Cyanate
The cyanate ion is an anion with the chemical formula written as [OCN]− or [NCO]−. In aqueous solution it acts as a base, forming isocyanic acid, HNCO. The cyanate ion is an ambidentate ligand, forming complexes with a metal ion in which either the nitrogen or oxygen atom may be the electron-pair donor. It can also act as a bridging ligand. Organic cyanates are called isocyanates when there is a C−NCO bond and cyanates when there is a C−OCN bond.
The three atoms in a cyanate ion lie on a straight line, giving the ion a linear structure. The electronic structure is described most simply as
with a single C−O bond and a triple C≡N bond. The infrared spectrum of a cyanate salt has a band at ca. 2096 cm−1; such a high frequency is characteristic of a triple bond. The cyanate ion is a Lewis base. Both the oxygen and nitrogen atoms carry a lone pair of electrons and either one or the other, or both can be donated to Lewis acid acceptors. It can be described as an ambidentate ligand.
Isocyanic acid, HNCO, is produced when a cyanate salt is acidified. Although the electronic structure according to valence bond theory can be written as HN=C=O, as illustrated, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which clearly requires the C≡N bond to be a triple bond. In valence bond theory the canonical form HN+≡C-O− is the major contributor to the resonance hybrid. The pure compound has a melting point of −86.8 °C and a boiling point of 23.5 °C, so it is volatile at ambient temperatures. In aqueous solution it is a weak acid.
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