# Question eb53a

Nov 30, 2015

"Ca"["FeBr"("CN")_2("H"_2"O")_3]_2

#### Explanation:

I assume that you're fairly familiar with the nomenclature used for naming coordination compounds, so I won't go too much into that.

So, start by breaking down the name of the coordination compound

"Calcium " color(red)("triaqua")color(green)("bromo")color(blue)("dicyano")color(purple)("ferrate(II)")

The first important things to notice here is that calcium is the cation and the complex ion is the anion. Now, you know calcium forms ${\text{Ca}}^{2 +}$ cations - keep this in mind.

Now focus on the complex ion. The iron cation is in its $\left(+ 2\right)$ oxidation state, as shown by the $\textcolor{p u r p \le}{\text{(II)}}$ Roman numeral used in naming the complex ion.

As you know, the overall charge of the complex ion will be determined by the charge of the metal cation and the charges of the ligands. In this case, your ligands will be

• color(red)("triaqua") " "-> three water molecules, $\text{H"_2"O}$
• color(green)("bromo") " "-> one bromide anion, ${\text{Br}}^{-}$
• color(blue)("dicyano") " " -> two cyanide anions, ${\text{CN}}^{-}$

The water molecules are neutral, which means that the overall charge of the complex ion will be

${\overbrace{1 \times \left(2 +\right)}}^{\text{iron cation") + overbrace(3 xx 0)^("water") + overbrace(1 xx (1-))^("bromide anion") + overbrace(2 xx (1-))^("cyanide anions}} = 1 -$

The complex anion will thus look like this

[color(purple)("Fe")color(green)("Br")(color(blue)("CN"))_color(blue)(2)(color(red)("H"_2"O"))_color(red)(3)]^(-)

Now, remember that the calcium cation has a $\left(2 +\right)$ charge. This means that in order to have a neutral compound, you need to have two complex anions to balance the charge of the calcium cation.

Therefore, the chemical formula for calcium triaquabromodicyanoferrate(II) will be

"Ca"["FeBr"("CN")_2("H"_2"O")_3]_2#

SIDE NOTE The order of the ligands in the actual formula is not that important. Most of the time, you'll see complex compound formulas being written like I did here, i.e. the anion ligands first, followed by the neutral ligands

As far as I know, that is not a must. However, you must use alphabetical order for the ligands when naming the complex ion.