# Why is Na^+ a WEAKER Lewis acid than Fe^(3+)?

This is because the sodium ion in ${\left[N a {\left(O {H}_{2}\right)}_{6}\right]}^{+}$ is not particularly polarizing.
We would normally write ${\left[N a {\left(O {H}_{2}\right)}_{6}\right]}^{+}$ as $N {a}^{+} \left(a q\right)$. Nevertheless, here the sodium ion is an aquated ion, and not particularly polarizing with respect to electron density. Compare this to ${\left[F e {\left(O {H}_{2}\right)}_{6}\right]}^{3 +}$ or $F {e}^{3 +} \left(a q\right)$. Here, the valence electrons poorly shield the nuclear charge, and electron density is shunted towards the iron centre. The result is that the $O - H$ bond is weakened, and the iron complex is weakly acidic.
I do not know offhand what the $p {K}_{a}$ values of the iron complex is, but I suspect it would be a reasonable Bronsted acid. If you find quantitative data, please post them here.