From the reactions between 2,2′-ethylidenebis(4,6-di-tert-butylphenol) and 2,2′-methylidenebis(4-dimethyl-6-di-tert-butylphenol) and Et 2AlCl the biphenoxide complexes [(O∼∼CHMe∼∼O)AlCl]2, 1, and [(O∼∼CH2∼∼O)AlCl]2, 2, have been isolated and characterized. These dimers are broken up by donor ligands, and the molecular structure of ethylidenebis(4,6-di-tert-butylphenoxide)AlCl(THF), 3, has been structurally characterized. Racemic 5,5′-6,6′-tetramethyl-3,3′-di-tert-butyl-1,1′-biphen-2,2′-diol and Et2AlCl react in hexane to give [(O∼∼O)AlCl]2, compound 6, as a hydrocarbon insoluble white precipitate. In the donor solvent THF monomeric species are formed, and (O∼∼O)AlX(THF) has been crystallographically characterized, X ) 20% Cl and 80% Et occupancy. Refluxing in THF favors X ) Cl, compound 4. The reaction of Et2Al(OEt) with the biphenol gives (O∼∼O)AlEt(THF), 5, in the presence of THF by displacement of one ethyl and one ethoxide ligand. Compounds 1, 2, 3, 4, 5, 6, [(O∼∼CHMe∼∼O)Al-(O i Pr-d7)]2, and [Cp2Zr(OEt)(OEt2)] + [HB(C6F5)3]act as propylene oxide, PO, polymerization catalyst precursors. The polymers have been examined by MS techniques and NMR spectroscopy, and these results are compared with polypropylene oxide, PPO, formed by base catalysis and by porphyrin-and salen-AlCl catalyst precursors. The new Al compounds and the cationic zirconium alkoxide give close to 50:50 HH to TT junctions with end groups C-Cl, OH, and )CH 2 being identified by MS and NMR. Polymerizations employing [(O∼∼CHMe∼∼O)Al(O i Pr-d7)]2 give HO-(PO)n-O i Pr-d7 oligomers, in addition to vinylterminated species. Polymerization of S-PO and 50:50 mixtures of S-PO and rac-PO reveals that the stereoirregular polymer is formed by a stereoselective ring-opening step. An analysis of the HH and TT junctions at the triad level is made, extending the earlier assignments of Tonelli and Schilling. This analysis leads us to suggest that polymerization occurs by a cationic coordinate mechanism wherein ring opening occurs by backside attack on an activated PO molecule which leads to inversion at the methine carbon. The rac-biphenoxide-Al complexes show a preference for ii and i linkages in (HT)(HT)(HT) units. These results are compared to coordinate catalysis polymerizations of PO employing the Union Carbide calcium amide-alkoxide system and (porphyrin)AlCl and lead us to predict that a cis-migratory ringopening polymerization process is not likely to be developed for polymerization of PO. † Dedicated to Dr. Walter Reichle, Corporate Research Fellow, Union Carbide, on the occasion of his retirement. * Corresponding author. E-mail Chisholm@chemistry. ohio-state.edu.

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Abstract[{(CO)5Cr}2Sn(Cl)2]2, 1, kann aus [(CO)5Cr]2− oder [(CO)5CrSnCl2 · THF] in guten Ausbeuten dargestellt werden. Alternativ ist 1 aus [Cr2(CO)10]2− und SnCl2 zugänglich. Dieses Verfahren kann auf die Synthese von [{(CO)5M}2Sn(Cl)2]2− (M = Mo, 2; M = W, 3) übertragen werden. Die Dianionen 1–3 werden als Alkalimetall‐(12‐Krone‐4)2 oder ‐[2,2,2]Kryptand‐Salze kristallisiert. Einkristallröntgenstrukturanalysen zeigen jeweils die Verbrückung durch die SnCl2‐Einheit mit MSnM‐Winkeln nahe bei 130°. Die Beziehung der Bindungsverhältnisse in 1–3 zu denen, die in Stannylen‐ oder „Iniden”︁‐Komplexen beobachtet werden, wird diskutiert.Die Umwandlung von 1 in die rhombendo‐dekaedrische (Röntgenstrukturanalyse) Käfigverbindung [{(CO)5CrSn}6(μ3‐O)4(μ3‐OH)4], 4, belegt die Reaktivität der Dianionen 1–3.

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