Al2Cl6 - Different length of bonds
We know that the number of electrons in both the bonds are same. One of the bond involves a covalent bond $\ce{Al-Cl}$ where as the other is a 3-centred bond $\ce{Al-Cl\bond{-&gt;}Al}$. Thus while providing a dative bond, the effective atomic charge of bridged chlorine decreases in comparison with that at the terminal position. Due to this difference in atomic charge, the terminal ones attract more and closer that the bridges.