Structural and anion binding studies have provided an insight into the conformational role of an allosteric CuII ion in the trinuclear catalyst (L 2–2H). L 2 is a novel trinucleating ligand with pyridyl, pyrimidyl and amide donor groups. (L 2–2H)Cu (1) has been characterized by X-ray crystallography. The Cu ion is located at the allosteric site and coordinated by two amide N and two pyrimidine N atoms, the CuN4 plane is tetrahedrally distorted, and the complex adopts a helically twisted conformation. In contrast, the (L 2–2H)Cu3(μ4-C2O4) subunit of the dodecanuclear complex [(L 2–2H)4Cu12(μ4-C2O4)2(μ-OH)4(μ-Cl)4Cl4(H2O)2] (5) is roof-shaped, and the allosteric Cu is located on the top of a square-based pyramid. The oxalate coligand is coordinated by the two catalytic Cu ions in an unusual 1,4-μ-O,O bridging mode with an O ⃛O “bite length” of 2.6 Å and a Cu ⃛Cu distance of 6.4 Å. Intramolecular transesterification of the phosphodiester 2-(hydroxypropyl)-p-nitrophenyl phosphate (HPNP) by [(L 2–2H)Cu3]4+ was investigated, in comparison with the closely related complex [(L 3–2H)Cu3]4+ in which the ligand framework is somewhat less flexible. From a kinetic analysis of cleavage rate at varying HPNP concentrations, K HPNP (the equilibrium constant for binding of HPNP to the complex) and k cat (first-order rate constant for cleavage of HPNP when bound to the complex) parameters were derived: K HPNP=190 M-1 ([(L 2–2H)Cu3]4+) and 305 M-1 ([(L 3–2H)Cu3]4+), k cat=10×10-3 s-1 ([(L 2–2H)Cu3]4+) and 3.3×10-3 s-1 ([(L 3–2H)Cu3]4+). Anion binding constants of the complexes were determined by monitoring competitive inhibition of HPNP cleavage. The complexes have a high affinity to , , and , which appear to be of the appropriate size for bridging coordination, while “smaller” anions and are bound less efficiently. [(L 3–2H)Cu3]4+ has a higher affinity than [(L 2–2H)Cu3]4+ to HPNP but a lower affinity to the rather large anion . This is interpreted as a consequence of the reduced flexibility of [(L 3–2H)Cu3]4+, which slightly disfavours widening of the Cu ⃛Cu distance for incorporation of perrhenate. Similarly, the somewhat lower reactivity of [(L 3–2H)Cu3]4+ is attributed to the larger energy gap between the ground state and sterically more demanding (and less efficiently stabilized) transition state.