The compound (CH 3 CH 2) 3 Al 2 Cl 3 is called a sesquichloride, because, on average, each Al centre has 1.5 chloride ligands.
The compound (CH 3) 3 Al 2 Cl 3 is called methylaluminium sesquichloride. The term sesquichloride refers to the fact that, on average, each Al centre has 1.5 chloride ligands.