M-C bond strength

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

The elements form compounds R2M and RMX, where R is an alkyl or aryl group and X a halide. M C bond strengths are in the order Zn > Cd > Hg but nevertheless the mercury compounds are the most easily formed for example, from... 

Of particular interest are o-organyls having empty orbitals of it symmetry (with respect to the M-C vector) available for overlap with occupied metal orbitals of n symmetry (t2t,). Such ligands, e.g. alkenyls (vinyls), acyls and alkynyls (acetylides), can in principle supplement their o-bonding with a Jt-retrodative component. This may be reflected in an increased M-C bond strength (Table 1.5), a decrease in the M-C bond length (Figure 4.2) or in restricted rotation about the M-C bond, as sometimes observed in VT NMR experiments. 

Limited information is available on how the reactivity toward SO2 varies within a given triad, all other factors being constant. The SO2 insertion, like the CO insertion (J37), proceeds much faster with CpMo(CO)3R than with CpW(CO)3R (57, 87). Furthermore, the complexes CpFe(CO)2R (R = Me and CHgPh) react with SO2 more rapidly than do their CpRu(CO)2R counterparts (71, 76). Based on the above comparisons, the observed reactivity order MeRe(CO)5 > MeMn(CO)5 (71, 76) comes as a surprise. It would be desirable to correlate the rates of SO2 insertion with the relative M—C bond strengths unfortunately, the latter data are not available. 

For main group metals, which exhibit relatively small differences between M—H and M—C bond strengths and no strong metal-alkene interactions, the thermodynamics of hydrometallation should be even more favorable than for early transition metals. This does not appear to be the case, especially for hydroalumination (Volume 8, Chapter 3.11). The reason is almost certainly the additional stability of the metal hydride reagent conferred by aggregation organoaluminum hydrides exist as rather tight dimers. 

The naked metal ion insertion reaction seems to indicate that high M-C bond strengths allow easy C-C bond cleavage for the bare ions. Apparently this is not the case for coordinated metals. Although C-C bond breaking appears to be kinetically facile as the initial step for the unhindered metal complexes, in the case of usual metal complexes, steric congestion at the metal center seems to retard such a process. " C-H activation is generally both thermodynamically and kinetically favored over C-C activation nevertheless, appropriate selection of reaction conditions and catalyst systems may allow C-C activations. " ... 

Perutz and Eisenstein have used a computational (DFT) approach to estimate the M-C bond energies of a series of partially fluoronated aryl ligands. They show that the main structural predictor of the M-C bond strength is the number of F atoms in the ortho positions, np. Higher M-C bond strengths are associated with larger np. This study shows the growing importance of computational work, particularly in cases like this where experimental determinations of the data are unreliable or unavailable. 

This result indicates that the M-C bond strength increases in the order Mo< CrRaman intensity study of these compounds. On the other hand, Hendra and Qurashi related the Raman intensity ratio of two modes, /(f, CO stretching)//(po, MC stretching), to the TT-character of the M-C bond, and concluded that the M-C bond strength increases in the order Cr< W< Mo < Re(l). 

Comparison of M-H bond strengths with M-C bond strengths allows one to predict the energetics of -hydrogen elimination and thus the stability of alkyl ligands. BDE(M-H)... 

This chapter presented studies of C-H activation of sp, sp, and sp hybridized carbon containing substrates by reactive [Tp RhL] precursors (L = CNneopentyl, PMes, P(OMe)3). By using the relationship between the kinetics of hydrocarbon reductive elimination and the competition for C-H activation, the thermodynamics for the various activations could be determined. Knowledge of the driving force for a reaction (AG°) allows the determination of the relative rhodium-carbon bond energy. Examination of the trends in M-C bond strength showed four important features. 

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