The preparation and characterization of a series of closely related magnesium and zinc compounds are reported: LMg((NPr2)-Pr-i)(THF), 1; LZn((NPr2)-Pr-i), 2; LMg((OBu)-Bu-t)(THF), 3; LZn((OBu)-Bu-t), 4; and LZn(OSiPh3)(THF), 6; where L = CH-(CMeNC6H3-2,6-Pr-i(2))(2). Their dynamic solution behavior has been examined by variable-temperature NMR studies and reveals that THF reversibly dissociates in toluene-d(8) or CD2Cl2 and that exchange with free THF occurs by a dissociative process, Compounds 1-4 and 6 all initiate and subsequently sustain ring-opening polymerization (ROP) of lactides. For a related series of compounds LMX(THF)(n), where n = 1 or 0, the rate of initial ring-opening follows the order M = Mg > Zn and X = (OBu)-Bu-t > (NPr2)-Pr-i > NSi2Me6 > OSiPh3. In THF at 25 degreesC, compounds 3 and 4 polymerize 100 equiv of rac-lactide to >95% conversion in 5 and 80 min for M = Mg and Zn, respectively, and yield ca. 90% heterotactic PLA, (isi + sis tetrads). The reactions proceed faster in methylene chloride, but for M = Mg, a Bernoulian distribution of tetrads is formed from rac-lactide (3iii:2isi.sii.sis:iis) prior to trans-esterification. Polymerization of L-LA in toluene-d(8) and THF-d(8) by 3 and 4 have been studied by VT H-1 NMR spectroscopy: the resting state for zinc is proposed to be a monomeric species akin to LZn(eta(2)-OCHMeC(O)OMe), whereas the magnesium complex appears to be dimeric LMg(mu-OP)(2)MgL. None of the compounds is capable of initiating homopolymerization of propylene oxide (PO) or cyclohexene oxide (CHO), although the magnesium amide 1 effects ring-opening by allylic proton abstraction and the dimeric compound [LMg(mu-OC6H9)](2), 7, is formed. Reactions with carbon dioxide are also described, along with the characterization of (LZnO2CNPr2)-Pr-i, 8, which is shown to be inert with respect to CHO and PO at room temperature. All the compounds are hydrolytically sensitive, and LZn(mu-OH)(2)ZnL, 5, has been isolated from hydrolysis of compound 4. The crystal and molecular structures are reported for compounds 1-5, 7, and 8. These results are compared with those recently reported by Coates et al.