Alkane complexes stability

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

The variation of surface entropy for molecules with complex stabilizing forces other than alkanes requires extensive analysis at this stage, although preliminary analysis shows that for more complicated molecules such as alkyl-naphthalene or alkyl-diester homologous series the plots of y vs. temperature intersect at 30 dyn/cm and 150°C. ... 

The properties and reactivity of alkane and silane - - complexes are closely related to those of dihydrogen complexes. However, the thermodynamic stabilities of the complexes are much different. The silane complexes have been studied in less detail than dihydrogen complexes, but, broadly speaking, they are similar in stability to the dihydrogen complexes or only slightly less stable. Alkane complexes have been studied intensively as reactive intermediates, " but detailed structural and reactivity studies have not been conducted because alkane complexes are unstable in solution. The most detailed data have been gained by NMR spectroscopic studies of CpRe(CO)2(RH) complexes. These data imply that the metal binds in an Tj -mode to a single C-H bond of the alkane. 

The stability of metal ion-alkane adducts such as shown in Figure 11 remains an interesting question. The bonding in such systems can be regarded as intermolecular "agostic" interactions (46). Similar adducts between metal atoms and alkanes have been identified in low-temperature matrices (47). In addition, weakly associated complexes of methane and ethane with Pd and Pt atoms are calculated to be bound by approximately 4 kcal/mol (43). The interaction of an alkane with an ionic metal center may be characterized by a deeper well than in the case of a neutral species, in part due to the ion-polarization interaction. 

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