Rhodium-carbonyl carbide clusters

In the 1970s Union Carbide had reported the use of rhodium with promoters such as amines, carboxylates, etc. for the synthesis of ethylene glycol from CO plus H2. Manufacture of ethylene glycol by this route, however, was never commercialized. The mechanism of this reaction is not understood. Both mononuclear and polynuclear (cluster) rhodium carbonyls can be seen by NMR and IR spectroscopy under conditions approximating that of the catalytic reaction. The question as to whether the catalytic intermediates are mononuclear or cluster has not been answered with any certainty so far. 

Cluster Compounds of Co, Rh, and Ir. In addition to the above-mentioned neutral cluster compounds, there is a large number of anionic carbonyl clusters and metal carbonyl carbides. Carbonyl carbides are formed when the interstice inside the metal cluster is sufficiently large to accommodate the carbon atom. Carbonyl carbides possessing at least four metal atoms are known. The most thoroughly investigated carbides are those of rhodium because they are very stable and resist air oxidation. Carbonyl clusters of group 9 elements containing even more than 20 metal atoms are now known [M6(CO)i5] - (M = Co, Rh, Ir), lM CO)uT. [M6(CO)i5C]"-(M = Co,Rh), [Co8(CO),sC] -, [Rh,(CO)i,] -, [Rh8(CO)i,C], [Ir8(CO)22]"-,... 

In the carbide rhodium clusters, the Rh—C distances of the terminal carbonyl groups are significantly longer than those of the noncarbide derivatives, whereas the corresponding C—0 distances are shorter (see Table II). This trend indicates a decrease in backbonding to the carbonyl... 

The data in Table 4.3 correspond to a radius for the octahedrally coordinated carbon atom that Ues in the range 0.59-0.69 A. We noted earlier that the radius of the core carbon in osmium, rhenium, and rhodium clusters lie in the range 0.59-0.62 A. It appears likely that the enthalpy change ZE(M-C), needed to cleave the six M-C bonds in these molecular carbonyl clusters, will lie in the same range (239-306 kcal moT i.e., 38-51 kcal mol per MC link) that we have now calculated for the similarly coordinated carbon atoms in these extended lattice binary carbides MC or M2C. 

Another example for an investigation of multinuclear transition metal clusters is the SOS-DFPT-IGLO study of the C shift tensors for interstitial carbides enclosed in carbonyl clusters. The interstitial shifts are important, both as a proof for the existence of an interstitial atom, and as a potential probe of electronic structure. Table 2 compares computed and experimental shifts. For the two rhodium clusters, it has been possible to compare not only isotropic shifts but the entire shift tensors, as an independent solid-state NMR study given the first tensor data for a number of interstitial carbides and nitrides. The overall agreement between computation and experiment is good, both for the isotropic shifts and for the available tensors. The largest deviation (43 ppm) was found... 

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