The reaction of (p-cymene)Ru2(μ-S2)(S2C2B10H10)2 (I) with HC≡CCH(OH)(CH3)2 in dichloromethane led to addition complexes, (p-cymene)Ru2(μ-S2)(S2C2B10H10)2(R1C=CR2) (R1 = H, R2 = C(OH)(CH3)2 (II); R1 = C(OH)(CH3)2, R2 = H (III)). In boiling chloroform both complexes II and III spontaneously lose water to generate two geometrical isomers (p-cymene)Ru2(μ-S2)(S2C2B10H10)2(R1C=CR2) (R1 = H, R2 = C(CH3)=CH2 (IV); R1 = C(CH3)=CH2, R2 = H (V)), respectively. Complexes IV and V could be interconverted in boiling toluene. All these complexes were characterized by elemental analysis, mass spectrometry, and NMR spectroscopy. The molecular structure of complex IV has been determined by X-ray crystallography (CIF file CCDC no. 1443964). Complex IV crystallizes in monoclinic system, space group P21/c with a = 10.3717(9), b = 20.3982(17), c = 18.6428(13) A, β = 111.096(4)°, C19H40B20Ru2S6, M r = 879.27, V = 3679.8(5) A3, ρ c = 1.587 g/cm3, Z = 4, F(000) = 1752, μ(MoK α) = 1.179 mm–1, R = 0.0416 and wR = 0.0848 for 4602 observed reflections (I > 2σ(I)).