Rhodium, Ruthenium and Rhenium

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

Use of ruthenium catalysts [41] and rhenium heptoxide [42] is rare. Their specialty is reduction of free carboxylic acids to alcohols. 

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In a series of papers, Holah et al. studied the donor character of TPPO, TPPS, and TPPSe towards rhodium, ruthenium, and rhenium. Treatment of TPPO with rhodium(III) chloride hydrate in ethanol causes reduction to Rh(I) and gives the dimeric complex (XXVI) in which TPPO is j -bonded to the metal. 

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