Dimers carbonyl group coordination

The necessity for ethylene to compete with CO for a coordination site (at high CO pressures the rate of copolymerization becomes negative-order in CO) could also explain the lower rate of copolymer formation compared with dimerization. It is believed that this explanation cannot be the whole picture, but that internal coordination of the growing polymer chain to palladium may be responsible for the observations. Each time, after an ethylene insertion, the polymer chain-end acts as a chelating ligand, in which the oxygen atom of the chain-end s carbonyl group coordinates to the electrophilic palladium ion. 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

The addition paths of two separated MeMgCl molecules to the carbonyl group of chiral a-alkoxy carbonyl compounds, which are shown in Schemes 8 and 9 and in Figure 11 in Section IV, were traced. However, in these models, one MeMgCl molecule bridges two oxygen atoms and acts merely as a catalyst. Instead, the Schlenck dimer (MeMgCl)2 should be considered for the carbonyl reactant coordination. 

Transition-metal-catalysed epoxidations work only on allylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isopropoxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

Both sodium atoms have a coordination number of six. Na2 is bonded to four selenium atoms, an oxygen atom of a carbonyl group and a THF molecule. Unlike Na2, Nal is coordinated by three selenium atoms and three THF molecules. The THF molecules bonded to Nal have a bridging function. This leads to a ribbon like arrangement of the heterocubane dimers along the crystallographic c axis (Fig. 4). 

Oxidation of the [CpFe(CO)2]2 dimer leads to a cation which retains one or more bridging carbonyl groups, but in the presence of coordinating ligands this dissociates into the mononuclear 17e CpFe(CO)2 radical and CpFe(CO)2L+.150 For details on mixed-valent carbonyl dimers, please refer to the following section. 

The bonding in this dimer poses problems. The 18-electron rule predicts a triple Mo—Mo bond, and this is consistent with the short distance which is observed (2.45 A). X-ray dilfraction, however, reveals rather novel coordination of all four carbonyl groups. The Mo—C—0 systems are essentially linear, while the C—0... 

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