Carbide bromides are mixed anion compounds containing bromide and carbide anions. Many carbide bromides are cluster compounds, containing on, two or more carbon atoms in a core, surrounded by a layer of metal atoms, encased in a shell of bromide ions. These ions may be shared between clusters to form chains, double chains or layers.^[1]

The great majority of these carbide bromide compounds contain rare earth elements. Since these elements have similar properties, similar structures can be made by substituting the elements. R₂CBr₂ forms a structure with layers of R₆C clusters that contain one carbon atom. Each layer has bromide coating the top and bottom. Very similar is R₂CBr₂ which has layers of R₆C₂ clusters containing pairs of carbon atoms. This dicarbon is an ethenide (C₂⁴⁻), and contains a double bond. Layers have bromide on both sides, and so they are only weakly held together by van der Waals forces. If these layers are aligned with each other a 1T- form results with a small c measurement on the unit cell. In some compounds the layers are not quite aligned, but repeat after three layers giving a 3R form, with a larger c unit cell height. Where the layers align, the crystal system is trigonal. But if the layers never quite align at any height, a monoclinic crystal results. The C₂ unit sits at an angle to the layers, and thus reduces symmetry compared to compounds with single carbon atoms in the cluster.^[1]

In R₂CBr there are layers of R₆C that share bromide between layers.^[1]

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