Rhodium carbonyls structures

This has a folded structure (Figure 2.27) similar to that of rhodium carbonyl chloride (Figure 2.18) with ethene acting as a two-electron donor, but ethene is more weakly held and readily displaced by CO and certain alkenes (e.g. cycloocta-1,5-diene). 

Fig. 35. Skeletal structures of the rhodium-carbonyl carbide complexes Rh8(CO)i9C and [Rhis(CO)28C2]- (7).

S. Martinengo, G. Ciani, A. Sironi, and P. Chini, Analogues of Metallic Lattices in Rhodium Carbonyl Cluster Chemistry. Synthesis and X-ray Structure of the [Rh15 ( i-CO)14(CO)13]3 and [Rh14((j,-CO)15(CO)9]4 Anions Showing a Stepwise Hexagonal Close-Packed/Body-Centered Cubic Interconversion, J. Am. Chem. Soc. 100,7096-7098 (1979). 

The structure of the acyl complex was supported by analysis and molecular weight determination. The complex showed a sharp infrared band at 1715 cm. which was assigned to the acyl rhodium carbonyl. This complex is stable in air and does not react with water or alcohol. 

Structure 4 is an intermediate for manufaeturing vitamin A (Scheme 2). The annual demand for vitamin A is about 3000 tons. Major producers are BASF, Hoffmann-La Roche and Rhone-Poulenc Animal Nutrition [55]. At an early stage in the synthesis BASF and Hoffmann-La Roche are using a hydroformylation step to synthesize 4 starting from l,2-diacetoxy-3-butene (5) and 1,4-di-aeetoxy-2-butene (6), respectively [56, 57]. The selectivity toward the branched product in the BASF process is achieved by using an unmodified rhodium carbonyl catalyst at a high reaction temperature. The symmetry of 6 in La Roche s process does not lead to regioselectivity problems. Elimination of acetic acid and isomerization of the exo double bond (La Roche) yields the final product 4 in both processes. 

In iV-confused porphyrins, this agostic coordination mode has been observed frequently. To date, monomeric complexes of this macrocycle with divalent manganese (35, 26), divalent iron (36, 29), lanthanides (57), rhodium carbonyl (55), and group 12 elements (34), have been structurally characterized that exhibit this peculiar type of metal binding. A generic structure for several of... 

For some clusters such as Rh species, the number of electrons available may not match the number apparently required by the structure adopted. Two examples in rhodium carbonyl chemistry are [Rh5(CO)i5] and [Rh9(CO)i9]. The former possesses 76 valence electrons and yet has a trigonal bipyramidal Rh5-core, for which 72 electrons are usual. However, a look at the Rh—Rh bond... 

In the complex rhodium carbonyl anion Rh,o(CO)22(M8-P) the P atom, (formally P ) lies at the centre of a bicapped square antiprism of metal atoms and the CO groups are attached to the comers and edges of this polyhedron [12] (8.244a). The P atom is believed to stabilise the complex and the Rh-P distances indicate it is covalently bound to eight of the metal atoms. The related anion Rh9(CO)2i([t8-P) has a similar structure with an apex Rh atom and a CO group missing. 

Supported metal complexes and clusters with well-defined structures offer the advantages of catalysts that are selective and structures that can be understood in depth. Such catalysts can be synthesized precisely with organometallic precursors, as illustrated in this review. Synthetic methods are illustrated with examples, including silica-supported chromium and titanium complexes for alkene polymerization rhodium carbonyls bonded predominantly at crystallographically specific sites in a zeolite and metal clusters, including Ir4, Rhg, OsjC, and bimetallics. 

---