Sodium cyanide reacts with the cationic ruthenium vinylidene complexes [(eta-5-C5H5)L2Ru = C = C(Me)Ph]+ (L2 = (PMe3)2 or (PPh2CH2)2) to form ruthenium eta-1-vinyl complexes in high yield and with a high degree of stereoselectivity. Computer modelling has been used to rationalise the selectivity obtained.