Zur Elektronenstruktur von Phosphor(III)-p-π-Bindungssystemen: UV- und PE-spektroskopische Untersuchungen an Methylenphosphanen vom Typ R?PC(SiMe3)2

On the Electronic Structure of Phosphorus(III)-p-π-Bonded Systems: UV and PE Spectroscopic Investigation of Phosphaalkenes R? P?C(SiMe₃)₂ UV and He-I-p.e. spectra of phosphaalkenes R? P?C(SiMe₃)₂ 3 (R = Cl, F, OBu^t, SBu^t, NHBu^t, NHSiMe₃, NEt₂, NPr₂ⁱ, 2, 2, 6, 6-Me₄C₅H₆N, N(Bu^t)SiMe₃, N(SiMe₃)₂, Me, Bu^t) are discussed. Assignment of π(P?C)- and n(P) ionization potentials agrees with UV data as well as semiempirical MNDO calculations. The alkyl-substituted species 31, m exhibit an inverse sequence of π(P?C) and n(P)-i.p.'s compared to theoretical predictions and recent spectroscopic studies, due to the π-electron accepting effect of the silyl substituents. Introduction of RO and RS substituents results in formation of a mesomeric 3-center-4-electron-π-system. Two different conformers of amino-phosphaalkenes can be distinguished which depend on the steric demand of the substituent, and differ in the orientation of the amine ligand in relation to the plane of molecular symmetry. Coexistence of both isomers is proved in case of 3h by temperature dependant UV studies. The existence of conformational isomerism provides an explanation for the remarkable differences in n.m.r. data of 3e – k .