The coordination geometry of zeolite-encapsulated copper(II)-histidine (CuHis) complexes, prepared by ion exchange of the complexes from aqueous solutions into zeolite NaY, was determined by a combination of UV-vis-NIR diffuse reflectance spectroscopy (DRS), X-band EPR, electron-spin-echo envelope modulation (ESEEM), and high field (W-band) pulsed ENDOR techniques. X-band EPR spectroscopy detected two distinct complexes, A and B, which are different from the prevailing Cu(II) bis-His complex in the exchange solution (PH 7.3 with a His:Cu(II) ratio of 5.1). Moreover, the relative amount of complex B was found to increase with increasing Cu(II) concentration. The EPR parameters of complex A are g(perpendicular to) = 2.054, g(parallel to) = 2.31, and A(parallel to) = 15.8 mT, whereas those of complex B are g(perpendicular to) = 2.068, g(parallel to) =2.25, A(parallel to) = 18.3 mT, and A(perpendicular to)(N-14) similar to 1.3 mT. The presence of the N-14 superhyperfine splitting shows that in complex B three nitrogens are coordinated to the Cu(II). Furthermore, DRS exhibits a shift of the d-d absorption band of Cu(II) from 15 200 cm(-1) in complex A to 15 900 cm(-1) in complex B, indicating an increasing ligand field strength in the latter. The coordination of the imino nitrogen of the imidazole group was detected in the two complexes via the ESEEM frequencies Of the remote nitrogen. In contrast, only complex A exhibited Al-27 modulation, which indicates that the Cu(II) binds to zeolite framework oxygens. H-2 and H-1 W-band ENDOR measurements on samples where the exchangeable protons were replaced with H-2, and using specifically labeled histidine (His-alpha -d-beta -d(2)), lead to the unambiguous determination of the coordination configuration of the two complexes. Complex A is a mono-His complex when both the amino and imino nitrogens are coordinated and the other equatorial ligands are provided by a zeolite oxygen and a water molecule. Complex B is a bis-His complex, which is situated in the center of the supercage, and all equatorial coordination sites are provided by the His molecules. These are amino and imino nitrogens of one His molecule and the imino nitrogen and carboxylate oxygen of the second His molecule. Complex A can be converted into complex B by stirring the zeolite in a high pH solution, whereas complex B is converted into complex A by using a low pH solution, thus indicating that complex A is stabilized by the presence of intrazeolitic protons. On the basis of the structure of the complexes, the dependence of their relative amounts on the pH and Cu(II) concentration in the exchange solution, the His: Cu(II) ratio in the zeolite, the amount of exchanged Na(I) ions, and the steric constraints imposed by the zeolite framework, a model for the ion exchange processes and the intrazeolite reactions leading to the formation of the two complexes is presented.