Magnesium bis(hexamethyldisilazide), Mg(HMDS)(2), reacts with substoichiometric amounts of propiophenone in toluene solution at ambient temperature to form a 74: 26 mixture of the enolates (E)-and (Z)-[(HMDS)(2)Mg-2(mu-HMDS) {mu-OC(Ph) = CHCH3}], (E)-1 and (Z)-1, which contain a pair of three-coordinate metal centers bridged by an amide and an enolate group. The compositions of (E)-1 and (Z)-1 were confirmed by solution NMR studies and also by crystallographic characterization in the solid state. Rate studies using UV-vis spectroscopy reveal the rapid and complete formation of a reaction intermediate, 2, between the ketone and magnesium, which undergoes first-order decay with rate constants independent of the concentration of excess Mg(HMDS)(2) (Delta H--/+ = 17.2 +/- 0.8 kcal/mol, Delta S--/+ = -11 +/- 3 cal/mol.K). The intermediate 2 has been characterized by low-temperature H-1 NMR, diffusion-ordered NMR, and IR spectroscopy and investigated by computational studies, all of which are consistent with the formulation of 2 as a three-coordinate monomer, (HMDS)(2)Mg {eta(1)-O=C(Ph) CH2CH3}. Further support for this structure is provided by the synthesis and structural characterization of two model ketone complexes, (HMDS)(2)Mg(eta(1)-O=(CBu2)-Bu-t) (3) and (HMDS)(2)Mg {eta(1)-O=C(Bu-t) Ph} (4). A large primary deuterium isotope effect (k(H)/k(D) = 18.9 at 295 K) indicates that proton transfer is the rate-limiting step of the reaction. The isotope effect displays a strong temperature dependence, indicative of tunneling. In combination, these data support the mechanism of enolization proceeding through the single intermediate 2 via intramolecular proton transfer from the R carbon of the bound ketone to the nitrogen of alpha bound hexamethyldisilazide.