Hafsa, the answer to your question is AlCl3. AlCl3 exists as a dimer as the Al atom in the molecule is electron deficient due to presence of only 6 electrons in its' valence shell ie it has an incomplete octet due to which it is quite unstable. Now, every compound desires to attain stability and minimise any strains in its structure, so in order to gain stability and complete its octet, two AlCl3 molecules dimerise through formation of coordinate bonds between lone pair of electron on Cl atom in one AlCl3 molecule and the empty p orbital on Al in another AlCl3 molecule, thereby forming a dimer which consists of three-center-two-electron bonds known as banana bonds between electron deficient Al and Cl in each AlCl3 molecule and the coordinate bonds between bridging Cl atoms and the election deficient Al.

The dimer so formed (Al2Cl6) is more stable and undergoes milder reactions than the monomer AlCl3.

Hope this helps you in your understanding of the topic.