Acetonitrile is not "really" a carbonyl compound. Obviously, there is no C=O bond in acetonitrile! Yet, nitriles often are discussed in the context of the carbonyl compounds for two main reasons. First, nitriles can be seen as the products of dehydration (water elimination) of amides. Secondly, nitriles contain a very polar CN multiple bond which shows many of the chemical reactivity patterns also seen for the very polar CO multiple bond.