Metal-ligand bond rupture, coordinated

OH to produce the coordinated dimethylphosphate (DMP) anion and methanol in this case, however, the reaction is accompanied by considerable Rh-O bond cleavage. This reaction presumably occurs by the characteristic dissociative conjugate base, SNl(cb), mechanism (136) for metal-ligand bond rupture, which also has a first-order dependence on hydroxide ion concentration. At zero buffer concentration the rate follows the rate law ... 

There are basically two types of intramolecular rearrangements which are not easy to distiguish experimentally. In one type, intramolecular twisting occurs without any metal-donor atom bond rupture. In the other, rupture of one metal-ligand bond of the chelate occurs to give a five-coordinated intermediate. 

The basic classification of nucleophilic substitutions is founded on the consideration that when a new metal complex is formed through the breaking of a coordination bond with the first ligand (or water) and the formation of a new coordination bond with the second ligand, the rupture and formation of the two bonds can occur to a greater or lesser extent in a synchronons manner. When the mpture and the formation of the bonds occur in a synchronous way, the mechanism is called substitution nucleophilic bimolecular (in symbols Sn2). On the other extreme, when the rupture of the first bond precedes the formation of the new one, the mechanism is called substitution nucleophilic unimolecular (in symbols SnI). Mechanisms Sn2 and SnI are only limiting cases, and an entire range of intermediate situations exists. 

The mechanism depicted in Scheme 2 involves two main steps. Rupture of the first metal-nitrogen bond accompanied by coordination of a water ligand at the metal center is followed by reversible deprotonation and intramolecular reduction of the metal center. Under the experimental conditions wherein the concentration of base is much larger than the concentration of tris(diimine) complex, and, applying the steady-state approximation to the concentration of the intermediate species with the monodentate diimine ligand, Eq. (6) can be derived as... 

Fig. 3. Ball and stick model for the ligand-promoted pathway for the dissociation of the iron dimer. Cyanate ion coordinates with the iron metal, exchanging with the hydroxyl bridging bond, rupturing these bonds to produce two monomers...

Three effects are observed in the reactions of Os6(CO)18 with diphenylacetylene and ethylene. There is modification of the metal framework, rupture of a C=C triple bond, and dimerization of ethylene (228-230). When the activated clusters Os6(CO)17(MeCN) (230) and Os6(CO)20(MeCN) (231) react with mono- and disubstituted alkynes, different penta- and hexanuclear framework geometries are obtained, and the alkyne-derived ligands adopt a range of coordination modes (Fig. 8). 

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