Bond strength rhodium-carbon

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

Thermodynainic Determination of Rhodium-Carbon Bond Strengths in Tp Rh(CNR)(R)H 72... 

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. 

Detailed discussion of bulk metal carbides would be inappropriate here, but aspects of their structures and thermochemistry are worth noting. Many metal carbides are metallic-type conductors of electricity, and have structures very similar to those of the bulk metals, with similar metal-metal distances, but with carbon atoms occupying interstitial sites (commonly octahedral holes) in the metal lattice. Thermochemical information is available on enough of them to get some insight into the relative strengths of both their metal-metal and metal-carbon bonding. Unfortunately, the metals that would be of most interest (osmium, rhenium, and rhodium) for the purpose of comparison with the molecular metal carbonyl carbides already discussed are not known to form stable binary carbide phases M cCj, and the carbides of the 3d metals in the same groups as these have very complicated structures. We therefore focus below on carbides of early transition metals, about which more is known. ... 

The equilibrium constants vary with the structures of the organorhodium(III) complexes (Scheme 1.53) [69]. Whereas the acetyl complex favors the acyl form, the benzoyl complex favors the carbonyl form. This result can be ascribed to the strength of the resulting carbon-rhodium bond the phenyl-rhodium bond is stronger than the methyl-rhodium bond. 

It has been suggested that the number of cation palladium(u) also causes the isomerization of cubane to cuneane, like the silver(i) but unlike d rhodium(i) complexes, eliminates this idea. The redox properties of the respective complexes may be relevant - rhodium(i) is much readier to undergo oxidative addition than silver(i) or palladium(ii). One factor which may be of considerable importance is the relative strengths of metal-carbon bonds. It is energetically much more favourable to insert rhodium(i) into a carbon-carbon bond than to insert silver(i) into such a bond. Though the different courses of isomerizations catalysed by rhodium(i) and silver(i) complexes may be ascribed to the operation of a non-concerted mechanism for the former but... 

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