Nucleophilic methylene transfer to complexes of sulfur dioxide leads to the formation of metal-coordinated sulfene (thioformaldehyde-S,S-dioxide, H2C=SO2) (see above, Sulfur Oxides) [52, 62, 107]. The cationic ruthenium sulfene complexes are highly reactive electrophiles which add even weak nucleophiles such as Cl– or even the "non-nucleophilic" base NEt(i-Pr)2 . This high reactivity may be exploited in C–C coupling reactions with enamines or enolates [62, 88]. Demetalation of the coordinated sulfinate is achieved by alkylation at oxygen and substitution by acetonitrile . By using chiral ruthenium complexes, this sequence can be developed into an enantioselective synthesis of functionalized sulfinic acid esters .