Final answer:
The conjugate base of [Fe(H2O)6]3+(aq) is [Fe(H2O)5OH]2+(aq), which forms when the complex donates a proton from one of its coordinated water molecules.
Step-by-step explanation:
The conjugate base of the aqua complex [Fe(H2O)6]3+(aq) is formed when the complex donates a proton (H+) and becomes [Fe(H2O)5OH]2+(aq). This remaining species is the conjugate base of the original complex because it has one less positive charge and can potentially accept a proton. In acid-base chemistry, a conjugate base is what remains after an acid has donated a proton, and in this case, when one of the coordinated water molecules in the metal complex loses a hydrogen ion, the product is the conjugate base.