Several mononuclear ruthenium(IV) complexes with phosphinous acid ligands [RuCl₂(η³:η³-C₁₀H₁₆)(PR₂OH)] have been synthesized (78–86% yield) by treatment of the dimeric precursor [{RuCl(μ-Cl)(η³:η³-C₁₀H₁₆)}₂] (C₁₀H₁₆ = 2,7-dimethylocta-2,6-diene-1,8-diyl) with 2 equivalents of different aromatic, heteroaromatic and aliphatic secondary phosphine oxides R₂P(O)H. The compounds [RuCl₂(η³:η³-C₁₀H₁₆)(PR₂OH)] could also be prepared, in similar yields, by hydrolysis of the P–Cl bond in the corresponding chlorophosphine–Ru(IV) derivatives [RuCl₂(η³:η³-C₁₀H₁₆)(PR₂Cl)]. In addition to NMR and IR data, the X-ray crystal structures of representative examples are discussed. Moreover, the catalytic behaviour of complexes [RuCl₂(η³:η³-C₁₀H₁₆)(PR₂OH)] has been investigated for the selective hydration of organonitriles in water. The best results were achieved with the complex [RuCl₂(η³:η³-C₁₀H₁₆)(PMe₂OH)], which proved to be active under mild conditions (60 °C), with low metal loadings (1 mol%), and showing good functional group tolerance.