Any substance in which two or more chemical elements other than carbon are combined, nearly always in definite proportions (see bonding), as well as some compounds containing carbon but lacking carbon-carbon bonds (e.g., carbonates, cyanides). Inorganic compounds may be classified by the elements or groups they contain (e.g., oxides, sulfates). The major classes of inorganic polymers are silicones, silanes, silicates, and borates. Coordination compounds (or complexes), an important subclass of inorganic compounds, consist of molecules with a central metal atom (usually a transition element) bonded to one or more nonmetallic ligands (inorganic, organic, or both) and are often intensely coloured. See also organic compound