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Specific Interactions and the Nature of the Chemical Nonequivalence of Chlorine Atoms in the ECl3 Group (E = C, P) of Trichlorophosphonium Ylides and Isomeric Dichloro(trichloromethyl)phosphine

“…The structural (Dl, Da), spectral (Dn), and charge (Dq~Di, where i is the bond ionicity) parameters that relate to the chemical nonequivalence of the C3Cl 1(1<) and C3Cl 2 bonds of the trichloromethyl group of phosphine I are presented in the lower part of Table 1. The pattern of the trichloromethyl part of the 35 Cl NQR spectrum (77 K) of phosphine I is quite characteristic [2] and is determined by the C s symmetry of the CCl 3 group and the stronger ionicity of the transoid C3Cl 1 (C 1< ) bonds: The distance between the low-frequency (Cl 1 , Cl 1< ) and high frequency (Cl 2 ) components of the spectrum (Dn) is 0.6 MHz, and their intensity ratio is 2 : 1 (Fig. 2).…”

“…This mechanism is evidently responsible for the rehybridization of the bond orbital of the phosphorus atom of the P3Cl 2 bond, but it does not explain the weakening of this bond. The final elongation of the P3Cl 2 bond in ylides II and III is evidently mainly determined by the electron density transfer from the 2p orbital of the ylide carbon atom to the antibonding s* orbital of the P3Cl 2 bond by the mechanism of negative hyperconjugation [2]. The energy of the p(PC)3s*(PCl 2 )-interaction is 194.6 kJ mol !1 , which, in terms of the second-order pertrubation theory (MP2), is one of the highest in ylide II (Table 2) and comprises more than three times the interaction energy with transoid bonds (63.4 kJ mol !1 ).…”

“…Secondly, the symmetry of the lone electron pair of phosphorus is close to spherical (s 64%, p 36%), which excludes donor3acceptor interaction of this orbital with the antibonding orbital of the C3Cl 2 bond and weakening of the latter bond. Finally, rehybridization of the bonding orbital of the phosphorus atom of the P3C bond decreases the contribution of the s component (s 16%, p 81%), thus weakening this bond and makes the P3C dissociation pathway preferred over the C3Cl dissociation pathway in the chlorotropic transformation of phosphine I to ylide III [2].…”

“…The transition from the monochloromethyl to trichloromethyl derivative is accompanied by the transformation of the gauche to staggered conformation, noticeable distortion of the C 3v local symmetry of the CCl 3 group, and increase in the activation energy of hindered rotation of this group about the phosphorus3carbom bond [2].…”

“…Trichlorophosphonium ylides isomeric to dichloro-(chloromethyl)phosphines, unlike related phosphorus oxides and imines, have not yet been isolated pure, even though some of their representatives exhibit thermodynamic and kinetic stability [1,2]. Trichlorophosponium ylides present interest in terms of the investigation of peculiar details of interaction of their phosphonium and carbanionic fragments by following alterations in the local symmetry of the trichlorophosphonium group PCl 3 (analogous to those of the CCl 3 group of trichloromethylphosphines) as a specific probe.…”