Why do we write Cu2I2 instead of CuI? What type of bonding is present in dimer or trimer of CuI?

Copper(I) iodide (CuI) is unique because in its solid form it exists as a dimer or polymeric form rather than as separate monomeric units represented by CuI. This is why chemists write the empirical formula as Cu2I2 to reflect the molecular structure more accurately.

The reason behind this stems from the electronic structure and bonding behavior of copper(I). The Cu+ ion has a tendency to form metal-metal bonds or bridged structures through iodine atoms. This creates units where two copper atoms are bridged by two iodide ions, which is well represented as Cu2I2, denoting a dimeric molecule. The bonding involves a combination of ionic and covalent character: copper is positively charged, while iodide is negatively charged, but the Cu-Cu interaction and bridging iodides create a coordination environment better explained by covalent interactions with some metallic bonding character.

The dimer or trimer formations stabilize the structure by lowering the energy through metal–metal interaction and the bridging iodide ligands. This is contrasted to CuI existing as simple ionic units, which is not stable or correct due to copper’s electronic configuration constraints.