted in reversion back to the precursors, enabled by a sacrificial CO donor. It was observed that this reversion process becomes less selective as the group 6 metal becomes heavier, leading to greater amounts of unidentified byproducts as the group is descended.