Just aluminium cyanide, no Roman numeral is needed here.
When , you only need to use Roman numerals for that contain metals that can exhibit multiple , like transition metals or .
Aluminium is a special case because you’ll sometimes see it listed as a metalloid, but more often than not it is characterized as metal.
SImply put, aluminium will always have a ##+3## oxidation state, which implies that you do not need to use Roman numerals when naming luminium compounds.
In your case, break up the compound into cation and anion. The subscripts of the cation becomes the charge of the anion, and vice versa – this is known as the criss-cross rule.
##”Al”_color(blue)(1)(“CN”)_color(red)(3) -> “Al”^color(red)(3+) + 3″CN”^color(blue)(1-)##
The only rule you need to follow is the use of the suffix -ide for the anion, which in this case is the cyanide anion, ##”CN”^(-)##.
The name of the compound will thus be aluminium cyanide, ##”Al”(“CN”)_3##.