Stretching the P-C Bond. Variations on Carbenes and Phosphanes.

The stability and the structure of adducts formed between four substituted phosphanes (PX, X:H, F, Cl, and NMe) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable -type structures but carbene → phosphane adducts (-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene-pnictogen interaction. Especially noteworthy is the case of PCl, which has an extremely low energy LUMO in its T-shaped form. Although this PCl structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene-phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both and -type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (), the stability of the adducts formed from carbenes with hypovalent phosphorus (PX-phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX) are the most stable when X is electronegative, and the carbene is highly nucleophilic.