Most of the chemistry of boron is nonmetallic in nature. Unlike its heavier congeners, it is not known to form a simple B or hydrated [B(H 2 O) 4 ] cation. The small size of the boron atom enables the preparation of many interstitial alloy-type borides. Analogies between boron and transition metals have been noted in the formation of complexes, and adducts (for example, BH 3 + CO →BH 3 CO and, similarly, Fe(CO) 4 + CO →Fe(CO) 5), as well as in the geometric and electronic structures of cluster species such as [B 6 H 6 ] and [Ru 6 (CO) 18 ]. The aqueous chemistry of boron is characterised by the formation of many different polyborate anions. Given its high charge-to-size ratio, boron bonds covalently in nearly all of its compounds; the exceptions are the borides as these include, depending on their composition, covalent, ionic, and metallic bonding components. Simple binary compounds, such as boron trichloride are Lewis acids as the formation of three covalent bonds leaves a hole in the octet which can be filled by an electron-pair donated by a Lewis base. Boron has a strong affinity for oxygen and a duly extensive borate chemistry. The oxide B 2 O 3 is polymeric in structure, weakly acidic, and a glass former. Organometallic compounds of boron have been known since the 19th century (see organoboron chemistry).