The synthesis, structure, and olefin, metathesis activity of the first neutral and cationic W-oxoalkylidene-N-heterocyclic carbene (NFIC) catalysts are reported; Neutral W-oxo,alkylidene-NHC catalysts can be prepared in up to 90% isolated yield. Depending on the ligands used, they possess either an octahedral (O-h) or trigonal bipyramidal ligand sphere. They can be activated with excess AlCl3 to form cationic olefin metathesis-active W-complexes; however, these readily convert into neutral chloro-complexes. Well-defined stable cationic species can be prepared by stoichiometric substitution of one chloro ligand in the parent, neutral W-oxo-allcylidene-NHC complexes with. Ag(MeCN)(2)B(Ar-F)(4) or NaB(Ar-F)(4); B-(Ar-F)(4) = B(3,5-(CF3)(2)-C6H3)(4). They are highly active olefin metathesis catalysts, allowing for turnover numbers up to 10,000 in various olefin metathesis reactions including alkenes bearing nitrile, sec-amine, and thioether groups.