Unusual Bonding and Properties in Main Group Element Chemistry: Rational Synthesis, Characterization, and Experimental Electron Density Determination of Mixed-Valent Tetraphosphetes

Five dispirocyclic λ3,λ5-tetraphosphetes [{R2Si(NR1)(NR2)P2}2] (R1 = R2 and R1 ≠ R2) are easily prepared in almost quantitative yields via photolysis of the respective bis(trimethylsilyl)phosphanyldiazaphosphasiletidines with intense visible light. These deep-yellow low-coordinate phosphorus compounds can be considered as the first higher congeners of the well-known cyclodiphosphazenes. The tetraphosphetes are remarkably stable in air and show unexpected molecular properties related to the unique bonding situation of the central four-π-electron four-membered phosphorus ring. The extent of rhombic distortion of the central P4 ring is remarkable due to an unusually acute angle at the σ2-phosphorus atoms. All of the PP bonds are approximately equal in length. The distances are in the middle of the range given by phosphorus single and double bonds. The anisotropic absorption of visible light that can easily be observed in the case of the yellow/colorless dichroic crystals of [{Me2Si(NtBu)(NtBu)P2}2}] and the exceptional 31P NMR chemical shift of the σ2-phosphorus atoms are the most remarkable features of the λ3,λ5-tetraphosphetes. In the case of [{Me2Si(NtBu)(NtBu)P2}2], the Hansen–Coppens multipole model is applied to extract the electron density from high-resolution X-ray diffraction data obtained at 100 K. Static deformation density and topological analysis reveal a unique bonding situation in the central unsaturated P4 fragment characterized by polar σ-bonding, pronounced out-of-ring non-bonding lone pair density on the σ2-phosphorus atoms, and an additional non-classical three-center back-bonding contribution