Ferrocene carboxylic acid 2-cyclohexa-1,4-dienyl-ethyl ester (1) is prepared from ferrocene carboxylic acid and 2-cyclohexa-1,4-dienyl-ethanol. This diene reacts with RuCl3.nH(2)O in refluxing ethanol to afford quantitatively [Ru(Fc-arene)Cl-2](2) (2) (Fc-arene=ferrocene carboxylic acid phenethyl ester). The dinuclear complex 2 reacts with triphenylphosphine to give the mononuclear complex [Ru(Fc-arene)(PPh3)Cl-2] (3). The trinuclear arene-ruthenium cluster cation [H3Ru3(Fc-arene)(C6Me6)(2)(O)](+) (4) is synthesised from the dinuclear precursor [H3Ru2(C6Me6)(2)](+) and the mononuclear complex [Ru(Fc-arene)(H2O)(3)](2+) accessible from 2 in aqueous solution. The water-soluble trinuclear cluster cation 4 catalyses the hydrogenation of benzene to give cyclohexane under biphasic conditions. (C) 2003 Elsevier Science B.V. All rights reserved.