Keggin polyoxometalate (POM)-supported two (arene)Ru2+ complexes, i.e., the monomeric species (type-m) [{(arene)Ru(H2O)}PW11O39]5− and the dimeric species (type-d) [({(arene)Ru}PW11O39)2(μ-WO2)]8−, were prepared as water-soluble Et2NH2+ salts by pH-controlled reactions in aqueous solutions of the in situ-generated, mono-lacunary Keggin POM [PW11O39]7− with the organometallic precursor [{RuCl2(arene)}2]. The effects of the arene (benzene (1), toluene (2), p-cymene (3), and hexamethylbenzene (4)) on the proportion of the type-m to type-d complex produced were examined. pH-Varied 31P NMR spectroscopy showed that there was a pH-dependent interconversion between type-m and type-d complexes, i.e., the ratio of the dimeric species increased as pH of the solution was lowered. Under the reaction conditions, the dimeric species d1, d3, and d4 as water-soluble Et2NH2+ salts were successfully isolated as the major products. It was concluded that (1) steric repulsion between the two (arene)Ru2+ fragments in the dimeric species was not significant, (2) the proportion was strongly dependent on the pH of the solution rather than the bulkiness of the arene, and (3) the use of the in situ-generated [PW11O39]7−, but not the isolated lacunary species, such as K7[PW11O39]·nH2O, had an effect on the reaction.

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