Hidden aryl-exchange processes in stable 16e RhIII [RhCp*Ar2] complexes, and their unexpected transmetalation mechanism

Experiments mixing the stable 16e 5-coordinate complexes [RhCp*Ar2] (Cp* = C5Me5; Ar = C6F5, C6F3Cl2-3,5) uncover fast aryl transmetalations. Unexpectedly, as supported computationally, these exchanges are not spontaneous, but catalyzed by minute amounts of 18e (μ-OH)2[RhCp*Ar]2 as a source of 16e [RhCp*Ar(OH)]. The OH group is an amazingly efficient bridging partner to diminish the activation barrier of transmetalation.