Why are lewis bases good ligands?

Given that Lewis bases are electron pair donors, they can certainly bind to Lewis acidic centres (such as ${H}^{+}$ and metal ions), which ACCEPT electron density. Metal-ligand ligation formally involves donation of an electron pair from the ligand to the metal. For a complex such as ${\left[F e {\left(O {H}_{2}\right)}_{6}\right]}^{3 +}$, what was the Lewis acid, and what was the Lewis base before the complex was formed?