The aqueous chemistry of molecules containing the Cp 2 Mo 2+ unit (referred to as "molybdocenes" in this paper) is reviewed. Aqueous molybdocenes are generated by hydrolysis of the Mo-X bonds in Cp 2 MoX 2 complexes (X = halide or pseudo-halide) or by dissolving isolable dimers of the form [Cp 2 Mo(-OH) 2 MoCp 2 2+ ][OTs -] 2 in water. The nature of the molybdocene species in solution is pH dependent: Cp 2 Mo(H 2 O) 2 2+ has pK a1 = 5.5 and pK a2 = 8.5; thus, at neutral and physiological pH, Cp 2 Mo(OH)(OH 2 ) + is the dominant monomer in aqueous solution. This monomer is in equilibrium with [Cp 2 Mo(-OH)] 2 2+ (K eq = 3.5 Γ 10 -2 M Β± 1.3 Γ 10 -3 M at pD 3.5). Cp 2 Mo(OH)(OH 2 ) + and Cp 2 Mo(H)(OH 2 ) + (and the Cp analogs) are catalysts for a variety of reactions in aqueous solution, including H/D exchange reactions that proceed through C-H bond activation pathways, transfer hydrogenation reactions of ketones and aldehydes, nitrile hydration, and the hydrolysis of ethers, carboxylic esters, phosphate esters, and thiophosphinates. In these reactions, the molybdenum center acts as a Lewis acid, activating substrates toward intra-or intermolecular nucleophilic attack by a bound hydroxo ligand or a free water molecule. Mechanistic evidence suggests that the intramolecular hydration and hydrolysis reactions proceed via strained, four-membered ring intermediates.