Are RR ' C-PR ''(BH3)(2) 'electron poor' phosphorus ylides ? - an ab initio-NMR study

The parent RR'C-PR"(BH3)(2), 3 (R, R', R"=H), has no minimum geometry with ylide structure. In contrast to 'normal' ylides, RR'C-PR"(3), which are des tabilised when the substituents (R, R') have pi donor character, the invest igated RR'C-PR"(BH3)(2) require at least one amino group to form a stable y lidic structure (e.g. 4NH with R=NH2, R'=H, and R"=H). Without pi donor sub stituents the molecules lack a tautomerisation barrier for the hydroborylat ion (RR'C=PH(BH3)-BH2H'--> (RR'H'C- PH(BH3)=BH2). Compound 6, cyclo-1-(C-PH (BH3)(2))- 2,5-(NH)(2)-3,4-(CH)(2), is a model for the ylide recently obtai ned by Arduengo. Analysis of the electronic structure confirms his suggesti on that a considerable delocalisation from the carbene moiety into the 'ele ctron poor' PR"(BH3)(2) group occurs. This electron delocalisation is refle cted by the partial charge of the ylide group (-0.50 in 4HN, -0.51 in 4NN, and -0.85 in 6) which is negative (positive in 'normal' ylides). The term ' inverse ylides' could express this special bonding situation.