Piperazine compounds of the formula I in which the variables are defined according to the description, their agriculturally suitable salts, processes and intermediates for preparing the piperazine compounds of the formula I, compositions comprising them and their use as herbicides, i.e. for controlling harmful plants, and also a method for controlling unwanted vegetation which comprises allowing a herbicidally effective amount of at least one piperazine compound of the formula I to act on plants, their seed and/or their habitat.