Thermochemistry metal carbonyls

It is not intended to discuss the details of the various methods of thermochemical measurement and the evaluation of results. This has been done in authoritative articles by Skinner1 and by Pilcher2) which have appeared recently, and which deal specifically with the thermochemistry of organometallic compounds. Instead this article will survey the results which may be derived from the information which is available and relate them to features of metal carbonyl chemistry in particular. 

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. ... 

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. 

Ribeiro da Silva, M. A. V., Reis, A. M. M. V., Thermochemistry of the Metal-Carbon Bond. Critical Review of Bond Energy Data of Organometallic Compounds and of Transition Metal Carbonyls, Rev. Port. Quim. 20 [1978] 47/62. 

F. A. Adedeji, J. A. Connor, C. P. Demain, J. A. Martinho-Simoes, H. A. Skinner, and M. T. Z. Moattar, J. Organometallic Chem., 1978,149, 333. Thermochemistry of Group VI metal carbonyl-pyridine and-acetonitrile complexes. 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

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