# Which is the stronger lewis acid Cu+ or Cu2+?

Sep 28, 2016

Given the definition of $\text{Lewis acid}$, surely it is $C {u}^{2 +}$?

#### Explanation:

A $\text{Lewis acid}$ is an electron pair acceptor. A priori we would expect that a dication, $\text{cupric ion}$, is more Lewis acidic than $\text{cuprous ion}$. In order to quantify this prediction we might compare complex formation constants for ${\left[C u {L}_{6}\right]}^{2 +}$ versus ${\left[C u {L}_{6}\right]}^{+}$.

Sep 29, 2016

This can also be explained via HSAB theory.

A harder acid holds onto its electron density more than a softer acid. The hardness of a lewis acid increases with increasing positive charge, as the electron density is more concentrated.

Therefore, ${\text{Cu}}^{2 +}$ is the harder acid, being less polarizable than a softer acid.

That means it accepts electron density more easily, because it holds onto it more easily (it is less polarizable), and consequently has greater polarizing ability (can distort other electron clouds more easily than a softer acid can).

Thus, $\boldsymbol{{\text{Cu}}^{2 +}}$ is the stronger Lewis acid.