No abstract

The parent RRЈC-PRЉ(BH 3 ) 2 , 3 (R, RЈ, RЉ = H), has no minimum geometry with ylide structure. In contrast to 'normal' ylides, RRЈC-PRЉ 3 , which are destabilised when the substituents (R, RЈ) have π donor character, the investigated RRЈC-PRЉ(BH 3 ) 2 require at least one amino group to form a stable ylidic structure (e.g. 4NH with R = NH 2 , RЈ = H, and RЉ = H). Without π donor substituents the molecules lack a tautomerisation barrier for the hydroborylationis 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' PRЉ(BH 3 ) 2 group occurs. This electron delocalisation is reflected by the partial charge of the ylide group (Ϫ0.50 in 4HN, Ϫ0.51 in 4NN, and Ϫ0.85 in 6) which is negative (positive in 'normal' ylides). The term 'inverse ylides' could express this special bonding situation.