Question

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

Answer 1

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 `"NH"_3` ligands bound to iron ion. Each coordinated `"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 `"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 `"NH"_3` ligands bound to silver ion. Each coordinated `"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 `"Ag"^(+)` is `2`.

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

(3) `"CoCl"_4`

Simply from the formula, we see that there are four `"Cl"` ligands bound to cobalt ion. Each coordinated `"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 `"Co"^(2+)` is `4`.

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