Are RR′C–PR′′(BH₃)₂‘electron poor’ phosphorus ylides?— an ab initio–NMR study

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.

The 1,4-Diphosphabuta-1,3-diene Ligand for Coordination of Divalent Group 13 and 14 Elements: A Density Functional Study

“…Also the isoelectronic anions with E = Al and Ga were reported. The latter, the group 13 element homologues, were first proposed on the basis of quantum chemical calculations. , For the group 14 element species a variety of physicochemical investigations were performed − (Chart ).

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Chemical Bonding to N, P, and As in Ylides and Their Boron Analogues

“…Although bonding in ammonium ylides has been clearly determined as a single one, 7 in recent years, the controversy for phosphonium ylides still continues. 8 Although these compounds have been extensively studied, a clear characterization for the central P-C bond remains elusive. On the other hand, there is a clear agreement on its covalency, with a strong polar character, its bond length being determined mainly by electrostatic interactions.…”

“…The typical bonding scheme for these compounds is explained by contributions to the ylide and ylene forms depicted in Scheme , which accounts for the relative multiple character of some ylides. Nevertheless, the ylene form requires a dsp 3 hybridization to allow a multiple bond for P and As atoms, but not for N. This possibility was discarded, and there is general agreement that the computational use of a d shell is necessary only as polarization functions, not participating in the bonding. − Different molecular orbital (MO) calculations support the possibility of a double bond, describing a HOMO that consists of a carbon p x orbital extended clearly to the phosphorus. Another fact that reveals a strong interaction is the short P−C bond distance (ca.…”

“…Although bonding in ammonium ylides has been clearly determined as a single one, in recent years, the controversy for phosphonium ylides still continues . Although these compounds have been extensively studied, a clear characterization for the central P−C bond remains elusive.…”

Computational study of the bonding structure in ylide compounds

“…As can be seen in Tables 2 and 3, the calculated geometries of H m DAH n complexes agree quite well with previously reported theoretical data. [6][7][8][9][10][21][22][23][24][25][26][27][28][29][30][31][32][33] For the H m DAH 2 complexes (A = C, N + , Si, P + ), there is a reasonable correlation between the t-angle, which measures the degree of planarity at the A atom, and the SDE ST values (DE ST is the energy difference between the singlet and triplet electronic states for DH m and AH n . Alternatively, H 3 DAH 2 bonding can also be described as the interaction of H 3 D with an n 2 singlet AH 2 (as shown in structure 3), or with a p p 2 singlet excited AH 2 (as shown in structure 4); 34 specifically, an ylidic bond can be viewed as a dative bond.…”