# Question 56859

Jun 29, 2016

${\left[{\text{Co"("NH"_3)_5("OH")] ("NO}}_{3}\right)}_{2}$

#### Explanation:

The first thing to note here is that the name you wrote in the question is wrong.

The correct name should be pentaamminehydroxocobalt(III) nitrate, with the emphasis here being on ammine, as opposed to amine.

Ammine is the name given to ammonia when it acts as a ligand, which appears to be the case here. Amine is used to refer to the $- {\text{NH}}_{2}$ group in an organic compound.

Since you're dealing with a coordination compound, the first thing to do here is separate the complex ion and the anion.

Let's start with the complex ion. The prefix penta is equivalent to five, so right from the start you know that you're dealing with $\textcolor{red}{5}$ ammine ligands.

$\textcolor{red}{5} \times {\text{NH}}_{3}$

The name hydroxo is given to the hydroxide anion, ${\text{OH}}^{-}$, when it acts as a ligand. Since a prefix is not given, you know that you're dealing with one hydroxo ligand

$1 \times {\text{OH}}^{-}$

As you know, Roman numerals are used to name cations that can have multiple oxidation states. In this case, the cobalt(III) cation is a cobalt cation that carries a $3 +$ charge, ${\text{Co}}^{3 +}$.

So, the cobalt(III) cation acts as the central ion for this complex ion. Since the ammine ligands are neutral, their presence will not affect the charge of the complex ion.

On the other hand, the hydroxo ligand carries a $1 -$ charge, so it will affect the overall charge of the complex ion, which will be

${\overbrace{\textcolor{w h i t e}{a} \left(3 +\right) \textcolor{w h i t e}{a}}}^{\textcolor{b l u e}{{\text{from Co"^(3+))) + overbrace(color(white)(a)(1-)color(white)(a))^(color(purple)("from 1" xx "OH}}^{-}}} = 2 +$

As you can see from the name of the coordination compound, the ligands are usually listed in alphabetical order while disregarding the prefixes. The complex ion will thus be

["Co"("NH"_ 3)_ (color(red)(5))("OH")]^(2+)

Now focus on the anion. The nitrate anion, ${\text{NO}}_{3}^{-}$, carries a $1 -$ charge, so you will need $\textcolor{b l u e}{2}$ to balance the $2 +$ overall positive charge of the complex ion.

You will thus have

["Co"("NH"_ 3)_ (color(red)(5))("OH")]^(2+)" "# and ${\text{ "color(blue)(2) xx "NO}}_{3}^{-}$

The chemical formula of the coordination compound will be

${\left[{\text{Co"("NH"_ 3)_ (color(red)(5))("OH")] ("NO}}_{3}\right)}_{\textcolor{b l u e}{2}} \to$ pentaamminehydroxocobalt(III) nitrate