The kinetics of the ligand exchange in (PPh 4 ) 2 [Mo(CN) 3 O(salhy)] AE 6H 2 O (Hsalhy = salicylaldehyde hydrazone) by a solvent molecule and by 2,2¢-bipyridine (bpy) have been studied in EtOH. For the ligand exchange by a solvent molecule the pseudo-first order rate constant equals k obs = 3.2 (±0.2) · 10 )3 s ), while for the exchange by a bpy molecule k obs ¼ 3.5 (±0.2) · 10 )3 s )1 (t ¼ 25°C), DH à ¼ 56 (±7) KJ mol )1 , DS à = )104 (±8) J mol )1 K )1 . It was found, that all reactions proceed via the same mechanism which involves the chelate ring opening cis to the Mo@O bond. The mechanism of the reaction was proposed and was proved by the synthesis of (PPh 4 ) 2 [Mo(CN) 3 O(N-pic)] AE 2.5H 2 O (N-pic denotes that the nitrogen of picolinic acid is trans to Mo@O) by ligand exchange in EtOH, while in aqueous solution the O-pic analogue is formed exclusively.