Concerted or stepwise? Ion-pair separation is often proposed as a mechanism for enantiomerization of organolithium reagents in ethereal solvents. But what is the timing of the solvation and bond-cleavage events? Are they concerted, or does one precede the other? Reaction order and Eyring studies of Grignard 1 in Et2O/toluene mixtures clearly demonstrate addition of solvent occurs before rupture of the MgC bond.