Answer
$[Cr(NH_{3})_{6}](NO_{3})_{3}$
Work Step by Step
The formula is written according to the rules of writing the formula for complex compounds.
1. Hexaamminechromium(III) is a cation whereas nitrate is the anion. Cations are written first then anion.
2. Hexaammine is the ligand and chromium is the metal. Thus, $[Cr(NH_{3})_{6}]$
3. Chromium(III) means the oxidation state of chromium ion is +3. $NH_{3}$ is a molecule so the oxidation state is zero.
4. Nitrate $NO_{3}^{-}$ has a -1 charge. In order to balance the charge on the cation and anion, 3 atoms of nitrate ion should be present. Therefore the chemical formula is:
$[Cr(NH_{3})_{6}](NO_{3})_{3}$
