From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [Ni-2(II)(L)(H2O)(2-)(CH3OH)][ClO4]CH3OH (1) (H3L = 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N',N"-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH3CN/CH3OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [Ni-5(II)(H2L)(2)(mu(3)-OH)(2)(mu-O2CCH3)(4)][ClO4](2)CH3CO2C2H5 (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [Ni-2(II)(HL)(mu-O2CCH3)(H2O)(2)][ClO4]4H(2)O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H3L with 2 equiv of Ni-II(O2CCH3)(2)4H(2)O, followed by the usual workup, and recrystallization from CH2Cl2 led to the isolation of [Ni-2(II)(H2L)(mu-O2CCH3)(2)][ClO4]CH(2)Cl(2)2H(2)O (4). Complex 4 is structurally different from 3, confirming that the reaction of Ni-II(O2CCH3)24H2O with H3L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by Ni-II-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [Ni-2(II)(HL)(mu-O2CCH2CH3)(H2O)(2)][ClO4] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = -25.0(1) cm(-1) for 1, J = -20.0(1) cm(-1) for 4, and J = -18.80(8) cm(-1) for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (J(a) = +16.02 cm(-1), and J(b) = +9.02 cm(-1)) and an antiferromagnetic (J(c) = -49.7 cm(-1)), have been identified.