Answer
$[Cr(NH_{3})_{6}]^{3+}$
Work Step by Step
Amine stands for $NH_{3}$ and hexa means 6.
As the name of metal is in the name of the coordination compound without any change, the complex ion is a cation. The metal is chromium and its symbol is Cr. Since the name of metal is followed by the Roman numeral for 3, the oxidation state of chromium is +3.
The charge of the complex ion=
[Charge of Cr + 6(charge of $NH_{3}$)]
$=(+3)+6(0) =+3$
Therefore, formula for the given complex ion is
$[Cr(NH_{3}) _{6}]^{3+}$
