Carbidocarbonyls

An additional significance for carbidocarbonyl clusters has appeared more recently with the discovery of the fascinating reactivity of carbon atoms in clusters when they are exposed to reactive molecules in low nuclearity carbidocarbonyl clusters. These observations followed on the heels of the recognition of the crucial role played by surface bound carbon atoms in metal-catalyzed carbon monoxide hydrogenation, and so a new area of overlap between cluster chemistry and surface chemistry has arisen. Moreover, in this case the comparisons between organometallic and surface chemistry may lie in reactivity and not merely structural similarities. 

The rapid expansion in carbidocarbonyl cluster chemistry has enabled the chemistry of these compounds to be addressed in its own right rather than as a subgroup of cluster chemistry as a whole. A recent survey of this area by Muetterties has appeared (/), covering the literature up to the end of 1979, and in it some of the aspects of carbide clusters referred to in this introduction are described. However, the rate of growth of the field in the past three years in both the synthesis and reactivity of these molecules makes a second review appropriate. I have incorporated results reported up to mid-1982, and although the earlier work is included with the hope of providing a comprehensive survey, emphasis is given to the most recent observations. 

The first carbidocarbonyl transition metal cluster to be recognized was Fe5C(CO),5 (1), which was isolated in very low yield from the reaction of triiron dodecacarbonyl with methylphenylacetylene and characterized by X-ray diffraction by Dahl and co-workers (2). The molecule (Fig. 1) comprises a square pyramidal Fe5 core with the carbide situated. 08 A below the center of a square face. Each iron atom bears three terminal carbonyls. Improved syntheses of 1 by protonation (5) or oxidation of [Fe6C(CO)l6]2-... 

Iron is so far unique in that it alone forms carbidocarbonyl clusters containing only four metal atoms, and the discovery of this class of compounds and the reactivity of its members has provided one of the most... 

The lowest nuclearity carbidocarbonyl clusters of ruthenium and osmium are RujC(CO)1J( 10, andOs5C(CO),s, 11. The former is synthesized in high yield by the carbonylation of Ru6C(CO),7 (see below) under precisely determined conditions of temperature and pressure [Eq. (9)] (29). 

The first reported examples of M6C carbidocarbonyl clusters were originally found in ruthenium chemistry. Ru6C(CO),7,16, and itsarene derivi-ties RueQCO) (arene) (arene = toluene, xylene, and mesitylene) (36, 37) were synthesized in modest yields by refluxing Ru3(CO)l2 in the requisite arene, 16 becoming the major product when a saturated hydrocarbon such as decane was the solvent. 

The chemistry of the carbidocarbonyl clusters of cobalt and rhodium (none is known for iridium) is predominantly the work of Italian school of the late Paolo Chini and colleagues. The first rhodium cluster of this type to be reported was [Rh6C(CO)ls]2, 24 (59), isolated as a minor by-product in the synthesis of [Rh7(CO)t6]3- (and originally misformulated as [Rh3(CO)l0]-). The formation of 24 resulted from the reaction of [Rh7(CO)l6]3- with chloroform (the source of the carbon atom), present as an impurity in the reaction solvent, and 24 is now synthesized by this reaction [Eq. (17)]. 

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