Alkyl-bridged species

Metals can attain a more stable electronic configuration by accepting electrons from n-donor and n-donor systems. As discussed in Chapter 2, a variety of compounds such as alkylfithiums, aluminum, beryllium, magnesium, and other related systems form stable alkyl bridged species where the metal draws upon the electrons in a bonds in the alkyl substituent to attain electronic stability. 

Selective binding of ethene and butadiene between the two rhodium sites of a tethered bimolecular bis(por)Rh species with steric demands comparable to TMP, results in selective intramolecular coupling (Fig. 51) (142). This gives further support to the idea that formation of alkyl bridged species from olefins and [M por)] requires a concerted action of two metalloradical sites (157). 

There is self exchange in Me2Cd and with Zn, Ga and In alkyls via alkyl bridged species.41... 

The complexes MZnEt are monomeric in benzene. In cyclopentane as solvent, Hn.m.r. spectra over a range of temperatures indicate the presence of free dialkyls and a complex CaZnEt, which is probably present as a contact ion-pair. The exchange reaction presumably occurs via an alkyl-bridged species ... 

Reactions leading to a series of remarkable dizirconium alkyls and hydrides are outlined in Scheme 213. In solution, the tetrahydride undergoes a rapid fluxional process (observed by NMR spectroscopy) that exchanges the four hydrides. At low temperature three distinct hydrides can be observed, which is consistent with either a dibridged structure, or a tri-bridged species undergoing... 

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

For the aluminum systems so far described, only the monomer-dimer equilibrium has been observed. For very bulkyl R groups, the monomer predominates for small alkyls, cyclopropyl, and phenyl-bridged species, the symmetrical dimer is the stable form. When the phenylethynyl group is present, this equilibrium is complicated further by the additional feature arising from the different possible types of bridge bonds, which may be described in the following manner ... 

The facile isomerization of strained bridged molecules was inconsistent with the involvement of triadsorbed species 15. Instead, a a-alkyl adsorbed species was proposed. Its further transformation leads to a it-complex-like species (17) with a three-center, two-electron bonding with its simultaneous attachment to the surface152,157 (McKervey-Rooney-Samman mechanism) ... 

The kinetics of the carbon disulfide elimination reaction were studied using PMR and visible spectroscopy (16). This spontaneous reaction was found to be first order in the M(RSXant)2 complexes (M = Ni, R = Et, r-Bu, Bz M = Pd, R =/ -Bu), both in the disappearance of the starting material and in the formation of the mercaptide-bridged species in CHC13 and THF. The pseudo-first-order kinetics observed in CS2 are attributed to an equilibrium between the M(RSXant)2 complexes and this solvent. Rate constants for the reaction are of the order of 10 3 to 10"1 min"1 depending on solvent, temperature, alkyl... 

These intermediates contain Nd-Nd and Nd-Al species which are clustered by Cl/alkyl-bridges as well as by Cl/Cl-bridges. In various papers Monakov et al. emphasize the oligomeric and polymeric nature of these species [609-... 

Alkyl halide can oxidatively add to both metal atoms in a bridged dinuclear species, e.g., CH3I with the Au2(H)complex, Au2[CH2PCH2]2- Here, as with the Rh2(I)-bridged species, formation of a new metal-metal bond accompanies one-electron oxidation at each center ... 

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