Structural properties singlet carbenes

Reviews have appeared on the preparation, structure and typical singlet carbene reactivity of phosphinocarbenes (1). Their properties as ligands for metal cations have been covered. The shorter history of another class of stable carbenes, the diaminocarbenes (2), has also been reviewed. ... 

Carbene structures and properties can now be computed with chemical accuracy , despite the difficulties associated with multi-reference species such as singlet carbenes. This is particularly encouraging since the determination of singlet-triplet energy separations and accurate structures of carbenes, which very often are, at best, fleetingly observable intermediates, is extremely difficult experimentally. Transition structures also can be computed apparently with good accuracy and complement experimental studies of the reactions as well as the complex rearrangements commonly found in carbene chemistry. 

---