solvation isomerism occurs when an inner sphere ligand is replaced by a solvent molecule.
British Dictionary definitions for solvation
the process in which there is some chemical association between the molecules of a solute and those of the solvent. An example is an aqueous solution of copper sulphate which contains complex ions of the type [Cu(H2O)4]2+
(chem) to undergo, cause to undergo, or partake in solvation