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 destabilised when the substituents (R, R′) have π donor character, the investigated RR′C–PR′′(BH3)2 require at least one amino group to form a stable ylidic structure (e.g.4NH with R = NH2, R′ = H, and R′′ = H). Without π donor substituents the molecules lack a tautomerisation barrier for the hydroborylation (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 obtained by Arduengo. Analysis of the electronic structure confirms his suggestion that a considerable delocalisation from the carbene moiety into the ‘electron poor’ PR′′(BH3)2 group occurs. This electron delocalisation is reflected 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.