Spectroscopic Properties of Carbide Clusters

The application of spectroscopic techniques familiar in organometallic chemistry to the unique features of carbidocarbonyl clusters has become more evident since the previous review of this field (1). Vibrational spectroscopy and 13C NMR have yielded useful data relevant to the geometric and 

In their original paper (2) on the structure of Fe5C(CO)l5, Dahl and co-workers assigned two bands in the infrared spectrum of hydrocarbon solutions of the cluster, at 790 and 770 cm-1, to vFeC modes. This assignment has been confirmed by a recent study of the infrared spectra of the series M5C(CO)15, (M = Fe, Ru, Os) (78). The room temperature spectra of the compounds (Table II) in the solid state are quite similar to each other, comprising three bands assigned as the a, and e modes (split in the solid state) expected for the C4 symmetry of the isostructural clusters. At low temperature the ruthenium and osmium clusters exhibit five absorptions associated with M-C stretches, whereas the iron cluster retains its room temperature spectrum. This is ascribed to the presence of two types of cluster molecule in the crystal lattices of the ruthenium and osmium clusters which are isostructural with, but not isomorphous with, the iron analog in which all the molecules are identical. 

This technique has also been applied to the products of hydrohalogena-tion of RusC(CO)[ 5. The three vRuC absorptions of the parent cluster at 757, 738, and 730 cm-1 are reduced to two for HRu5C(CO)l5X at 823 and 685 cm-1 (X = Cl) and 823 and 687 cm-1 (X = Br). These observations were interpreted on the basis of a rearrangement of the square pyramidal Ru5C(CO)l5 to a distorted bipyramidal geometry similar to that found for [Os5C(CO)15I]-(Fig. 12) (32). 

The vibrational frequencies of the encapsulated carbon atom in [Osl0C(CO)24]2-, 21 (Fig. 26), and its protonated derivative H2OS 0C(CO)24 have been identified by Oxton et al. (57) and the assignments confirmed by isotopic enrichment. The carbon atom in the tetrahedral Os,0 dianion resides in an octahedral cavity, and at room temperature a band at 753 cm-1 is observed and assigned to Vos.c- On protonation three absorptions of approximately equal intensity are observed in this region and 3C enrichment of the central carbon atom identified these absorptions, at 772.8, 

and 735.4 cm-1, as associated with the motion of the carbido carbon atom. The increase in the number of infrared active modes in H2Osl0C(CO)24 over the dianion is attributable to a lowering of the symmetry of the cluster on protonation, and since the carbide carbon is in the octahedral cavity of the cluster (56), the osmium atoms defining this cavity were suggested as possible sites of protonation. 

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