The macrobicyclic mixed donor cage ligand AMME-N(3)S(3)sar (1-methyl-8-amino-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]eicosane) is capable of binding to Cu(II) as either a hexadentate (N3S3) or tetradentate (N2S2) ligand. The "Cu-in" (hexadentate)/"Cu-out" (tetradendate) equilibrium for the {Cu(AMME-N(3)S(3)sar)}(2+) units is strongly influenced by both solvent (DMSO, MeCN, and water) and halide ions (Br- and Cl-). We have established a crucial role of the solvent in these processes through the formation of intermediate solvato complexes, which are substituted by incoming halide ions triggering a final isomerization reaction. Surprisingly, for reactions carried out in the usually strongly coordinating solvent water, the completely encapsulated N3S3-bound Cu in form is dominant. Furthermore, the small amounts of the Cu out form present in equilibrated DMSO or MeCN solutions revert entirely to the Cu in form in aqueous media, thus preventing reaction with halide anions which otherwise lead to partial or even complete decomposition of the complex. From the kinetic, electrochemical, and EPR results, the existence of an outer-sphere H-bonded network of water molecules interacting with the complex inhibits egress of the Cu(II) ion from the cage ligand. This is extremely relevant in view of outer sphere interactions present in strongly hydrogen bonding solvents and their effects on Cu(II) complexation.