I think officially the SMILES string for this format would be `[C+]#[O-]` ?
See eg. [this quote in a book](http://books.google.com/books?id=pTrwTKzECOsC&lpg=PA82&dq=%22smiles%20for%20carbon%20monoxide%22&pg=PA82#v=onepage&q=%22smiles%20for%20carbon%20monoxide%22&f=false) and compare the cactus resolver response for [[C+]#[O-]](http://cactus.nci.nih.gov/chemical/structure/[C+]%23[O-]/names/xml) compared to [C#O](http://cactus.nci.nih.gov/chemical/structure/C%23O/names/xml) (hydroxymethylene) or [[C]=O](http://cactus.nci.nih.gov/chemical/structure/[C]=O/names/xml) (which it gets wrong and thinks is formaldehyde!)
Cactus also interpret [[C]#[O]](http://cactus.nci.nih.gov/chemical/structure/[C]%23[O]/names/xml) is carbon monoxide. Wikipedia say it should be `[C+]#[O-]`.