3 HalidesTrihalides, MX3As shown in [LINK] all the combinations of Group 13 element (M) and halogen (X) exist for the trihalides (MX3), except thallium(III) iodide. It should be noted that while there is a compound with the general formula TlI3, it is actually a thallium(I) compoundThe trihalides of boron are all MONOMERS with a coordination number of 3 ([link]), as evidence from their low melting points ([link]). In contrast, the fluorides and chlorides of the heavier Group 13 elements (except GaCl3) are GENERALLY ionic or have a HIGH ionic character, with a coordination number of 6 ([link], [link] and [link]). The bromides and iodides (except InBr3) are generally dimeric with a coordination number of 4 ([link]) and have molecular structures involving HALIDE bridging ligands ([link] and [link]). AlCl3 is unusual in that in the solid state it has an ionic structure, but it is readily sublimed, and in the vapor phase (and liquid phase) it has a dimeric structure ([link]).Hope this helps you too Mark as a brainliest