Aqueous copper(II) N,N',N"-trimethyl-cis,cis-1,3,5-triaminocyclohexane (Cu(tach-Me-3)(2+)(aq)) promotes the hydrolysis of activated phosphate diesters in aqueous medium at pH 7.2. This complex is selective for cleavage of the phosphate diester sodium bis(p-nitrophenyl) phosphate (BNPP), the rate of hydrolysis of the monoester disodium p-nitrophenyl phosphate being 1000 times slower. The observed rate acceleration of BNPP hydrolysis is slightly greater than that observed for other Cu(II) complexes, such as [Cu([9]aneN(3))Cl-2] ([9]aneN(3) = 1,4,7-triazacyclononane). The rate of hydrolysis is first-order in phosphate ester at low ester concentration and second-order in [Cu(tach-Me3)]2+(aq), suggesting the involvement of two metal complexes in the mechanism of substrate hydrolysis. The reaction exhibits saturation kinetics with respect to BNPP concentration according to a modified Michaelis-Menten mechanism: 2CuL + S reversible arrow LCu-S-CuL --> 2CuL + products' (K-M = 12.3 +/- 1.8 mM(2), k(cat) = (4.0 +/- 0.4) x 10(-4) s(-1), 50 degreesC) where CuL = [Cu(tach-Me3)]2+, S = BNPP, and LCu-S-CuL is a substrate-bridged dinuclear complex. EPR data indicate that the dicopper complex is formed only in the presence of BNPP; the active LCu-S-CuL intermediate species then slowly decays to products, regenerating monomeric CuL.