Ligand exchange processes, bridged

Three types of stereochemical lability have been observed (1) rotations about M-M bonds, (2) cis—r- trans isomerizations at each metal center and (3) bridge = terminal ligand exchange processes. 

Another example is tetrameric methoxy(methyl)zinc, (MeZnOMe)4. The mechanism can be treated both as an aggregative process as well as a ligand coordination exchange process. It reveals aggregation due solely to oxygen bridging, with no direct participation of the C—Zn bond, and will be mentioned only briefly. 

NOM-mineral interactions involve a number of processes such as cation and water bridging, cation and anion exchange, ligand exchange, van der Waals interactions, and hydrophobic bonding (Mortland, 1970 Greenland, 1971 Theng, 1979 Sposito, 1984). 

By D-NMR spectroscopy, up to three hindered ligand movements were detected in [Cr(CO)2L(tj4 CH-diene)] complexes, a situation comparable to [Mn(CO)3(t/3 CH-cyclohexenyl)] complexes (46-51). Only small activation barriers are found for exchange processes of the C—H—Cr bridge. Higher barriers are observed for the site exchange of the three monodentate ligands. Similar energies are necessary to activate 1,5-H shifts (Table VI). 

It is necessary to distinguish between two different H-bridge exchange processes. Complexes with cyclic, symmetrical diene ligands like 37g, 37o, 38g, 38o. 39g, and 39o show an exchange between endo-H-5 and endo-H-6 or endo-H-7, respectively. By this process, the R and S configurations are interconverted via an achiral 16-electron transition state. The relative positions of the monodentate ligands and the diene unit are unaffected by this... 

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