The catalytic activity of [CpRu(L)(2)(MeCN)] PF6 (L = 2-diphenylphosphinopyridine with bulky groups at C-6) for anti-Markovnikov hydration of terminal alkynes to aldehydes is retained when one heterocyclic ligand L is replaced by L' = PPh3. Equal amounts of CpRuCl(PPh3)(2) (1) and phosphane L in acetone solution equilibrate to a mixture of 1, CpRuCl(L)(PPh3) (2), and CpRuCl(L)(2) (3), which acts as highly active in situ catalyst for preparative anti-Markovnikov hydration of alkynes in water-rich media (2 mol % [Ru], 60 degrees C, 3-18 h in 4:1 (v/v) acetone/water). Reactions were completed in <15 min at 160 degrees C.