Metallic moieties

Table 2. Framework Electron Contributions for Metal Moieties ...

The mechanism for aldehyde-derived enamines involves a Michael-type 1,4-addition of the enamine to the alkenylcarbene complex to generate a zwit-terionic intermediate which evolves to the final product by cyclisation. On the other hand, ketone-derived enamines react through an initial 1,2-addition to the carbene carbon to generate a different zwitterionic intermediate. Then, a [l,2]-W(CO)5 shift-promoted ring closure produces a new intermediate which, after elimination of the metal moiety, furnishes the corresponding cyclopen-tene derivatives (Scheme 30). 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

In addition to Refs, [la-d] there have been several recent reviews that have dealt with various aspects of multiple bonding to the group 13 elements (a) Robinson GH (1999) Acc Chem Res 32 773 (b) Downs AJ (1999) Coord Chem Revs 189 59 (c) Robinson GH (2000) Chem Common 2175. Low-valent heavier group 13 species and their bonding to transition metal moieties have also been reviewed recently (d) Linti G, Schnockel H (2000) Coord Chem Rev 206-207 285... 

A peptide, once released, is not subject to reuptake like most transmitters, but is broken down by membrane peptidases. There are no known peptide transporters so that reuptake and re-use are not likely. The peptidases are predominantly membrane bound at the synapse and many are metalloproteases in that they have a metal moiety, most often zinc, near the active site. These enzymes are generally selective for particular... 

Silastannative cyclization of bis(diene) catalyzed by PdC cod) achieves formal 1,8-silastannative G-G bond formation (Equation (118)).125 Poor stereoselectivity at the allylic metal moieties (E/Z) is observed. 

It can be seen that application of the 18-electron rule to clusters necessitates this arbitrary assignment of the number of orbitals of a particular predominant character. The number of orbitals per metal used for cluster skeletal bonding is a consequence of N and E. It varies from two for the M(CO)4 unit in Os3(CO)12 to three for an M(CO)3 unit in Ir4(CO)12. Since these two metal moieties differ, this variation seems reasonable. However, the distribution for an M(CO)3 unit varies with cluster shape as in Ir4(CO)12 and [Os6(CO)18]". ... 

Table l. Coordination mode of transition metal C02 complexes and electronic structure of the metal moiety [20]. 

The detailed study of the molecular orbitals in the different species allowed a better understanding of the interactions under way. It was proved that the charge-transfer from the HOMO of the metal moiety to the n orbital of C02 is the most important interaction in the transition state and that the anti-bonding mixing of the n orbital of C02 also plays a significant role. The leading role of this HOMO-LUMO interaction also explains why the M-OCOH species is more easily formed than the M-COOH species. 

The action of other reagents leading to this cleavage is not clearly imderstood, since the transition metal moiety decomposes ... 

In cobalt-germanium complexes, substitution of one carbonyl by electron-donating ligands should increase the nucleophilicity of the transition metal moiety . ... 

Structures of immobilized rhodium complexes on the sihca support have been proposed on the basis of the data obtained from C, P and Si MAS-NMR. NMR spectra of the rhodium-modified solid materials confirmed that trimethylsiloxide ligand was removed from the rhodium coordination sphere during the immobilization process. Formation of a new covalent bond between the rhodium organo-metallic moiety and the silica support occurs, probably with evolution of trimethylsilanol, which is rapidly converted into disiloxane (Me3Si)20. The presence of this molecule in the solution obtained after the silica surface modification process was confirmed by GCMS analysis. 

XXXVII we also see a bridging group with Al—C distances very close to that observed in other bridged aluminum compounds. The distance between the metal centers in this compound is similar to that observed in the simpler aluminum derivatives but greater than the sum of the covalent radii of the two metals (2.54 A), which may be an indication that Ti—Al interactions do not increase the stability of the bridged system. Structures on Cp2MMe2AlMe2 (M = Y, Er, and Yb) have been recently completed and clearly show stable electron-deficient bonds between the aluminum and the transition metal moiety (XXXVIII) (12). 

Of the few cases of reported scalar couplings between C60 and a metal moiety, the value was comparable with analogous metal-alkene molecules. 

Oxidation waves associated with the metal moiety often are observed. 

The hexaadducts [C60 M(PEt3)2 6] (M = Ni, Pd, Pt) involve an octahedral array of metal moieties in a similar fashion to C60 C(CO2Et)2 6 and possess the very rare point group Th (16). It is believed that each metal fragment binds to one fullerene C=C bond and sterically blocks the neighboring four, so that six moieties will block all 30 C=C bonds of C60. Hence, 7r-adducts involving more than six addends are likely to be difficult to prepare. 

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