What is the overall charge of a complex ion and how do you determine the oxidation state of the central metal ion?

For the complex ion $[\text{CrCl}_2(\text{H}_2\text{O})_4]^+$, the overall charge is given as $+1$. This means that when you add up the charge of the chromium ion, the charges of the two chloride ligands, and the charges of the four water ligands, the total must equal $+1$.

We used this in Step 2 to find the oxidation state of Chromium:
Let $x$ be the oxidation state of $\text{Cr}$.

• Each $\text{Cl}^-$ ligand has a charge of $-1$.
• Each $\text{H}_2\text{O}$ ligand has a charge of 0. The sum of these charges equals the overall charge of the complex:

$$x + 2(-1) + 4(0) = +1$$ $$x - 2 = +1$$ $$x = +3$$

So, the central chromium ion has an oxidation state of $+3$.