Take carbenes. Everybody likes a carbene--it's a lone pair on carbon, but it's formally neutral, and it does interesting reactions like alkene insertion (cyclopropanation), C-H insertion, rearrangements, and the like.
Carbenes are typically highly reactive and short-lived, though examples of persistent carbenes are now well-known. N-heterocyclic carbenes (NHCs) make a prime example. The electron-rich di-adamantyl NHC shown above, for instance, was described in 1991 and can be crystallized (it melts, by the way, at 240 degrees Celsius).
Grubbs' second-generation catalyst, which bears an NHC in lieu of one of the phosphine ligands of the first-generation counterpart. Besides olefin metathesis, though, persistent carbenes (as NHCs) are quite useful ligands for tricky C-C cross-couplings. Specifically, the so-called Pd-PEPPSI complexes are useful for coupling of tetrahedral carbon centers to each other.
thiamin diphosphate (ThDP), which, if you didn't guess, is thiamin with diphosphate attached (it also goes by the name thiamin pyrophosphate, or TPP, which is definitely not confusing at all). ThDP is a coenzyme for pyruvate decarboxylate and pyruvate oxidase, among other enzymes.
short-lived carbene. A recent report in Nature Chemical Biology provides evidence for the latter. The authors examined thiamin diphosphate with the enzyme pyruvate oxidase (from bacterial origins). Phosphate was employed as a mimic of the substrate (pyruvate) that would bind similarly but not form a covalent adduct--this was to see if substrate binding might correspond with the formation of a carbene.
Via circular dichroism (CD), and X-ray diffraction, the authors give evidence that although the coenzyme is C-protonated in its resting state, binding of phosphate results in accumulation of either the carbanion/carbene form. This is narrowed down to the carbene chiefly through analysis of the XRD structure. Not only was the electron density consistent, but the bond lengths and angles were similar to synthetic thiazolium carbenes previously reported. The authors mention similar results upon analysis of the ThDP/cyclohexane-1,2-dione hydrolase complex.
Essentially, under physiologically relevant equilibrium conditions, the thiamine/enzyme complex can accumulate a carbene. In water. That's quite cool.