# Can you predict the coordination number of an ion from the formula of an ionic compound?

Jun 5, 2016

From the formula? Yeah. Let's take three examples.

(1) ["Fe"("NH"_3)_6]^(2+)

Simply from the formula, we see that there are six ${\text{NH}}_{3}$ ligands bound to iron ion. Each coordinated ${\text{NH}}_{3}$ contributes $0$ charge, so the oxidation state of iron is $+ 2$. Therefore, the metal ion is called iron(II).

Since six ligands are bound, the coordination number of ${\text{Fe}}^{2 +}$ is $6$.

The full name of this compound is hexammineiron(II).

(2) ["Ag"("NH"_3)_2]^(+)

Simply from the formula, we see that there are two ${\text{NH}}_{3}$ ligands bound to silver ion. Each coordinated ${\text{NH}}_{3}$ contributes $0$ charge, so the oxidation state of silver is $+ 1$. Therefore, the metal ion is called silver(I).

Since two ligands are bound, the coordination number of ${\text{Ag}}^{+}$ is $2$.

The full name of this compound is diamminesilver(I).

(3) ${\text{CoCl}}_{4}$

Simply from the formula, we see that there are four $\text{Cl}$ ligands bound to cobalt ion. Each coordinated $\text{Cl}$ contributes $- 1$ charge, so the oxidation state of cobalt is $+ 4$. Therefore, the metal ion is called cobalt(IV).

Since four ligands are bound, the coordination number of ${\text{Co}}^{2 +}$ is $4$.

The full name of this compound is tetrachlorocobalt(II).