The complexes (BDI)MgX(THF), where X = Bu-n, NEt2, and OBut, are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bun, Delta H-# (kcal mol(-1)) = 13.4 +/- 0.4 and Delta S-# (cal mol(-1) K-1) = 6.3 +/- 1.6; X = NEt2, Delta H-# (kcal mol(-1)) = 15.2 +/- 0.5 and Delta S-# (cal mol(-1) K-1) = 11.4 +/- 2.3; X = OBut, Delta H-# (kcal mol(-1)) = 16.4 +/- 0.3 and Delta S-# (cal mol(-1) K-1) = 9.5 +/- 1.3. The apparent aryl group rotations involving the BDI ligands have, within experimental error, the same activation parameters as the THF dissociation, which suggests that the two are correlated involving a three coordinate reactive intermediate akin to what is well-known for related (BDI)ZnR compounds involving three-coordinate trigonal planar Zn2+. At higher concentrations of THF for X = Bu-n and OBut, but not for X = NEt2, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-d8, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the Ia mechanism to have activation parameters: Delta H-# (kcal mol(-1)) = 5.4 +/- 0.1 and Delta S-# (cal mol(-1) K-1) = -20.9 +/- 0.3 for X = Bu-n and Delta H-# (kcal mol(-1)) = 8.3 +/- 0.8 and Delta S-# (cal mol(-1) K-1) = -19.8 +/- 3.0 for X = OBut. For the complex (BDI)MgBun(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs more readily than for its THF analogue, as expected for the more sterically demanding Lewis base O-donor: Delta H-# (kcal mol(-1)) = 12.8 +/- 0.5 and Delta S-# (cal mol(-1) K-1) = 8.6 +/- 1.8. At very low temperatures in toluene-d(8), restricted rotation about the Mg-O(THF) bond is observed for the complexes where X = NEt2 and OBut but not for the complex where X = Bu-n. These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution.