Ketenes are highly electrophilic at the carbon atom bonded with the heteroatom, due to its sp character. Ketene can be formed with different heteroatom bonded to the sp carbon atom, such as O, S or Se, respectively named ketene, thioketene and selenoketene.
Ethenone, the simplest ketene, has different experimental lengths for each of the double bonds; the C=O bond is 1,160Å and the C=C bond is 1,314Å. The angle between the two H atoms is 121.5°, similar to the theoretically ideal angle formed in alkenes between sp carbon atom and H substituents.
Ketenes are unstable and cannot be stored. In the absence of nucleophiles with which to react, ethenone dimerises to give β-lactone, a cyclic ester. If the ketene is disubstituted, the dimerisation product is a substituted cyclobutadione. For monosubstituted ketenes, the dimerisation could afford either the ester or the diketone product.